literature review on longwall mining - waternsw · s y d ne c a t c h ment a u t h o r i t y may...

SYDNEY CATCHMENT AUTHORITY

MAY 2007

LITERATURE REVIEW ON LONGWALL MINING Collaborative Research Program: Impacts of

Longwall Mining in the Waratah Rivulet

A356 2114288A

Literature Review on Longwall Mining

May, 2007

Sydney Catchment Authority (SCA)

Parsons Brinckerhoff Australia Pty Limited ABN 80 078 004 798

Ernst & Young Centre, Level 27, 680 George Street Sydney NSW 2000

Australia

Email [email protected]

GPO Box 5394 Sydney NSW 2001

Telephone +61 2 9272 5100 Facsimile +61 2 9272 5101

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2114288A PR_3818 RevB

© Parsons Brinckerhoff Australia Pty Limited (PB) [2007].

Copyright in the drawings, information and data recorded in this document (the information) is the property of PB. This document and the information are solely for the use of the authorised recipient and this document may not be used, copied or reproduced in whole or part for any purpose other than that for which it was supplied by PB. PB makes no representation, undertakes no duty and accepts no responsibility to any third party who may use or rely upon this document or the information.

Author: Elizabeth Cohen, Kimberly Saflian, Liz Webb..............................

Signed: .....................................................................................................

Reviewer: Brian Rask....................................................................................

Signed: .....................................................................................................

Approved by: Brian Rask....................................................................................

Signed: .....................................................................................................

Date: .....................................................................................................

Distribution: SCA (3) PB (2) .............................................................................


Contents Page Number

1. Introduction..........................................................................................................................................1

1.1 Background 1 1.2 Longwall Mining 1 1.3 Environmental Impacts 3

1.3.1 Ground and Surface Water 4 1.3.2 Gas Release 4 1.3.3 Rock and Cliff Falls 4 1.3.4 Erosion and Vegetation Loss 4

1.4 Regulation of Longwall Mining in NSW 5 1.5 Geology of the Southern Sydney Basin and the Southern Coalfields 7

2. Geomechanics of Longwall Mining Subsidence ............................................................................12

2.1 Effects of Topography on Subsidence 14 2.2 Effects of Geological Features on Subsidence 15 2.3 Limit of Mining Influence 16 2.4 Subsidence Modelling and Prediction 17

3. Impacts of Longwall Mining on Surface Water...............................................................................19

3.1.1 Rock Bars 20 3.1.2 Recovery of Surface Water 20

4. Impacts of Longwall Mining on Groundwater ................................................................................22

4.1 Limit of Mining Influence on Groundwater 25 4.2 Groundwater Recovery after Mine Completion 25

5. Impacts of Longwall Mining on Water Quality................................................................................27

5.1 Impacts on Groundwater Geochemistry 27 5.2 Impacts on Surface Water Chemistry 28

6. Investigative Methods .......................................................................................................................33

6.1 Geophysical Methods in Subsidence Monitoring 33 6.1.1 Geophysical Methods 35 Gravity 35 Magnetics 37 Seismic Refraction/Reflection 37 Resistivity 38 Electromagnetics 40 Ground Penetrating Radar 41 Radiometrics 42 Induced Polarisation (IP) 42 Self Potential (SP) 42 Borehole geophysics 43 6.1.2 Geophysical Modelling 43

6.2 Predicting Longwall Mining Impacts on Hydraulic Properties 44 6.3 Tracer Investigations 45

7. Previous Hydrological Investigations..............................................................................................47

7.1 Australia 47 7.1.1 Southern Coalfields 47

Cataract River................................................................................................................................................. 47 Upper Georges River...................................................................................................................................... 48 Bargo River..................................................................................................................................................... 49 Upper Nepean River ....................................................................................................................................... 49 Flying Fox Creek, Wongawilli Creek, and Native Dog Creek .......................................................................... 49 Waratah Rivulet .............................................................................................................................................. 49

7.1.2 Western Coalfields 50 Farmers Creek................................................................................................................................................ 50 Cox’s River ..................................................................................................................................................... 50

7.1.3 Hunter Valley and Newcastle Coalfields 50 Hunter Valley .................................................................................................................................................. 50 Diega Creek.................................................................................................................................................... 50

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Contents (continued) Page Number

7.2 United States 51 7.2.1 Illinois 51

Jefferson County ............................................................................................................................................ 51 Saline County ................................................................................................................................................. 51

7.2.2 Pennsylvania 52 7.2.3 Virginia 53 7.2.4 Utah 53

8. Limitations of Available Information ...............................................................................................54

9. Summary of Findings ........................................................................................................................55

10. Acknowledgements...........................................................................................................................57

11. References .........................................................................................................................................58

List of tables Table 5-1 Chemical reactions affecting surface water and groundwater 31 Table 6-1 Geophysical methods (Gascoyne and Eriksen, 2005; Reynolds, 1997) 33

List of figuresFigure 1-1 Longwall mining operations (NSW DPI PrimeFacts, 2006) 2 Figure 1-2 Typical longwall layout (Holla and Barclay, 2000) 3 Figure 1-3 Storage restricted zone for dams (Holla et al., 1993) 6 Figure 1-4 Structure restricted zone (Holla et al., 1993) 6 Figure 1-5 NSW Coalfields (Holla and Barclay, 2000) 8 Figure 1-6 Stratigraphy of the Southern Coalfields (NSW Mineral Resources, 1999) 9 Figure 1-7 Stratigraphic Column of the Illawarra Coal Measures in the Southern Coalfield (NSW

Mineral Resources, 1999) 10 Figure 2-1 Formation of a subsidence trough (Holla and Barclay, 2000) 12 Figure 2-2 Characteristics of trough-like subsidence: left half vertical components, right half

horizontal components (Holla and Barclay, 2000) 13 Figure 2-3 Profiles over a series of longwall panels (Holla and Barclay, 2000) 14 Figure 2-4 Notch effect on horizontal stress field (Holla and Barclay, 2000) 15 Figure 2-5 General effect of faults on subsidence troughs (Holla and Barclay, 2000) 16 Figure 3-1 Formation processes of migratory and stationary ponds (Peng et al., 1996) 20 Figure 4-1 Schematic of areas above a longwall mine (Booth, 2003) 23 Figure 4-2 Typical overburden zones above a longwall mine (Judall, Platt, Thomas and

Associates, 1984) 23 Figure 4-3 Cross section of a longwall mine and the corresponding water level drop (Karaman et

al., 1999) 24 Figure 5-1 Iron precipitate in the Georges River (Ecological Australia, 2004). 29 Figure 5-2 Gas release in Georges River (Ecological Australia, 2004). 32 Figure 6-1 Scintrex CG3 Gravity meter in operation (Scintrex Ltd) 36 Figure 6-2 Sting R1 resistivity meter and Swift Electrode switching box. 39 Figure 6-3 Resistivity; data modelling and interpretation showing sand aquifers within clay geology

(Christensen and Sorensen, 1994). 40 Figure 6-4 Electromagnetic Method 41 Figure 7-1 NSW Southern Coalfield (Total Environment, 2007) 47 Figure 7-2 Illinois Coal Basin (USGS, 1996) 51 Figure 7-3 Appalachian Coal Basin (USGS, 2007) 52

PARSONS BRINCKERHOFF 2114288A PR_3818 RevB Page ii


Glossary of terms ACIDITY. Base neutralising capacity. Determined by titrating up to a pH of about 8.3. Equal to H2CO3

concentrationin most samples except when Al3+ or Fe3+ are present.

ALKALINITY. Acid neutralising capacity. Determined by titrating with acid down to a pH of about 4.5. Equal to the concentrations of mHCO3- + 2mCO32- (mmol/l) in most samples.

ALLUVIUM. Sediments (clays, sands, gravels and other materials) deposited by flowing water. Deposits can be made by streams on river beds, floodplains, and alluvial fans.

AQUICLUDE. A formation that will not transmit water fast enough to furnish an appreciable supply for a well or spring.

AQUIFER. Rock or sediment in a formation, group of formations, or part of a formation that is saturated and sufficiently permeable to transmit significant quantities of water to wells and springs. Aquifers generally occur in formations which can also store large volumes of water such as sands, gravels, limestone, sandstone, or highly fractured rocks.

AQUIFER, CONFINED. An aquifer that is overlain by a confining bed. The hydraulic conductivity of the confining bed is significantly lower than that of the aquifer.

AQUIFER, SEMI-CONFINED. An aquifer confined by a low-permeability layer that permits water to slowly flow through it. During pumping of the aquifer, recharge to the aquifer can occur across the confining layer. Also known as a leaky artesian or leaky confined aquifer.

AQUIFER, UNCONFINED. Also known as a water-table and phreatic aquifer. An aquifer in which there are no confining beds between the zone of saturation and the surface. The water table is the upper boundary of unconfined aquifers.

AQUITARD. A low-permeability unit that can store groundwater and also transmit it slowly from one aquifer to another.

BASEFLOW. The part of stream discharge that originates from groundwater seeping into the stream, and supports stream flows during long periods of no rainfall.

BEDDING PLANE. In sedimentary or stratified rocks, the division plane which separates the individual layers, beds or strata.

BORE. A structure drilled below the surface to obtain water from an aquifer system.

CALCITE. The carbonate mineral calcite is a calcium carbonate corresponding to the formula CaCO3 and is one of the most widely distributed minerals on the Earth's surface. It is a common constituent of sedimentary rocks, limestone in particular. It is also the primary mineral in metamorphic marble. It also occurs as a vein mineral in deposits from hot springs, and also occurs in caverns as stalactites and stalagmites. Calcite is often the primary constituent of the shells of marine organisms (e.g. plankton, bivalves, etc.).

CARBON DIOXIDE. An atmospheric gas comprised of one carbon and two oxygen atoms. A very widely known chemical compound, it is frequently called by its formula CO2.

CHEMICAL OXYGEN DEMAND. A measure of the total quantity of oxygen required to oxidise all organic material into carbon dioxide and water.

PARSONS BRINCKERHOFF 2114288A PR_3818 RevB Page 1


CLOSURE. is the reduction in horizontal distance between the valley sides, and is expressed in units of millimetres (mm). Closure also results from the redistribution of, and increase in the horizontal stresses in the strata as mining occurs.

CONFINING LAYER. A body of relatively impermeable material that is stratigraphically adjacent to one or more aquifers. It may lie above or below the aquifer.

COMPRESSION. A system of forces or stresses that tends to decrease the volume or shorten a substance, or the change of volume produced by such a system of forces.

COMPRESSIVE STRAIN. A strain that tends to push together the material on opposite sides of a real or imaginary plane.

CURVATURE. Is the second derivative of subsidence, or the rate of the change of tilt, and is calculated as the change in tilt between two adjacent sections of the tilt profile divided by the average length of those sections. Curvature is usually expressed as the inverse of the Radius of Curvature with units of 1/kilometre (1/km), but the values of curvature can be inverted, if required, to obtain the radius of curvature, which is usually expressed in kilometres (km).

DEUTERIUM. also called heavy hydrogen, is a stable isotope of hydrogen with a natural abundance of one atom in 6,500 of hydrogen. The nucleus of deuterium, called a deuteron, contains one proton and one neutron, whereas a normal hydrogen nucleus just has one proton.

DILATION. Deformation that is change in volume; but not in shape.

DISCHARGE. The volume of water flowing in a stream or through an aquifer past a specific point in a given period of time.

DISCHARGE AREA. An area in which there are upward components of hydraulic head in the aquifer.

DISSOLUTION. process of dissolving a substance into a liquid. If the saturation index is less than zero, the mineral is undersaturated with respect to the solution and the mineral might dissolve.

DYKE. A tabular body of igneous rock that cuts across the structure of adjacent rocks. Although most dykes result from the intrusion of magma, some are the result of metasomatic replacement.

EC. An acronym for Electrical Conductivity unit. 1 EC = 1 micro-Siemens per centimetre, measured at 25'C. It is used as a measure of water salinity (see salinity below).

EROSION. The group of processes whereby earthy or rock material is loosended or dissolved and removed from any part of the earth’s surface. It includes the processes of weathering, solution, corrosion, and transportation. The mechanical wear and transportation are affected by running water, waves, moving ice, or winds, which use rock fragments to pound or grind other rocks to powder or sand.

FAULT. A fracture in rock along which there has been an observable amount of displacement. Faults are rarely single planar units; normally they occur as parallel to sub-parallel sets of planes along which movement has taken place to a greater or lesser extent. Such sets are called fault or fracture zones.

FERROMAGNETIC MINERALS. Refers to those paramagnetic minerals having a magnetic permeability considerably greater than one. They are attracted by a magnet.

FLOW PATH. The path of two dimensional flow through porous media. PARSONS BRINCKERHOFF 2114288A PR_3818 RevB Page 2


FRACTURE. Breakage in a rock or mineral along a direction or directions which are not cleavage or fissility directions.

FRACTURED ROCK AQUIFER. These occur in sedimentary, igneous and metamorphosed rocks which have been subjected to disturbance, deformation, or weathering, and which allow water to move through joints, bedding plains and faults. Although fractured rock aquifers are found over a wide area, they generally contain much less groundwater than alluvial and porous sedimentary aquifers.

GOAF. That part of a mine from which the mineral has been partially or wholly removed; the waste left in old workings; - called also gob.

GROUNDWATER. The water contained in interconnected pores located below the water table.

GROUNDWATER-DEPENDENT ECOSYSTEMS (GDEs). For the purposes of defining ecosystem dependence, groundwater may be defined as that water in the system that would be unavailable to plants and animals were it to be extracted by pumping (Hatton and Evans, 1998).

GROUNDWATER FLOW SYSTEM. A regional aquifer or aquifers within the same geological unit that are likely to have similar recharge, flow, yield and water quality attributes.

HARDNESS. Sum of the ions which can precipitate as “hard particles” from water. Sum of Ca2+ and Mg2+, and sometimes Fe2+.

HYDRAULIC CONDUCTIVITY. The rate at which water of a specified density and kinematic viscosity can move through a permeable medium (notionally equivalent to the permeability of an aquifer to fresh water).

HYDRAULIC HEAD. is a specific measurement of water pressure or total energy per unit weight above a datum. It is usually measured as a water surface elevation, expressed in units of length, but represents the energy at the entrance (or bottom) of a piezometer. In an aquifer, it can be calculated from the depth to water in a piezometric well (a specialized water well), and given information of the piezometer's elevation and screen depth. Hydraulic head can similarly be measured in a column of water using a standpipe piezometer by measuring the height of the water surface in the tube relative to a common datum. In both cases, although the measurement is made at the water surface, the hydraulic head measurement represents the total energy at the entrance (base) of the piezometer. The hydraulic head can be used to determine a hydraulic gradient between two or more points.

HYDRAULIC GRADIENT. The change in total head with a change in distance in a given direction, which yields a maximum rate of decrease in head.

HYDROLYIS. Decomposition of a chemical compound by reaction with water, such as the dissociation of a dissolved salt or the catalytic conversion of starch to glucose.

JOINT. A fracture or crack in a rock mass along which no appreciable movement has occurred.

MACROINVERTEBRATES. aquatic invertebrates including insects (e.g. larval Ephemeroptera and Trichoptera), crustaceans (e.g. amphipods), molluscs (e.g. aquatic snails) and worms (e.g. Platyhelminthes), which inhabit a river channel, pond, lake, wetland or ocean.

MAGNETIC SUSCEPTIBILITY. A measure of the degree to which a substance is attracted to a magnet; the ratio of the intensity of magnetization to the magnetic field strength in a magnetic circuit.

METHANE. An odorless, colorless, flammable gas, CH4, the major constituent of natural gas, that is used as a fuel and is an important source of hydrogen and a wide variety of organic compounds. OXIDISNG CONDITIONS. Conditions in which a species loses aelectrons and is present in oxidised form.



OVERBURDEN. Material of any nature, consolidated or unconsolidated, that overlies a deposit of useful materials, ores, or coal especially those deposits that are mined from the surface by open cuts.

PANEL. A coal mining block that generally comprises one operating unit.

PERIPHYTON. Sessile organisms, such as algae and small crustaceans, that live attached to surfaces projecting from the bottom of a freshwater aquatic environment.

PERMEABILITY. The permeability of a rock is its capacity for transmitting a fluid. degree of permeability depends upon the size and shape of pores and the size, shape and extent of interconnections.

PH. p(otential of) H(ydrogen); the logarithm of the reciprocal of hydrogen-ion concentration in gram atoms per liter; provides a measure on a scale from 0 to 14 of the acidity or alkalinity of a solution (where 7 is neutral and greater than 7 is basic and less than 7 is acidic).

PILLAR. A column of rock remaining after solution of the surrounding rock.

POROSITY. The proportion of interconnected open space within an aquifer, comprised of intergranular space, pores vesicles and fractures.

POROSITY, PRIMARY. The porosity that represents the original pore openings when a rock or sediment formed.

POROSITY, SECONDARY. The porosity that has been caused by fractures or weathering in a rock or sediment after it has been formed.

POTENTIOMETRIC SURFACE. Surface to which water in an aquifer would rise by hydrostatic pressure.

PRECIPITATION. (1) in meteorology and hydrology, rain, snow and other forms of water falling from the sky (2) the formation of a suspension of an insoluble compound by mixing two solutions. Positive values of saturation index (SI) indicate supersaturation and the tendency of the water to precipitate that mineral.

PYRITE. or iron pyrite, is iron disulfide, FeS2

RECHARGE. The process which replenishes groundwater, usually by rainfall infiltrating from the ground surface to the water table and by river water entering the water table or exposed aquifers. The addition of water to an aquifer.

RECHARGE AREA. An area in which there are downward components of hydraulic head in the aquifer. Infiltration moves downward into the deeper parts of an aquifer in a recharge area.

REDOX POTENTIAL. The redox potential is a measure (in volts) of the affinity of a substance for electrons – its electronegativity – compared with hydrogen (which is set at 0). Substances more strongly electronegative than (i.e. capable of oxidising) hydrogen have positive redox potentials. Substances less electronegative than (i.e. capable of reducing) hydrogen have negative redox potentials.

REDOX REACTION. Redox reactions, or oxidation-reduction reactions, are a family of reactions that are concerned with the transfer of electrons between species, and are mediated by bacterial catalysis. Reduction and oxidation processes exert an important control on the distribution of species like O2, Fe2+, H2S and CH4 etc. in groundwater.

REDUCING CONDITIONS. Conditions in which a species gains electrons and is present in reduced form.

REMNANENT MAGNETISM. Permanent magnetisation induced by an applied magnetic field.

RIPARIAN. Of, on, or relating to the banks of a natural course of water.



SALINITY. The concentration of sodium chloride or dissolved salts in water, usually expressed in EC units or milligrams of total dissolved solids per litre (mg/L TDS). The conversion factor of 0.6 mg/L TDS = 1 EC unit is commonly used as an approximation.

SEDIMENTARY AQUIFERS. These occur in consolidated sediments such as porous sandstones and conglomerates, in which water is stored in the intergranular pores, and limestone, in which water is stored in solution cavities and joints. These aquifers are generally located in sedimentary basins that are continuous over large areas and may be tens or hundreds of metres thick. In terms of quantity, they contain the largest groundwater resources.

SEDIMENTATION. That portion of the metamorphic cycle from the separation of the particles from the parent rock, no matter what its origin or constitution to and including their consolidation into another rock.

SEISMOLOGY. The geophysical science of earthquakes and the mechanical properties of the earth.

SIDERITE. is a mineral composed of iron carbonate FeCO3.

STORATIVITY. The volume of water an aquifer releases from or takes into storage per unit surface area of the aquifer per unit change in head. It is equal to the product of specific storage and aquifer thickness. In an unconfined aquifer, the storativity is equivalent to specific yield. Also called storage coefficient.

SUBSIDENCE. usually refers to vertical displacement of a point, but the subsidence of the ground actually includes both vertical and horizontal displacements. These horizontal displacements in some cases, where subsidence is small, can be greater than the vertical subsidence. Subsidence is usually expressed in units of millimetres (mm).

SURFACE WATER-GROUNDWATER INTERACTION. The interaction between surface water and groundwater occurs in two ways: (1) streams gain water from groundwater through the streambed when the elevation of the water table adjacent to the streambed is greater than the water level in the stream (2) streams lose water to groundwater by outflow through streambeds when the elevation of the water table is lower than the water level in the stream.

TENSILE STRAINS. A normal strain that tends to pull apart the material on opposite sides of a real or imaginary plane.

TENSION. A system of forces tending to draw asunder the parts of a body, especially of a line, cord or sheet, combined with an equal and opposite system of resisting forces of cohesion holding the parts of the body together; stress caused by pulling. Opposed to compression, and distinguished from torsion.

TILT. is the change in the slope of the ground as a result of differential subsidence, and is calculated as the change in subsidence between two points divided by the distance between those points. Tilt is, therefore, the first derivative of the subsidence profile. Tilt is usually expressed in units of millimetres per metre (mm/m). A tilt of 1 mm/m is equivalent to a change in grade of 0.1%.

THROW. 1. The amount of vertical displacement occasioned by a fault. 2. More generally, used for the vertical component of the net slip.

TOTAL DISSOLVED SOLIDS (TDS). A measure of the salinity of water, usually expressed in milligrams per litre (mg/L). See also EC.

TRACER (applied). Applied tracers are non-natural constituents that are intentionally introduced, and are used to characterise groundwater flow paths, estimate groundwater velocities, measure subsurface properties and measure physicochemical properties insitu.



TRANSMISSIVITY. The rate at which water is transmitted through a unit width of aquifer of confining bed under a unit hydraulic gradient. The product of saturated thickness and hydraulic conductivity.

UPSIDENCE. is the reduced subsidence, or the net vertical movement within the base of a valley. Upsidence results from the buckling of near surface strata in the base of the valley which results from the redistribution of, and increase in the horizontal stresses in the strata immediately below the base of the valley as mining occurs.

WATER TABLE. The upper level of the unconfined groundwater, where the water pressure is equal to that of the atmosphere and below which the soils or rocks are saturated. It is the location where the subsurface becomes fully saturated with groundwater, the level at which water stands in wells that penetrate the water body. Above the water table, the sub-surface is only partially saturated (often called the unsaturated zone). The water table can be measured by installing shallow wells extending just into the zone of saturation and then measuring the water level in those wells.



1. Introduction

1.1 Background A review of available literature has been undertaken by Parsons Brinckerhoff for the Sydney Catchment Authority (SCA) as part of an investigation into the hydrological impacts of longwall coal mining in an area of the Southern Coalfields of NSW. The literature review encompasses background information on the techniques and theory of longwall mining methods as well as an indication of past research into the impacts of longwall mining on hydrological systems. It is a precursor to a more extensive review of site specific literature and mine reports.

1.2 Longwall Mining There are two main methods of extracting coal by underground mining: room-and-pillar (or, bord-and-pillar) and longwall mining (panel-and-pillar). Longwall mining is a commonly used method of coal extraction that involves the complete removal of large, rectangular panels of coal (EIA, 1995). A section of the coal bed is blocked out in panels and passageways are dug on three sides on the panel, starting from the main entry of the mine (EIA, 1995). Coal is then mined on ‘retreat’, meaning the extraction process moves from the far end of the panel towards the main entry (EIA, 1995). The panels are extracted (Figure 1-1) using an extractor (either plow, or more commonly a shearer), which moves backwards and forwards across the coal face (O’Brien, 2007; Booth et al., 1998; EIA, 1995).

The coal removed falls onto a conveyor, generally armoured to support the weight of the shearer or plow and to protect from the impact of the falling coal, removing a section of coal at a time (O’Brien, 2007). The conveyor spans the entire face of the longwall and can be moved with the coal face without being dismantled. The conveyor carries coal to a conventional belt conveyor which moves the coal out of the mine.

The roof is supported by hydraulic roof supports, which move forward as the extraction progresses along the panel (O’Brien, 2007; Booth et al., 1998). Most commonly the roof supports are shield roof supports, which consist of a canopy, a caving shield, which prevents rock fragments from getting into the working area and two to four hydraulic legs set on a base (EIA, 1995). The shields are set side by side along the longwall face and can typically support up to 600 to 800 tonnes each (EIA, 1995).

The coal panels are typically extracted as a series of panels laid adjacent to each other, separated by chain pillars (Figure 1-2). The panel and pillar widths are variable, in NSW panels are generally between 110 m and 250 m and pillars between 20 m and 50 m (Holla and Barclay, 2000). In the Southern Coalfields panel widths are from 145 m (Elouera) to 300 m (West Cliff) (Cram, 2006).

The longwall mining method results in a high percentage of coal recovery from within the panel as no pillars of coal are left behind in the panel to support the mine roof (Booth et al., 1998). However, as there are no supports left behind as the longwall progresses this method also results in the collapse of the overlying rock strata (overburden) into the areas from which the coal was extracted, know as the goaf (O’Brien 2007; Booth et al., 1998). The subsidence progresses through the overlying strata to the ground surface, where it results in a trough-like depression that mimics the mined out panel (Booth et al., 1998).



The extent of subsidence at the ground surface depends on a number of factors including the depth of mining, the width of the panels and pillars, and thickness of the coal seam mined (O’Brien, 2007); for instance, when the longwall panels are narrow compared to the depth of the overburden, the overburden strata bridges across the panels, thus reducing subsidence. As a result, full subsidence does not develop until many adjacent panels have been mined (Mills and Husskes, 2004).

Subsidence caused by longwall mining is generally more uniform and predictable than subsidence due to other methods (EIA, 1995). Generally, steeper environments, such as gorges, are more highly affected by mining subsidence than flatter areas (Total Environment Centre, 2007). It has been stated that 80% of subsidence occurs within a 2 month period of longwall mining. In NSW, the majority of this subsidence is typically between 1 and 2 metres (Total Environment Centre, 2007).

Figure 1-1 Longwall mining operations (NSW DPI PrimeFacts, 2006)



Figure 1-2 Typical longwall layout (Holla and Barclay, 2000)

1.3 Environmental Impacts The alteration of habitat as a result of longwall mining subsidence was listed by the independent NSW Scientific Committee as a key threatening process under the Threatened Species Conservation Act 1995 (Total Environment Centre, 2007). Species that depend on aquatic or semi aquatic habitats are particularly affected as a result of cracks beneath streams or other water bodies that lead to a temporary or permanent loss of water flow. Subsidence may also impact and cause permanent changes to riparian community structure and compositions (Total Environment Centre, 2007).

In developed areas, longwall mining induced subsidence can have a severe impact on infrastructure, roads, bridges, piping and other structures as well as cause flooding of the lowered ground surface (Bell and Genske, 2001).



1.3.1 Ground and Surface Water

Subsidence induced cracking and fracturing can cause surface water loss and diversion into subsurface routes if occurring beneath a stream or other surface water body, as well as groundwater decline due to increased aquifer permeability. Water quality can also be affected due to redirection of the water flow into groundwater system as the water interacts with various subsurface strata allowing compounds and sediments in the rocks to dissolve into the water. The impact of longwall mining on surface and groundwater is discussed in greater detail in Sections 1 and 5.

1.3.2 Gas Release

When water levels decline in an aquifer, either by mine water drainage or other subsidence related changes in groundwater flow pathways, gases (mainly methane in the case of coal mining but also some C4 and C6 hydrocarbons) become mobile and can migrate to the surface or to nearby openings such as wells or bores (Atkinson, 2005; Total Environment Centre, 2004). When methane seeps through soil, it displaces the soil oxygen leading to anoxic soil conditions, which can result in the death of vegetation. Furthermore, an increase of methane in the soil can lead to an increase in microbial activity. Since soil microbes feed on the gas, this may cause temperatures in the soil to reach between 31 - 44˚C (CSIRO Media Release, 2003).

In many cases the release of methane following mining subsidence is attributed to methane escaping from the coal seam. However, based on an analysis of the composition of the gas, which is similar to that found naturally in the underlying sandstone, methane in the Cataract River (NSW) was attributed to underlying sandstone layers rather than from the mined coal seam (CSIRO Media Release, 2003).

1.3.3 Rock and Cliff Falls

Rock benches and overhangs are common features that are susceptible to cracking and fracturing due to mining induced subsidence, resulting in an increase of rock falls and cliff collapses (Total Environment Centre, 2007; Zhahiri, 2006). This has been evident at a number of longwall mining locations in the Southern Coalfields (Total Environment Centre, 2007; Zhahiri, 2006; Parkin, 2002; Holla and Barclay, 2000)

Rock falls and cliff collapses can have significant impacts on the cliff line ecology and often also sites of aboriginal significance (Total Environment Centre, 2007).

1.3.4 Erosion and Vegetation Loss

In steeper areas where longwall mining occurs there is an increased change of unconsolidated surface materials moving down the slopes when cracking and subsidence occurs; increasing localised soil erosion and can result in higher levels of sedimentation and loss of vegetation (Total Environment, 2007).



1.4 Regulation of Longwall Mining in NSW The environmental impacts of longwall mining have lead to the implementation of legislative provisions in NSW for protecting the environment, including:

• Dams Safety Act, 1978

• Mining Act, 1992

• Coal Mines Regulation Act, 1982

• Mine Subsidence Compensation Act, 1961.

In NSW there are large coal reserves which lie beneath stored water in the Southern Coalfields. In 1977 a Commission of Inquiry was undertaken which reviewed available data and made recommendations, known as the Reynolds Inquiry (Reynolds, 1977). The recommendations were:

• A marginal zone for the reservoirs to be fixed at an angle of 26.5˚ from the Full Supply Level.

• No mining permitted in the marginal zone where the cover was less than 60 m.

• No total extraction to be permitted anywhere within the marginal zone.

• Bord and Pillar mining to be permitted within the marginal zone where the cover is more than 60 m with 5.5 m wide bords and pillars of a minimum width 15 times the height of extraction or one-tenth of the cover depth, whichever is greater.

• Longwall mining to be permitted in the marginal zone, where cover is more than 120 m, panel width is to be less than one-third of the depth of cover and pillar size to be less than one-fifth of the depth of cover or 15 times the extracted seam thickness, whichever is the greater.

However, these recommendations were not adopted in 1978 when the Dams Safety Act was introduced (Holla and Barclay, 2000). The responsibility of ensuring that mining under stored water bodies was undertaken safely fell to the Dams Safety Committee, which may declare a Notification Area for any prescribed dams under the Mining Act 1992. The Notification Area sets the limit of the committee’s interest in mining around the dam and includes the storage and structure restricted zones, with mining outside of the Notification Area considered far enough away to pose negligible risk to the dam (Holla and Barclay, 2000). The storage restricted zone comprises the areas of the reservoir, and the marginal zone at an angle of draw of 35˚ (Figure 1-3). The structure restricted zone extends 1.2 times the depth from the structure of non rigid dams to the base of the coal seam, and 1.7 times for rigid dams (Figure 1-4).



Figure 1-3 Storage restricted zone for dams (Holla et al., 1993)

Figure 1-4 Structure restricted zone (Holla et al., 1993)

The Coal Mines Regulation Act (1982 Act) was introduced in 1982 to control the mining methods utilised for coal extraction, in most cases requiring longwall mining methods. The 1982 Act stipulates the dimensions of the pillars and boards at certain depths in order to protect surface features and control mining subsidence.

The Mine Subsidence Compensations Act (1983 Act), provides payment of compensation by the Mine Subsidence Boards for damage due to mine subsidence following coal or oil shale extraction, if these features were constructed in accordance with the Board’s approval, or existed prior to proclamation of the Mine Subsidence Area. However, the 1983 Act only



allows compensation for areas that have been improved from the natural landscape, and does not cover damage due to vibrations, only subsidence due to settlement strains. Furthermore, it does not cover economic loss due to subsidence.

1.5 Geology of the Southern Sydney Basin and the Southern Coalfields The sediments of the Sydney Basin consist of Permian and Triassic aged sediments that have been uplifted, gently folded, and eroded over time. They have been described as horizontally bedded sandstones, capped with shale, and underlain by coal measures and older sandstones (Haworth, 2003). The Southern Coalfields occur in the southern most section of the Greater Sydney Basin, and are one of the five major coalfields that lie within the Sydney Basin (Figure 1-5). The stratigraphy of the Southern Sydney Basin is given in Figure 1-6 and Figure 1-7, and is primarily Permo-Triassic sedimentary rock (Parkin, 2002), which is underlain by undifferentiated sediments of Carboniferous and Devonian age.



Figure 1-5 NSW Coalfields (Holla and Barclay, 2000)



Figure 1-6 Stratigraphy of the Southern Coalfields (NSW Mineral Resources, 1999) PARSONS BRINCKERHOFF 2114288A PR_3818 RevB Page 9


Figure 1-7 Stratigraphic Column of the Illawarra Coal Measures in the Southern Coalfield (NSW Mineral Resources, 1999)

The basal unit of the Southern Sydney Basin is the Shoalhaven Group, and is typically lithic sandstones with interbedded siltstones, and is between 300 and 900 metres thick. The Shoalhaven Group is overlain by the Illawarra Coal Measures which consist of interbedded sandstones, shale and coal seams, and have a thickness of approximately 300 metres. The Narrabeen Group then overlies the coal measures, and consists largely of lithic to quartz lithic sandstones, claystones and shales with a thickness of between 300 to 500 metres (Haworth, 2003). The Narrabeen Group is overlain by the Hawkesbury Sandstone, which consists of quartzose sandstones, typically medium to coarse grained, and is approximately 190 metres thick across the southern areas of the Greater Sydney Basin. The Hawkesbury Sandstone is capped by the thinner strata of the Wianamatta Group Shales across the middle portion of the plateau in the southern area of the Sydney Basin (Haworth, 2003).

The coal seams within the Southern Coalfields area are persistent over large areas when compared to other area of the basin and Australia in general (Sherwin and Holmes, 1986). The coal in this area is mined principally from the Illawarra Coal Measures (Figure 1-7), which consists of four coal seams, the Bulli Seam, Balgownie Seam, Wongawilli Seam and Tongarra Seam. The Bulli Seam is the youngest and uppermost seam in the Southern Coalfields area (Holla and Barclay, 2000).

The Hawkesbury Sandstone is the dominant outcropping formation across the Southern Coalfields, and generally constitutes a surface layer of thickness greater than 100 m above most mine workings in the area (Parkin, 2002; Holla and Barclay, 2000). The Hawkesbury Sandstone thus constitutes the major geological unit in surface subsidence.



The Hawkesbury Sandstone is predominantly quartzose (approximately 90%) with siltstone/fine-sandstone laminate, and siltstone and claystone interbeds, making up the remaining 10% (Moffit, 2000).

The Illawarra Escarpment is topped by Hawkesbury Sandstone, and erosion by rivers and creeks to form incised valleys is a common feature of the Hawkesbury Sandstone, with valley side slopes often ranging from 10 to 15˚ (Holla, 2000; 1997). The Hawkesbury Sandstone contains significant cross bedding and vertical jointing, on average 7-15 metres wide, which leads to the formation of large blocks (Holla and Barclay, 2000).

Quartz grains in the Hawkesbury Sandstone are generally tightly packed and void spaces are infilled so that the majority of groundwater is held within zones of weakness such as fractures, joints, faults and bedding planes (Branagan, 2000). The Hawkesbury Sandstone behaves as a dual porosity aquifer and as such displays complex hydraulic behaviour when stressed. The movement of groundwater is generally controlled by elevation with the potentiometric surface typically mirroring topography (Parkin, 2002).



2. Geomechanics of Longwall Mining Subsidence Coal seam extraction over a large area, such as that which occurs in longwall mining, causes the immediate seam roof to break and fall into the void created by mining. The convergence of the surrounding strata towards the mining void causes subsidence on the surface. In additional to the vertical subsidence which is evident at the surface, there are also horizontal movements inwards toward the centre of the void, which cause the surface to tilt, bend, stretch and compress. Compression and bending (curvature) occurs in the central part of the trough, and other areas undergo stretching (tension) and convex bending (Figure 2-1).

Figure 2-1 Formation of a subsidence trough (Holla and Barclay, 2000)



The primary surface subsidence parameters are:

• Maximum subsidence (Smax)

• Maximum ground tilt (Gmax)

• Maximum tensile and compressive ground strains (+Emax and –Emax)

• Minimum radius of ground curvature (Rmin).

The relationships between these parameters are given in Figure 2-2. The magnitude of subsidence is dependant upon the extracted seam thickness (T), cover depth (H), width of the underground opening (W) and the type of goaf material. Tilt is determined by dividing the difference in subsidence at two points by the distance between them, with the maximum tilt occurring where subsidence is roughly equal to one half of Smax.

Strain is caused by horizontal movements, and is the change in length per unit of the original horizontal length of the ground surface (Holla and Barclay, 2000). Compressive strains occur above the extracted areas and tensile strains over the goaf edges. Strain and tilt are related by the equations:

H-1+Emax = 100K1Smax (1)

H-1-Emax = 100K2Smax (1)

H-1+Emax = 100K3Smax (1)

where K1, K2, K3 are constants.

Figure 2-2 Characteristics of trough-like subsidence: left half vertical components, right half horizontal components (Holla and Barclay, 2000)

When mining longwall panels as a series, the resulting subsidence profile is dependant on the size of pillars and panels in relation to the mining depth (Figure 2-3). If the panels are



narrow (W/H less than 0.33) and the pillars are large (pillar width /H >0.2) then subsidence is small, as the main roof strata bridges across the extraction void over the pillars, resulting in a final subsidence profile that is flat and shallow. If the pillars were smaller (pillar width/H = 0.06) then a deeper subsidence profile would develop, however it would still be smooth. As the ratio of W/H increases, then a much larger subsidence profile occurs over the panel centres, resulting in a ‘wavy’ subsidence profile.

Figure 2-3 Profiles over a series of longwall panels (Holla and Barclay, 2000)

2.1 Effects of Topography on Subsidence Gullies, streams, rivers, gorges, and topographical high and low points can have a significant effect on the extent of mining subsidence (Holla and Barclay, 2000; Ewy and Hood, 1983). In rugged terrains maximum subsidence occurs at ridge tops and minimum



subsidence at topographic lows (Holla and Barclay, 2000; Holla 1997; Ewy and Hood, 1983). The trend is attributed to the forces associated with the mechanics of the movement of valley sides, rather than due to any unique geological characteristics of the rock mass in and around the valleys (Holla, 1997).

Streams and gorges create a notch effect by transferring the horizontal stress previously carried by the eroded strata to the stream floor. Movements in and along the valley sides caused by the collapse of strata result in an increase in horizontal stresses in the notched zone, the compressed mass at the stream floor is unconfined in the vertical direction and therefore becomes ‘pushed’ up, causing a hump which reduces subsidence (Holla and Barclay, 2000) (Figure 2-4 ). The increased compression in this area also results in increased strain in the bed of streams and gullies (Holla, 1997; Goultry and Al-Rawhay, 1996).

Figure 2-4 Notch effect on horizontal stress field (Holla and Barclay, 2000)

2.2 Effects of Geological Features on Subsidence Geological structures like faults and dykes provide weak planes of discontinuity in the overburden layer, along which a slip may be triggered by mining (Holla and Barclay, 2000) causing ground movement at the surface. A step or concentrations of strains may then result on the surface; faults tend to be locations where strain is concentrated (Bell and Genske, 2001; Holla and Barclay, 2000). If a fault is encountered during seam extraction and its throw is large then workings are often terminated against the fault (Bell and Genske, 2001). Depending on the location of the fault relative to mining, different subsidence profiles can occur (Figure 2-5).



Figure 2-5 General effect of faults on subsidence troughs (Holla and Barclay, 2000)

2.3 Limit of Mining Influence The limit of mining influence is defined by the angle of draw; it refers to the limit of subsidence movement relative to the edge of the longwall panel (Booth, 2006; Booth, 2003). This is the angle between the vertical and the line joining the edge of the mining void with the edge of the subsidence trough (Holla and Barclay, 2000,) and is often in the range of 20 – 40˚ (Booth, 2002). Mining induced subsidence has been noted at up to 25 mm even up to 1.5 km away from the edge of the mining panel (Hebblewhite, 2001; Holla, 1997). In the NSW Southern Coalfields horizontal displacements have been observed extending more than 1 km away from the mine workings, and in extreme cases up to 3 km away (Krogh, 2007). However, the extent of these impacts is ultimately dependant on the depth of mining, the thickness of the mined seam, the panel and pillar thickness and configuration, and the topographic and geological features in the area.

In NSW subsidence of up to 20 mm can occur in areas which are not impacted by mining, in response to changes in factors such as soil moisture content, variations in water table and other climatic fluctuations (Holla and Barclay, 2000). Therefore, a cut-off subsidence of 20 mm is used to fix the limit of mining influence in NSW.



2.4 Subsidence Modelling and Prediction Subsidence prediction methods can be classified into theoretical models, empirical methods (which including graphical methods, profile function and influence functions), physical models and numerical models (Asadi et al. 2005; Alejano et al.1999).

Empirical models involve analysing data gathered from a study on existing subsidence in order to predict future subsidence effects (Alejano et al., 1999). This method can be useful in predicting subsidence in areas of similar geology and topography but can have limited value in areas removed from the initial data collection. Numerous empirical methods exist for predicting ground movement due to longwall mining. Most of these methods deal with vertical subsidence and horizontal ground strains (Holla, 1997).

Physical models require the construction of a scaled physical model of the geological strata and topographic conditions in order to recreate conditions in the field. This tool is valuable for understanding strata mechanics, but has limited use in predicting displacements (Alejano et al., 1999).

Theoretical models are generally based on continuum mechanic principles that attempt to explain the mechanisms behind the subsidence.

Numerical methods employ iterative techniques to determine rock movements and/or changes in conditions based on mathematical criteria. These models can closely simulate a mining sequence but rely on accurate determination of in-situ geologic properties, (Agioutantis and Karmis, 1988). Commonly available numerical models include BEM (Boundary Element Method), DEM (Discrete Element Method) FEM (Finite Element Method) and FDM (Finite Difference Method) (Alejano et al. 1999)

The finite element method (FEM) is used for approximate solutions for partial differential equations, it is commonly used in structural engineering and can handle (and relies upon) knowledge of the elastic properties of the media it is describing (Agioutantis and Karmis, 1988) The FDM is sometimes considered a subset of the FEM as it is an approximation of the partial differential equation rather than an approximation of the solution. The FDM is restricted to handling rectangular shapes only, so that complex geometries can only be approximated. The FEM is considered to be more mathematically sound than the FDM with better quality estimates, however the FDM is easier to implement.

The discrete element method (DEM) which was developed for rock mechanics can also be considered a generalised FEM (Williams et al., 1985). The fundamental assumption of this method is that the materials consist of separate discrete particles which may have different shapes or properties.

The BEM (boundary element method) is also related to the FEM in that it is a method of solving partial differential equations which are formulated into integral equations. Given boundary conditions are used to fit boundary values into the integral equation. Results can then be given for any point within the given data “boundaries”.

In modelling mine subsidence, more than one type of modelling may be undertaken depending on which information is available and required, and modelling should always be combined with the calibration of real data (Alvarez-Fernandez et al. 2005). It should also be noted that many empirical and theoretical formulae have been developed into commercial subsidence modelling software packages. Donelly et al. 2000, describe the irregularities



which occur when applying the SWIFT software package developed using mine subsidence data from Great Britain to mining located in the Andes.



3. Impacts of Longwall Mining on Surface Water If subsidence induced cracking occurs beneath a stream or other surface water body, flow paths can be altered causing dewatering and/or rerouting of surface water, often by loss of water to near-surface groundwater flows (Total Environment Centre, 2007; Sidle et al., 2000; Zipper et al. 1997; Peng et al., 1996). Depending on the depth of coal mining and the extent of cracking, the water may be lost either permanently or temporarily, with the possibility that some water may re-emerge further down stream (Total Environment Centre, 2007). Generally, there is no a significant loss of stream flow to mines deeper than 100 metres as surface cracking and goaf zones are unlikely to be connected; however surface water can still be transferred to the groundwater system (Dawkins, 1999).

Cracks at the ground surface occur as either faceline or ribline cracks. Faceline cracks develop as a longwall advances and the overburden behind the working area subsides in a dynamic wave, they run parallel to the longwall face, and generally close up as the longwall advances and the ground subsides to its maximum depth at greater distances back from the face (Dawkins, 1999). Ribside cracks develop along the axis of the extracted panel, as the overburden falls in towards the subsidence trough. Generally these cracks do not close up, although they may be reduced as adjacent panels are extracted (Dawkins, 1999).

When these types of cracks occur in the surface layers the rate of channel water drainage to the groundwater table can increase. Generally, it is thought that such fractures are contained within a shallow zone close to the surface (Booth, 2003; Judall, Plat, Thomas and Associates, 1984), however if they extend to a sufficient depth they are able to provide a connection between surface water and deeper permeability zones (Parkin, 2002). Connection with permeable regional aquifers could lead to a net loss of water to the local catchment if the regional aquifer has discharge locations outside the catchment of flow origin.

There is some evidence to suggest that in Australia these fractures in the shallow zone are limited to between 15 and 20 metres from the surface (Dawkins, 1999; Seedsman 1996). However, as discussed in more detail in Section 2, this too is dependent upon too many site specific details to be a general “rule” applied to all areas of practice. Substantial cracking of streams and tributaries due to longwall mining has been noted on the Woronora Plateau (Krogh, 2007). In some of these cases water that re-emerges downstream was notably deoxygenated with high concentrations of iron (Krogh, 2007; Everett et al., 1998), thus indicating some water is maintained in the shallow fracture network.

Groundwater discharge (baseflow) to a river also contributes to flow and baseflow in mining impacted catchments can vary significantly with both loses and gains occurring (Dawkins, 1999). Development of new fissures in influent streams can transfer surface water to shallow groundwater, causing water to flow in the subsidence cracks rather than along the surface.

Stream ponding occurs when there is a change in stream flow rate and water depth due to a subsidence trough and water accumulation in this trough. The process is illustrated in Figure 3-1 (Peng et al., 1996). An area of subsidence is flooded by a pond, as mining progresses, and subsidence advances, the resulting pond can gradually move down the stream until subsidence in the stream bed stabilises. A pond of this type, which moves as



the longwall face advances, is called a migratory pond. If a pond is relatively stable, and does not experience further subsidence it becomes a stationary pond (Peng et al., 1996).

The occurrence of stream ponding is dependent on two main factors, the angle of stream flow and the change in streambed gradient (Peng et al., 1996).

Figure 3-1 Formation processes of migratory and stationary ponds (Peng et al., 1996)

3.1.1 Rock Bars

Surface cracking at rock bars allows surface water flow to preferentially pass under the rock bars rather than over them, as would be the normal pattern, resulting in accelerated rock pool drainage. If the dominant morphology in the area is a chain of ponds, it will also result in decrease in river flow rate (Parkin, 2002). This type of surface water impact has been observed at Jutt’s Crossing on the Georges River and at rock bars on the Waratah Rivulet, where an estimated 1.7 to 2 Ml/day was lost from a rock pool above a rock bar due to leakage in to the groundwater system (Krogh, 2007; Mills and Huuskes, 2004; Parkin 2002).

3.1.2 Recovery of Surface Water

The ability of a surface water body to recover is dependent on the extent of cracking, surface gradient, substrate composition and the presence of organic matter. If the extent of cracking is such that there is no groundwater loss to a mine or other deeper fractured zone which removes water from the area, then recharge to the water table will eventually saturate the



subsidence fractures (Dawkins, 1999). The new subsidence related flow paths may also close with subsequent ground movement.

The surface gradient is important as a low surface gradient will improve the potential for surface cracks to close. While the steeper the gradient, the more likely that solids transported by water will be moved downstream allowing the void to remain open and the potential loss of flows to the subsurface to increase (Total Environment Centre, 2007). A lack of thin alluvium in the stream bed may also prolong stream dewatering as indicated by two case studies; one showing at least 13 years to recovery (Total Environment Centre, 2007) and another study in the Central Appalachian region showing no indication that the physical, chemical or biological impacts to streams recovered to reference conditions with time (Stout, 2004).

Because the impacts to surface water flows are dependent upon many variables, most of which are either mining activity specific or local characterisation specific, or some combination of the two, there is no overall general understanding of what the quantitative impacts of longwall mining are on surface water flows and thus very little transferable knowledge of recovery potentials. What is known is that fracturing increases the interaction of surface and groundwater, at least temporarily, if not permanently. The amounts of potential flow losses, or increase, are then dependent upon site specific activities and local geologic, geomorphologic, and hydrologic conditions.

During the period of mining and recovery to a new equilibrium, water is likely to be lost from the surface water system to fill the new voids and storage capacity within the ground. Surface water that is filling the increased groundwater storage voids\capacity during the recovery and mining phases is not likely to be recovered at the surface water body. Thus, if mining has created, or increased the capacity, of a system where surface water is flowing into the groundwater than a temporary loss of surface flow associated with the filling of increased void space would be expected.

Once a new equilibrium is reached, it is unknown if the net water balance of the surface water system is maintained. The new conditions of equilibrium will dictate overall losses or gains temporally within the system. It is likely to shift water from high flow conditions to baseflow.



4. Impacts of Longwall Mining on Groundwater During subsidence the strata undergoes fracturing and opening of existing fractures and joints and also a separation of the bedding planes, which causes an increase in fracture permeability and porosity (Booth, 2003). These changes in turn cause variations in hydraulic heads, gradients, and groundwater flow patterns (Dawkins, 1999; Booth et al., 1998; Aston and Singh, 1983) which can have significant impacts in nearby wells and on water supplies in the surrounding communities (Karaman et al., 2001; Hill and Price, 1983).

Although there is some potential for fracturing in the overlying aquifer to cause or increase drainage into the mine, it is generally accepted that variations in hydraulic gradients and groundwater levels are due to the changes in fracture porosity and permeability caused by longwall mining subsidence (Booth, 2003; Karaman et al., 1999; Booth et al., 1998). Normally, mine drainage only impacts wells that penetrate the lower fractured zone, and\or other zones where natural fractures are present (Booth, 2006).

The overburden above a longwall panel (Figure 4-1 and Figure 4-2) can be described as three zones (Booth, 2002; Booth, 2003; Judall, Platt, Thomas and Associates, 1984):

1. Fractured Zone: the lowest, most severely fractured zone, which greatly increases permeability and drains directly into the mine. The height of this zone can be described as a third to a half of the width of the panel, or between 20 and 60 times the extraction thickness. The fracture zone is typically dewatered; thus wells completed across this zone typically lose their water.

2. Aquiclude Zone: An intermediate compressional zone that subsides with little extensive fracturing. It typically forms in a shale interval and maintains overall lower permeability and confining characteristics between the mine and the shallow aquifer system.

3. Surface Zone: Is the uppermost zone and consists of shallow strata subject to extensional stress and fracturing. Aquifers are affected by in situ fracturing but not necessarily by drainage into the mine from which they are hydraulically separated by the Aquiclude Zone. Wells in this area have significant water-level decreases, which may be followed by recovery.



Figure 4-1 Schematic of areas above a longwall mine (Booth, 2003)

Figure 4-2 Typical overburden zones above a longwall mine (Judall, Platt, Thomas and Associates, 1984)



The typical potentiometric response to longwall mining is a drop in groundwater levels in bedrock aquifers (Booth, 2003). Fracturing and dilation of joints and bedding planes in the subsidence areas causes an increase in aquifer void volume and porosity. This in turn increases the aquifer permeability and storage coefficient (Dawkins, 1999), which creates a drop in water level and a local potentiometric depression (Booth, 2006; Karaman et al., 1999). Groundwater levels can begin to fall even before a longwall panel passes under a well, as drawdown is transmitted through the aquifer ahead of the longwall (Dawkins, 1999). Groundwater typically flows along the induced hydraulic gradient towards the depression over the subsiding zone (Figure 4-3) due to any or all of the following (Booth, 2003; Hill and Price, 1983):

• Direct drainage to the mine, if the well bottom is within the lower fractured zone.

• Increases in fracture porosity in strata, which causes large head drops in confined bedrock aquifers because of their low storativity and low fracture porosity.

• Increases in fracture permeability, causing decreases in hydraulic gradient from the site. Groundwater levels in valley wells may rise and groundwater discharges to springs and streams may increase.

• Draining of upper level aquifers through fractured aquitards down to lower levels.

• Drawdown expands outwards from the primary potentiometric flow.

Figure 4-3 Cross section of a longwall mine and the corresponding water level drop (Karaman et al., 1999)



Elsworth and Lui (1995) found that for the same geometric setting the impacts of longwall mining on groundwater were controlled, in the majority, by topography. It was found that wells located in upland areas experienced dewatering to a greater extent than wells within the valley base. The conclusion is supported by the mechanics of subsidence (Section 2.1) and the greater compressional forces within the topographic lows resulting in comparatively less void space created.

4.1 Limit of Mining Influence on Groundwater The lateral limit of the influence of mining on groundwater can be compared to the angle of draw. The angle of draw defines the limits of mining on subsidence and also the edge of the primary hydrologic response to fracture dilation. The extent of the groundwater impacts beyond the subsidence zone is controlled by the transmissive properties of the aquifer (Booth, 2003). Generally, the angle of groundwater influenced is within 40˚ of the angle of draw, depending on the slope of the terrain. However, coal aquifers, having relatively low transmissivities, only have groundwater effects which reach a few hundred metres from the longwall panel, even in areas where there is a steep head drop directly over the mine (Booth, 2006; Booth, 2003). The exception to this rule is where impacts are extended by fracture zones, or are shortened by areas of even lower transmissivity (Booth, 2006).

Previous studies have shown that the observed influence on hydrogeology is dependant on horizontal and vertical distance from mining operations, and the surrounding topography (Booth et al., 2000).

4.2 Groundwater Recovery after Mine Completion Longwall mining can cause permanent changes in groundwater flow due to the gradient changes and leakage associated with fractured aquitards. Long-term recovery is site-specific and difficult to predict (Booth, 2006). Upon completion of mining, groundwater levels may never stabilise, stabilise at depressed levels, or they may rebound approaching pre-mining levels (Hill and Price, 1983). There are two separate mechanisms of water level recovery after subsidence, these being compression and recharge (Booth, 2002). The initial subsidence phase is generally followed by a compressional phase, which results in a partial recovery of water levels. In this phase some of the tension fractures created during subsidence close back up and some settlement of the beds also results in closure of fractures. The compressional phase is followed by a gradual recovery as water flows back into the temporary potentiometric depression created by subsidence fracture effects. Later recovery depends on the ability of the aquifer to transmit water back into the effected areas (Booth, 2006; Booth, 2003; Booth, 2002).

Recovery is more likely in valleys, where the effects of the compressional phase are more pronounced than on hilltops (Booth, 2002).

Recovery of wells which penetrate the lower fractured zone does not usually occur, while water levels in the upper zone of the subsidence areas commonly recover quickly. The ability of an aquifer to recover is based on the transmissivity of the aquifer, connection to sources of recharge, the rate of water loss through fractured aquitards, and the continuation of mining activities (Booth, 2006). The extent of the recovery may be full or partial, more often than not permanent changes in groundwater flow occur (Booth, 2006). The recovery of an aquifer can be hampered by (Booth, 2002):



• Low transmissivity of the aquifer, which restricts lateral recharge through the aquifer.

• Natural barriers to hydraulic continuity with sources of recharge such as tight confining units and lateral boundaries in the aquifer.

• Mining operations that can interrupt the physical hydraulic continuity between the affected aquifer and recharge sources.

• Continuing loss of water to the mine or in high relief areas through fracture aquitards to lower aquifers of down gradient discharge zones. For this reason, wells which penetrate the lower fractured zone do not generally recover (Booth, 2006).

• The depth and thickness of the overburden. Where the overburden thickness is less than half the panel width, wells did not significantly recover over a period of years, except near streams (Booth, 2002).



5. Impacts of Longwall Mining on Water Quality Subsidence induced by longwall mining and the subsequent fracturing associated with it can impact on the quality of both groundwater and surface water. Surface water can be redirected through the subsurface via fractures and can come into contact with compounds and sediments which are not normally dissolved into the water. These compounds are then able to leach back into the drainage lines (Total Environment Centre, 2004).

Artificial hydraulic connection between aquifers and/or aquifers and surface water created by longwall mining can alter groundwater chemistry through water-rock interaction and mixing processes increasing rates of hydrogeochemical processes which mobilise major ions, metals and minor elements. Studies in the USA and Australia (Stout, 2004; Ecological Australia, 2004; DIPNR, 2003; Sidle et al., 2000; Booth and Bertsch, 1999; Booth et al., 1998) have shown that subsidence-induced deterioration of surface water quality is due to a reduction in dissolved oxygen, and to increased salinity, iron, manganese, turbidity and contamination of groundwater by acid drainage.

5.1 Impacts on Groundwater Geochemistry The impacts of longwall mining on groundwater geochemistry have been studied at two active longwall mine sites in Illinois, USA (Booth, 2006; Booth, 2002; Booth and Bertsch, 1999; Booth et al., 1998). Longwall mining was found to affect water chemistry due to increased rates of water-rock interaction as an effect of subsidence-related fracturing by increasing downward leakage from overlying units, and due to the temporary potentiometric depression and subsequent recovery, water from surrounding areas of the aquifer recharges the affected zones above and adjacent to the mine.

At the two sites in Jefferson County and Saline County, shallow glacial-drift aquifers were largely unaffected by mining but the geochemistry of the bedrock aquifers changed during the post-mining water-level recovery. At the Jefferson County site, brackish, sulphate-rich water present in the upper bedrock shale briefly had lower values of total dissolved solids after mining due to increased recharge from the overlying glacial-drift aquifers. The chemistry of the sodium-bicarbonate water present in the underlying sandstone changed substantially after subsidence, with a major increase in total dissolved solids and sulphate concentrations. The change in chemistry was due to downward leakage of brackish, sulphate-dominant water from the shale and lateral inflow of water through the sandstone, which may have liberated and mobilised sulphates from sulphide minerals present in the sandstone.

At the Saline County site, sandstones contained water ranging from brackish sodium-chloride to fresh sodium-bicarbonate. Similar but more subdued processes occurred locally at the site, and minor changes in the chemistry of groundwater in the Trivoli Sandstone suggest vertical leakage from the lower drift aquifers into the sandstone.

Similarly, in the Upper Silesian Coal Basin, Poland, changes in the natural chemistry and quality of groundwater have occurred due to water mixing processes caused by artificial hydraulic interconnection between aquifers (Różkowski, 1993). Underground mining of Carboniferous coal seams caused an increase in rock permeability and the hydraulic



connection of water from different aquifers, producing the interruption of isolating layers. The size of drainage was determined by geological structure, depth and duration of mining works. Desalination of natural brines has occurred due to active drainage effect of less mineralised waters from shallower, overlying aquifers in the Tertiary and upper Carboniferous formations.

There have been few studies investigating impacts of longwall mining on groundwater chemistry in the Southern Coalfields of New South Wales. Water quality studies have focused on the quality of surface water in creeks and rivers. Studies recognise that mine related subsidence induces local shears that can produce pathways for connection of surface waters to the groundwater table (Krogh, 2007) but they have not investigated the changes induced by mixing processes between surface water and groundwater. Water quality studies have focused on the quality of surface water in creeks and rivers, and the chemical changes brought about by enhanced groundwater discharge and re-emergence of surface water from the subsurface.

At the Centennial Tahmoor mine in the Southern Coalfields of New South Wales, groundwater quality in the Hawkesbury Sandstone ranges from 3,300 µS/cm to 13,030 µS/cm salinity, from 3.53 to 6.49 pH and from 0 to 36 mg/L of iron. No significant adverse groundwater quality changes (as opposed to water availability) have been reported from bores within subsidence affected areas in the Southern Coalfields, but it is noted that water quality changes are likely to have occurred, but not observed, particularly in regard to increased iron concentrations and precipitation of iron hydroxide in pipes, dams or other water transfer systems through pumping and aeration of groundwater (Geoterra, 2006). Other changes may include the increase in dissolved nickel, zinc, iron or manganese levels through oxidation and dissolution of iron sulphides (marcasites) after subsidence induced fracturing of bedrock around a bore (Geoterra, 2006).

5.2 Impacts on Surface Water Chemistry Substantial cracking of stream beds and tributaries as a result of longwall mining causes the loss of flow and redirection through the subsurface. In the majority of cases, surface waters lost to the subsurface re-emerge downstream but the chemistry of water is significantly altered due to mixing with groundwater and enhanced reactions during water-rock interaction with minerals and compounds present in the subsurface strata.

Changes in surface water quality have been noted in many rivers and creeks flowing through the longwall mining subsidence areas of the Southern Coalfields of New South Wales including Cataract River, Upper Georges River on the Woronora Plateau, and Bargo River (Krogh, 2007; Geoterra, 2006; Ecological Australia, 2004; DIPNR, 2003; Everett et al., 1998).

Changes in surface water quality are likely to have occurred in other Sydney Catchment Authority Special Areas including Flying Fox Creek, Wongawilli Creek, Native Dog Creek, Waratah Rivulet and a number of other smaller tributaries (Ecological Australia, 2007).

In these catchments, the Hawkesbury Sandstone is the dominant surface\shallow geologic unit, and groundwater from the sandstone typically has elevated concentrations of iron and manganese (McKibbin and Smith, 2000). Sources of iron include siderite (iron carbonate, FeCO3) and iron oxyhydroxides and oxides. Enhanced discharge of iron-rich groundwater and re-emergence into surface water, which has reacted with iron and manganese minerals during subsurface passage, is one of the major threats to surface water quality in longwall



mining affected catchments. When iron-rich water discharges to surface water bodies, iron is rapidly oxidised and precipitates as an orange-brown iron hydroxides and oxides (Figure 5-1). The precipitates have an impact on water chemistry and aesthetics as well as increasing the level of suspended solids, resulting in a significant reduction in the quality of water and aquatic habitat (Ecological Australia, 2004).

Figure 5-1 Iron precipitate in the Georges River (Ecological Australia, 2004).

Studies in Southern Coalfield upland streams determined that water quality can be adversely affected through (Geoterra, 2006):

• Increased groundwater discharge to a stream following direct undermining and subsidence.

• Lowered concentration of dissolved oxygen, lowered pH, elevated Fe/Ni/Zn/Mn, elevated sulphate and salinity from flow through fresh cracks in cliff and fracture rich stream bed sandstone following subsidence which manifest as orange-brown, low dissolved oxygen plumes in receiving waters.

• Increased rainfall recharge through cracked Wianamatta Shale generating elevated salinity level and increased concentration of dissolved iron, manganese, nickel and zinc. Waters affected by Wianamatta Shale discharge do not have the elevated sulphate or lowered pH noted in discharges from other sandstone formations. Some acidity can be generated, however, from shale leachates when dissolved iron and manganese form metal oxide and hydroxide precipitates on mixing with aerated water.

• Pool depth reduction that resulted in enhanced stagnation and increased evaporation which decrease dissolved oxygen levels and elevate salinity.



• Gas release (including methane, carbon dioxide and other gases) through cracked strata and emission at the surface create anoxic conditions and can alter water chemistry (Ecological Australia, 2004; ACARP, 2001).

• Decrease of oxygen level through formation of iron mats causing limited flux of atmospheric oxygen into water.

The main observable change in stream waters results from the dissolution of iron and manganese from iron and manganese bearing minerals from cracked sandstone which subsequently precipitates on discharge to a receiving body as orange-brown iron oxides and hydroxides and generates sulphuric acid, iron, manganese, nickel and zinc which can exceed the ANZECC 95% protection of aquatic species trigger values (Geoterra, 2006).

Excessive growth of iron oxidising bacteria results in growth of very thick mats of iron precipitate containing large number of micro-organisms. These have been observed in many of the longwall mining-affected rivers including the Cataract and Upper Georges rivers (Ecological Australia, 2004; DIPNR, 2003). Iron bacteria are tiny filamentous bacteria that take iron out of solution/suspension and use it to form protective sheaths. These microorganisms are related to reducing/oxidising conditions and occur commonly in the Hawkesbury Sandstone areas, where seepage through rocks is often rich in iron compounds (Jones and Clark, 1991). Iron bacterial growth is common in low flow streams and rock pools. At higher flows, they are stripped from the streambed, transported downstream and deposited in areas of low flow velocity. The iron bacterial mats have detrimental environmental effects on aquatic life including reduction of available habitat by growing in and around interstitial areas, smothering small macro invertebrates, clogging streams (making movement difficult), and reducing the available food (periphyton, macro invertebrates) and exchange of gases mainly flux of oxygen from the atmosphere (DIPNR, 2003).

Monitoring programs in the longwall mining affected catchments in the Southern Coalfields have indicated that water quality impacts as a result of mining induced subsidence are local and ameliorated naturally over relatively short distances down stream of the impact, although cumulative impacts across longer stretches (e.g. a 300 metre wide longwall panel) have the potential to degrade water quality across large areas (Ecological Australia, 2004). The effect of acidity produced by water-rock interactions such as pyrite/marcasite oxidation and metal hydrolysis (Equations 1 to 3,Table 5-1) is reduced mainly through dilution with receiving waters and proton consuming species such as bicarbonate (HCO3

-) (Equation 4, Table 5-1) and to a lesser degree hydroxide species (OH-).

Water rock reactions that consume protons and/or produce alkalinity include dissolution of carbonate minerals (Equations 5 and 6, Table 5-1), aluminosilicate weathering (Equation 7, Table 5-1) and clay hydrolysis (Equation 8, Table 5-1). Rates of carbonate and silicate dissolution increase with increasingly acidic pH conditions. However, typically, calcite reacts several orders of magnitude faster than pyrite/marcasite oxidise (especially at low pH) and most aluminosilicates react several orders of magnitude slower (Banks, 2004). The increased acidity (and lower dissolved oxygen) is generally only observed close to the discharge point, and is dependent on flow rate and volume at discharge point (Ecological Australia, 2004). Stout (2004) found that acidity produced by the dissolution of pyritic materials in longwall mining affected streams in northern West Virginia was buffered by carbonate minerals in fractured rock strata, thus pH in these streams remained similar to unmined streams.



Table 5-1 Chemical reactions affecting surface water and groundwater

Chemical reactions

Pyrite oxidation and metal hydrolysis

(1) 2FeS2 + 2H2O + 7O2 ↔ 2Fe2+ + 4 SO42- + 4H+

(2) FeS2 + 14Fe3+ + 8H2O ↔ 15Fe2+ + 2SO42- + 16H+

(3) Fe3+ + 3H2O ↔ 3H+ + Fe(OH)3

Proton-consuming, alkalinity generating reactions -(4) HCO3 + H+ ↔ H2CO3 ↔ H2O + CO2

Carbonate mineral dissolution -(5) CaCO3 + H+ ↔ Ca2+ + HCO3

-(6) FeCO3 + H+ ↔ Fe2+ + HCO3

Silicate weathering

(7) 2NaAlSi3O8 + 2H+ + 2H2O ↔ 2Na+ + Al2Si2O5(OH)4 + 4SiO2(aq)

Clay hydrolysis

(8) Al2Si2O5(OH)4 + 6H+ ↔ 2Al3+ + 2SiO2 + 5H2O

Water quality improves downstream from the emergence point (Geoterra, 2006) as a result of the following processes:

• dilution of discharge

• precipitation of iron and manganese oxides and hydroxides

• adsorption of dissolved Ni and Zn onto the hydroxides

• binding and adsorption onto dissolved/total organic carbon.

Surface release of methane and other gases into Cataract River, and to a lesser extent the Upper Georges River, have had a detrimental impact on surface water quality (Ecological Australia, 2004; DIPNR, 2003) (Figure 5-2). The extraction of coal and the subsequent cracking of strata surrounding the goaf may liberate methane, carbon dioxide and other gases. Most of the gas is removed by the ventilation system of the mine but some gas remains in the goaf areas. Gases tend to diffuse upwards through any cracks in the strata and be emitted from the surface (ACARP, 2001) (Figure 5-2). Release of methane gas into surface waters promotes the rapid growth of aerobic bacteria which can deplete the surrounding water of oxygen. Major fish kills have been noted in some impacted river systems, for example in the Cataract River in 1994, and this may be attributed to draining of pools and lack of flow, and/or by the pollution of the water (Everett et al., 1997).



Figure 5-2 Gas release in Georges River (Ecological Australia, 2004).



6. Investigative Methods Numerous investigative tools and techniques are amiable to predict and assess the impacts of longwall mining. Investigative methods can typically be classified into three general categories:

• Monitoring and statistical assessment: A baseline condition is established by which changes in statistical trends are measured, as is recovery. Statistical trends are considered spatially and temporally. All studies reviewed which could draw meaningful conclusions incorporated this basic technique.

• Predictive modelling: Based upon site knowledge tools are available to estimate changes in hydraulic properties based upon expected subsistence.

• Post-mining measurement: Techniques are used after the mining of longwall panels to measure the changes to hydraulic parameters and pathways.

The following is a summary of techniques found to be utilized in previous impact studies.

6.1 Geophysical Methods in Subsidence Monitoring Geophysics is the study of the earth defined through the measurement of its physical properties (Gascoyne and Eriksen, 2005). The development of modern geophysics was driven by oil and mineral exploration in the early 20th century, although geophysical techniques have been used since ancient times.

Geophysical methods can be split into two categories; passive and active. Passive geophysical methods measure natural occurring sources such as the earth’s gravitational or magnetic fields. Active methods require the application of an artificial signal and measuring the resultant earth response. Examples of active methods include seismology where waves are generated or resistivity where an electrical current is emitted into the ground (Gascoyne and Eriksen 2005; Reynolds, 1997).

A table of geophysical methods and the physical properties they measure is presented below.

Table 6-1 Geophysical methods (Gascoyne and Eriksen, 2005; Reynolds, 1997)

Geophysical Method

Gravity

Passive/Active

Passive

Property Measured

Density

Common Applications

Hydrocarbon and mineral exploration

Regional geology studies

Near –surface void assessment.

Magnetics

Seismic Refraction/Reflection

Passive

Active

Magnetic Susceptibility

Density/elasticity

Hydrocarbon and mineral

exploration


Location of buried metal objects

Archaeology

Hydrocarbon exploration




Geophysical Method Passive/Active Property Measured Common Applications

Engineering site investigations

Resistivity

Electromagnetic methods

Active

Passive/Active

Electrical Resistivity

Electrical Conductivity/Resistivity

Mineral exploration


Hydrogeological investigations

Detection of subsurface cavities

Contamination/leachate plume

investigations

Archaeology


Mineral exploration




Contamination/leachate plume investigations

Archaeology


Ground Penetrating Radar

Active Dielectric, (Permittivity) Engineering site investigations



Archaeology


Forensic geophysics

Radiometric (Gamma ray spectrometry)

Passive/Active Radioactivity Regional geology studies

Soil and geomorphology studies

Induced Polarization (IP)

Active Electrical Resistivity/complex resistivity and chargeability

Mineral exploration


Self Potential (SP) Passive/Active Electrokinetic Mineral exploration


Wireline/borehole Logging

Passive/Active Various Mineral exploration



Magnetic Resonance Sounding (MRS)

Passive/Active water content, porosity, and hydraulic permeability


A suitable geophysical survey can be designed using a prior knowledge of the physical earth properties of the local geological conditions and the anticipated properties of the investigation target. Often a variety of geophysical techniques will be used in combination to solve particular engineering problems. Sometimes a measurable contrast is unlikely



between the local geology and the target, thus requiring other physical properties associated with the target to be measured (Gascoyne and Eriksen, 2005).

The successful design and implementation of a geophysical survey requires consideration of the following factors:

• Target discrimination: the magnitude and nature of the differences in physical properties between local geology and the target.

• Detection distance (or depth penetration): the distance to size ratio that is able to be assessed by the various geophysical techniques. The deeper a target is the larger the volume or cross sectional area will have to be in order for it to be detected.

• Survey resolution: the distance between data points collected in a geophysical survey which will influence the scale and size of features that are able to be delineated. Survey resolution is commonly reduced, as distance to the target increases. The smaller the target or the smaller area in which geological changes are likely to occur will determine the survey resolution.

• Site conditions: how suitable a site is for a particular geophysical method. Presence of electrical powerlines will interfere with some geo-electrical methods. Vibrations produced from random man made sources (such as traffic or heavy machinery) may cause interference with sensitive detection equipment such as gravity meters and seismic geophones.

In subsidence monitoring using geophysics it is important to have an understanding of the local geology as well as the anticipated effect that the subsidence will have on the geology. Once the conditions are understood, the appropriate geophysical methods can be chosen. It is often the case with geophysics that the exact parameters are not measured directly but that an earth model is created which must then be interpreted with regards to geologic conditions. Methods which have good lateral and vertical resolution, and provide information that can be interpreted with regard to water movement; transmissivity and relative ground strength are highly regarded in subsidence monitoring.

6.1.1 Geophysical Methods

Gravity

The gravity geophysical method estimates the subsurface density distribution (or density anomaly) from the observed gravity field (Takahashi, 2004). Gravity is measured by a gravity meter (or gravimeter) in either gravity units (g.u.) or Gals. (0.1 mGal = 1 g.u.).

The measurement of gravity is resultant of the gravity acting in a line with the earth’s centre of mass from a given point (Reynolds, 1997). Gravity measurements differ with latitude and corrections need to be made to counteract this.



Figure 6-1 Scintrex CG3 Gravity meter in operation (Scintrex Ltd)

In engineering geophysics relative gravity measurements are taken. A base station is selected and given the arbitrary gravity value of 0. All other values are measured relative to this location. In order for an engineering gravity survey to be successful, the latitude of each location point must be known to the nearest 10 m, and the elevation to the nearest 10 mm (Reynolds, 1997).

Gravity measurements are taken with either a Scintrex CG-3 or Lacoste Romberg gravimeter (Figure 6-1). In the case of the Scintrex CG-3 the mass hangs between 2 capacitor plates. When the mass moves due to changes in gravity, a feedback circuit is activated moving the mass back to its original position and the changes in electrostatic force between the mass and the plates is measured. The Lacoste-Romberg utilises a hinged mass hanging from a spring with a light beam indicating the original position. The position of the spring is adjusted manually with a screw to return the mass to the original position. As well as being sensitive to vibrations, gravity meters are also sensitive to heat (heat will affect the length of the spring and the position of the mass).

In order for observed gravity measurements to be meaningfully interpreted they must be corrected for a number of factors:

• instrumental drift

• earth tides

• latitude

• elevation

• terrain

• acceleration due to movement (if the gravity meter is mounted on a vehicle)

• isostatic correction (removes the effect of larger scale regional changes in density giving a small scale density changes in the earths crust).

The resulting measurement after all gravity corrections have been made is known as the “Bouger anomaly.” A high density feature within low density geology will give a positive Bouger anomaly, and a low density feature (such as a void) within high density geology will give a negative Bouger anomaly.



Gravity data can be interpreted either directly by comparing Bouger anomalies, (normally by displaying as a colour contour plot over the study area) or indirectly by modelling and comparing resulting models with the observed Bouger anomaly (Reynolds, 1997).

Magnetics

Magnetic surveys in geophysics utilise differences in magnetic susceptibility of the earth materials. Rocks which have a high concentration of ferro or ferri – magnetic minerals will have the highest susceptibilities. Rock magnetic susceptibility can also vary according to the orientation of grains or minerals within the rock fabric. Many rocks also display a natural remnant magnetism, which is generally acquired either during the rock formation (either by sedimentary or igneous processes) or during some change in the rock state such as during metamorphic processes of heating and deformation.

The total magnetic field is measured during a magnetic survey in units of nano Tesla (nT). Proton procession and caesium vapour magnetometers are popular instruments used for magnetic surveys.

Like gravity numerous correction factors must be applied to magnetic data in order to be able to interpret it effectively. Magnetic objects should be kept away from the magnetometer as much as possible including watches, metal tools, knives and locations of any other obvious magnetic features like powerlines, metal vehicles or buildings.

Diurnal drift corrections are important and are measured by taking periodic measurements from a base station with a fixed location for the duration of the survey.

Terrain corrections are sometime applied in areas of very high magnetic susceptibility and large variations in topography.

Latitude and longitude corrections take into account differences in the earth’s magnetic field.

The final corrected magnetic data is expressed as anomalous total field strength. Magnetic data is interpreted in a similar way to gravity data, either as contour plot across the survey area or by modelling the data.

Magnetic data is often modelled in conjunction with gravity data as the two datasets provide constraints on each other. A model which fits theoretically with one type of data may not fit with another. The model which can fit with both types of data is more likely to be a better representation of the true subsurface conditions.

The magnetic and gravity methods are both mainly used to detect specific targets within the subsurface such as ore-bodies, geological faults, voids (for gravity data) or specific objects within a uniform media.

Seismic Refraction/Reflection

The principle of applied seismology is for a wave signal to be generated at a known time and for the resulting seismic waves to travel through the subsurface media and be reflected or refracted back to the surface where the returning signal are detected. The time taken for the waves to travel between the trigger point and the detection point is then used to determine the nature of the various subsurface layers.



In seismic refraction the P wave velocity is used to describe the subsurface and is expressed in metres per second. In seismic reflection the two way travel time of the waves is expressed in seconds.

Typically sources of seismic energy can be split into three groups, impact (sledge hammer or drop weight), Impulsive (explosive or shotgun source) or vibrator. The generated waves are detected by geophones, and then processed together by a seismograph. Geophones are typically comprised of a magnet suspended by a spring within a coil all housed within a protective casing. When the seismic wave returns to the surface, the ground surface moves; as a result, the magnet moves relative to the coil inducing a current. It is this current which is measured and relayed to the seismograph for processing (Reynolds, 1997).

Seismic refraction data is initially interpreted manually by picking the P wave arrival time, which is then displayed as a function of geophone distance from the source vs. time. The P wave velocity can then be calculated as the inverse of the slope on the graph.

Typically, interpreted seismic refraction data will be displayed as a cross section of a subsurface profile, showing changes in seismic velocities both laterally and with depth. The limitation of seismic refraction data is that resolution is limited to the geophone and source spacing and steeply dipping features are not easily defined. The interpretation methods of seismic refraction require that P wave velocity must increase with depth. As such seismic refraction is not commonly used for assessments of low density/strength units beneath units which may have a higher P wave velocity (Takahashi, 2004).

Resistivity

Electrical resistivity geophysical method delineates the subsurface structures through the distribution of their electrical properties (Takahashi, 2004). The objective of a resistivity survey is to obtain a resistivity model of the subsurface.

Ground resistivity imaging has been used for many years as an imaging tool for the earth’s subsurface. An electrical current is applied to the earth and the resulting current is measured across two potential electrodes. A geometric factor is applied to this potential current, depending on the placement of the current and potential electrodes to determine the apparent resistivity for the earth. The apparent resistivity must then be inverted to arrive at a true earth resistivity for the subsurface.

Resistivity data collected in the field is expressed as apparent resistivity; it is collected by applying a current to the ground via current electrodes and measuring the resultant gradient between potential electrodes. The common power source for engineering resistivity is a 12volt rechargeable car or marine battery. A resistivity meter is commonly used to control the source and measurement currents.

Resistivity data can be collected using a variety of different methods:

• Vertical resistivity soundings: resistivity measurements are taken at a fixed point, but with the spacing between the current and potential electrodes being expanded to take measurements at increasing depths. This provides information on ground resistivity of earth layers at one location.

• Horizontal resistivity profiling: the current and potential electrode spacing is fixed and the centre measurement location is moved. This provides information on lateral changes in resistivity at a fixed depth.



• 2D and 3D resistivity imaging: the measurement location and current and electrode spacing are changed in order to provide either a 2D (by having locations positioned over a line) or 3D (by arranging locations in a grid pattern) model of earth resistivity.

In an automated resistivity system, all the current and potential electrodes are placed in the ground and connected to the resistivity meter via an insulated multi-core cable. The sequence of measurements is programmed into the resistivity meter via a downloaded file. The file is then run and the resistivity meter switches electrodes on and off in sequence taking measurements at various locations and depths. When all the measurements have been taken for the array, the electrodes are moved to a new location and the process begins again.

Figure 6-2 Sting R1 resistivity meter and Swift Electrode switching box.

Resistivity is commonly used for groundwater investigations as the ground conductivity/resistivity is closely related to the pore fluid saturation and pore volumes (Reynolds, 1997).



Figure 6-3 Resistivity; data modelling and interpretation showing sand aquifers within clay geology (Christensen and Sorensen, 1994).

Electromagnetics

The electromagnetic method (EM) can be divided into two separate disciplines, frequency domain EM (FDEM) and time domain EM (TDEM). Both disciplines work on the same principal of inducing an electrical current into the ground, but are measured in different ways and have slightly different applications.

Frequency domain EM gives information on the average bulk conductivity of the earth over the study area. The technique works by generating an electromagnetic field of fixed frequency from a transmitter coil, this field induces a current in the earth which in turn creates an electromagnetic field. The earth induced electromagnetic field is measured by a receiver coil. The field is measured as an apparent conductivity for the earth between the two receiver coils in comparison with the known primary generated field.

A graphical representation of the technique is presented in Figure 6-4.



Figure 6-4 Electromagnetic Method

In Time Domain EM (TDEM) a primary electromagnetic field is applied to the ground via an induction coil or loop. The field induces an electric eddy current in the earth; the eddy current then generates its own secondary electromagnetic field. The primary electromagnetic field is switched off, and the change in the resulting secondary electromagnetic field is then measured over time. TDEM gives information on the variation of conductivity with depth over one location. It is similar to collecting a resistivity sounding. A series of TDEM sounds can be interpreted together to provide a 2D or 3D model of the earth.

Ground Penetrating Radar

Ground Penetrating Radar (GPR) relies on detecting differences in the dielectric constant of materials in the subsurface. The GPR system consists of a transmitter antenna which transmits pulses of electromagnetic radio-waves into the ground. Antenna frequencies can range from 50MHz to 1.5GHz depending on the depth of penetration required and resolution required. When the radio-wave reaches a change in materials the signal is reflected back to the ground surface and detected by the receiver antenna.

The amplitude of the reflected radio-wave can be described by the amplitude reflection coefficient R.

ε 2 − ε 1Where R =

ε 2 + ε 1



ε1 = the dielectric constant of the first material and

ε2 = the dielectric constant of the second medium

As the amplitude reflection co-efficient approaches 1, all of the radio-wave is reflected back to the receiving antenna and does not penetrate further into the strata; this is generally the case when metals (which have a dielectric constant approaching infinity) are encountered or in areas with a high saturated clay content.

GPR data is presented as wiggle trace displays of radio-wave amplitudes with a vertical scale of two-way travel time (similar to reflection seismic). If the radio-wave velocity of the medium is know (or can be estimated using know subsurface features), then the vertical scale can be displayed as depth. Various frequency bandwidth filters can be applied to the data to enhance resolution and aid interpretation.

Radiometrics

The radiometric method measures changes in the intensity of natural radiation emanating from the earth’s surface (Takahashi, 2004). Radiometrics is also know as Gamma Ray Spectrometry, as a radiometric survey measures the gamma rays produced during the natural radioactive decay of the elements potassium (K), thorium (Th) and uranium (U).

The gamma rays are measured using a spectrometer, usually mounted on an aircraft or a moving vehicle, but sometimes a hand held survey may be conducted. Typically, radiometric surveys are conducted over wide areas in order to provide information on regional geology and/or geomorphology. The radioactive elements K, Th and U occur naturally in different types of rocks and soils, by displaying the different elements an assessment of rock and soil distribution can be made over the area.

Induced Polarisation (IP)

Measurements of induced polarisation are made using conventional resistivity electrode configurations (Reynolds, 1997). In an IP survey, the electrical current is switched off, and the voltage between the two potential electrodes takes a finite time to decay to zero as the ground temporarily stores charge (polarises). When the current is switched back on again the voltage builds up over the same amount of time it takes to decay (time rise) before it is at a maximum applied value. The decay and time rise are dependant on both instrumental and geological factors and so can be a diagnostic tool for earth geology.

Self Potential (SP)

The self potential method measures the differences in natural ground potentials at different points on the ground surface (Reynolds, 1997). Electricity can be conducted within the earth in three different ways:

• Dielectric conduction: conduction of a current between soil/rock grains.

• Electrolytic conduction: conduction of a current within a fluid.

• Electronic conduction: conduction through metals.

Self potentials are measured with two porous pot electrodes connected to a precision multimeter. The electrodes are made up of a copper electrode dipped in a solution of copper



sulphate which can then percolate through the porous base of the electrode in order to make contact with the ground (Reynolds, 1997). There are two methods for collecting SP data, the gradient method where the separation between the electrodes is kept constant across the traverse, or the potential amplitude method where one electrode is fixed and the other is mobile.

The SP method is very simple in application; however can be subject to a number of noise factors which may make interpretation problematic, including the presence of surface water, which can cause streaming effects, regional anomalies and changes due to topography.

Borehole geophysics

Borehole geophysics, also known as wire-line or down-hole geophysics, uses combinations of geophysical methods mentioned above but deployed within a borehole. Borehole geophysics can be deployed in two different ways. When taking measurements directly within one borehole, the data will give information on conditions in the ground directly adjacent to that borehole only. Cross-hole tomography will use at least two different boreholes to collect data on the earth in between the boreholes; this is especially useful with seismic, resistivity and GPR surveys. Borehole geophysics can also be combined with traditional ground geophysics having the source located on the ground and the receivers within the borehole or vice versa.

6.1.2 Geophysical Modelling

Geophysical data can be interpreted in one of two ways, either by directly interpreting the data, as is the case with a colour contour map plot, or by using the interpreted data to provide an earth model, as is the case with a resistivity profile.

The challenge in modelling geophysical data is the issue of “equivalence” (Reynolds, 1997), where a number of different models can produce the same results. For example, since apparent resistivity is a function of the true earth resistivity and the earth layer thickness, a thick layer with a low resistivity can produce the same result as a thin layer with a high resistivity. Gravity, magnetic and seismic refraction all experience this challenge in interpretation. The equivalence issue can be alleviated by using either know geological data to provide constraints to the geophysical model or by modelling two different types of geophysical data together.

Geophysical modelling is typically a multi-step process, depending on the methods available to the interpreter. At the very least geophysical data is “forward modelled”. Forward modelling involves the interpreter providing an expected earth model. The response of this model is then tested (with respect to the geophysical parameters being measured) and compared to the original results.

The next step in modelling is “inverse modelling” whereby the model provided is then changed by using one of various numerical modelling techniques until the model approaches the original observed dataset. There are many commercial programs available which perform inverse modelling, using a number of different numerical modelling techniques.

Finite element method (FEM) and finite difference method (FDM) modelling is commonly applied. The popular Res2dinv program for resistivity interpretation uses a non linear least squares technique for forward modelling based on the following equation:



(JTJ + uF)d = JTg

where: F = fxfxT + fzfzT

fx = horizontal flatness filter

fz = vertical flatness filter

J = matrix of partial derivatives

u = damping factor

d = model perturbation vector

g = discrepancy vector

After: (Loke 2004)

The advantages of this technique are that constraints can be placed on the parameters (such as filters) based on prior geological knowledge.

The 1D Laterally Constrained Inversion method (Auken et al. 2005) is useful in imaging horizontally layered strata. The method operates by modelling each location as a 1D sounding, with each sounding constrained by the results of the ones beside. This method is mainly valid within sedimentary geological units, and in this setting provides better resolution of subsurface structure, which is displayed gradationally using traditional inversion techniques.

6.2 Predicting Longwall Mining Impacts on Hydraulic Properties During longwall mining, rock mass deformation and water/gas flow processes interact dynamically. Flow is controlled by the permeability of the porous medium, which remains a highly non-linear function of mining induced stress and resulting fractures (Adhikary and Guo, 2005). Therefore, any predictive methods to determine hydraulic behaviour requires the capability to accurately determine mining induced rock mass deformation, fractures and resulting changes in fluid flow parameters (Wook, 2005).

Prediction methods thus generally couple a geophysical model, which predicts the strains and deformations in the overlying strata (described in Section 2.3) and a groundwater flow field model (Kim et al., 1997; Matetic et al., 1995). Using the predicted strains and knowledge of the pre-mining hydraulic properties of the overlying strata, the change in hydraulic conductivity resulting from the strain field may be determined (Matetic et al., 1995). Once the post mining conductivity field has been determined it can be applied to a groundwater flow model, and then a post-mining hydrologic system defined (Matetic et al., 1995).

Numerical methods (Finite Difference and Finite Element models) are two of the more powerful tools for prediction and analysis of the impacts of longwall mining on hydraulic properties (Kim et al., 1997), but physical subsidence models in conjunction with flow modelling has been utilised successfully (Wang and Park, 2003)

Matetic et al., (1995) utilised a finite element geophysical model, to determine the strain field which developed around a longwall in Vinton County, Ohio. The finite element model applied gravitational load, removed material excavated from the panel and allowed overburden material to fail and deform according to the mining-induced strains creating subsidence and strain fields, which could then be calibrated against field observations. This



method assumed that the strains were uniformly distributed over each element (Matetic et al., 1995). After identifying the strain fields, conductivity distribution was determined and boundary conditions applied to the model to represent groundwater and surface recharge and infiltration rates. When comparing the model to field data it was found that the predicted effects of post mining groundwater system correlated well with field data collected at the site (Matetitc et al., 1995).

Other finite element numerical models, which follow a similar method of coupling strain and groundwater models have been utilised successfully by CSIRO (for example CSOFLOW; UDEC; FLOMEC) which allow the impacts of mine subsidence on creeks and river valleys to be studied (CSIRO, 2006; ACARP, 2001). These models have been successfully applied to study longwalls at Cataract Gorge, Georges River and Nepean River.

COSFLOW is a three-dimensional finite element computer code which was designed to model rock mass deformation, desorption, and two-phase flow (gas and water) problems arising in underground coal mines (Adhikary and Guo, 2005). It couples a mechanical (Cosserat) model with a flow model. An important feature of the Cosserat model is that it incorporates bending rigidity of individual layers in its formulation, and can simulate rock breakage and slip as well as separation along the bedding planes, making it different from other conventional models in use (Adhikary and Guo, 2005).

If changes to hydraulic properties are estimated external to a hydrogeologic flow model, then these parameters can then be input into the commonly used MODFLOW and FEFLOW modelling packages. MODFLOW is a finite difference model and FEFLOW is a finite element model. Each package has its advantages depending upon the complexities of the system to model and the questions being asked. Both of these software packages are currently being used to assess longwall mining impacts.

6.3 Tracer Investigations Only one study was identified where tracers were used to investigate the relationship between subsidence induced by longwall mining and groundwater occurrence. Mather et al. (1969) conducted four groundwater tracing experiments, using sodium chloride and sodium fluorescein as tracers, at Aberfan, South Wales to investigate the influence of mining subsidence on the pattern of groundwater flow.

Groundwater tracing experiments were initiated to obtain an estimate of the velocity of groundwater flow through the tensional corridor, and to define more precisely the importance of this tensional feature in the flow pattern of groundwater. Four tracing experiments were performed using injection boreholes and a number of sampling points including boreholes, surface water locations and springs located up to 618.7 metres (2,035 feet) from injection points.

In the first experiment, samples were taken from 22nd February to 1st March 1967. No trace of fluoroscein was detected at any sampling location and there was no significant increase in the concentration of the chloride ion above the background level.

In the second experiment, samples were taken for two days. No fluoroscein was detected but a significant increase in chloride was detected at sampling points located up to 584 metres (1,920 feet) away.



In the third experiment, samples were taken for 10 days. Coloration was first detected at the closest sampling point 172 metres (565 feet) away from injection point) at 3.5 hours after injection, and reached a peak after 9 hours. Colouration was observed in sampling points up to 336 metres (1,105 feet) away. The first sign of colouration at this point occurred after only 12 hours, with a peak at 30 hours.

In the fourth experiment, no fluorescein was detected at sampling points but a significant increase in chloride at the sampling point located 102 metres (1,225 feet) from injection point.

Measurements showed that areas subject to tensional strains were characterised by rapid groundwater movement through the fissure system. Tracers injected into an area subject to compressional strain were not recovered and it was suggested that boundaries between zones of tensile and compressional strains should be regarded as hydraulic discontinuities.

Of the two tracers used in the experiments, sodium chloride, in the form of a saturated brine solution made from rock salt, proved to be more successful than fluorescein due to the acidic nature of the groundwater involved.



7. Previous Hydrological Investigations Previous studies have been undertaken on the impacts of longwall mining on hydraulic properties. All of the studies available for review examined impacts of mining on ground and surface water in areas where mining occurred directly beneath the study area, or the study area was within the mining subsidence zone.

7.1 Australia Research in Australia is limited and focuses in the majority on the Southern Coalfield and some areas of the Western and Hunter Coalfields.

7.1.1 Southern Coalfields

Previous studies have been conducted in the NSW Southern Coalfield, including Cataract River, Upper Georges River, Bargo River, Upper Nepean River, Flying Fox Creek, Wongawilli Creek, Native Dog Creek, and the Waratah Rivulet (Figure 7-1).

Figure 7-1 NSW Southern Coalfield (Total Environment, 2007)

Cataract River The Cataract River is part of the Upper Nepean River Catchment, NSW, which is located in the Southern Coalfields. Sections of the river have been impacted by longwall mining at the Belambi West Colliery and also by the Douglas (Tower) Colliery (Total Environment Centre, 2007; Bennett et al., 2002; Parkin, 2002; Singh and Jakeman, 2001). Mining under the Cataract Gorge occurs at a depth of approximately 450 metres (Dawkins, 1999) and the



resultant subsidence has generated cracks of up to 35 mm wide, which has enabled loss of up to 5 ML/day of surface water to the shallow groundwater system (Dawkins, 1999). Significant upsidence (0.25mm) has also occurred in the centre of the valley.

The Cataract River was studied extensively by Bennett et al., (2002) over the period 19992002. Significant cracking of the river bed were observed, in particular at rock bars, which allowed surface water to pass preferentially under the rock bar rather than over it, leading to increased drainage from rock pools and a degeneration of flow rate. It was found that the Cataract River ceased to flow on at least 17.6% of occasions, during the study period, with an estimated loss of flow of up to 50% (Total Environment Centre, 2007). Major fish kills in 1994 occurred in the Cataract River due to draining of pools and lack of flow, and/or by the pollution of the water (Everett et al., 1998).

There was significant venting of methane gas, beginning in 1996 in the Cataract River and surrounds. Methane is a natural product arising out of the decay of organic matter. Methane in coal deposits are formed when, with increasing depths of burial, rising temperatures and rising pressures over geological time, a proportion of the methane produced was adsorbed by the coal (Coal Authority, 2007). Such adsorption is maintained by the lithostatic and hydrostatic pressures and the release of these pressures allows methane to escape from the coal (Coal Authority, 2007). Methane may also be trapped within the void spaces of sandstone, fracturing and cracking of the sandstone will thus allow trapped methane to escape to the atmosphere (Coal Authority, 2007).

CSIRO (2003) concluded that gas bubbling to the surface of the Cataract River was most likely coming from reservoirs in the Bulgo Sandstone, 200 – 400 metres below the river. A second possible source was the Hawkesbury Sandstone, which extended from the surface to 155 metres depth, and had been fractured close to the surface by mining activity in a way that might permit gases which are adsorped to escape. The composition of gas in the river was significantly different to that present in the Bulli coal seam mined by BHP. The CSIRO study showed that gas was bubbling out of the river at a rate of about 20 litres a second.

Water quality in the river was found to be highly seasonal and there were thick mats of iron-manganese reducing bacteria covering much of the river substrate (Bennett et al., 2002). Areas where water returned to the surface exhibited increased salinity, temperature variations and changes in dissolved oxygen (Dawkins, 1999).

Bentonite grouting of the riverbed cracks was undertaken by BHP in 2002. It has been determined that much of the water returned to the surface, further downstream, resulting in no net loss from the river to the water table (Everett et al., 1998).

Other nearby areas have been mined with little or no observable effects on stream flow (Parkin, 2002). This difference in behaviour between sites in the areas was attributed mainly to the steep gorge setting of the affected areas of the Cataract River, which had approximately 50-80 m of sub vertical relief (Parkin, 2002; Dawkins, 1999).

Upper Georges River The Upper Georges River Catchment has been affected by mining from two collieries (Appin and West Cliff Collieries). Approximately 1.5 km of the Georges River has been undermined by longwall from the West Cliff colliery.

In 2000 the Jutts Crossing rock bar on the Georges River at Appin cracked, resulting in increased drainage from adjacent rock pools, and consequently a drying of the river (Total Environment Centre, 2007; Parkin, 2002). In 2002 there was substantial cracking observed



downstream from Jutts Crossing at Marhnyes Hole, which resulted in a further disappearance of water. There were also rock fall collapses in this area, along with ongoing problems associated with leaching of oxides (Total Environment Centre, 2007).

While the topography is relatively moderate, there are high (-9.55mm/m) tensile strains (compressive strains) present at the base of the river valley which is thought to have contributed the severity of cracking (Parkin, 2002).

Bargo River The Bargo River was noted as being impacted by the Tahmoor Colliery in 1994 (Total Environment Centre, 2007). In 2002 a two kilometre section was reported as being completely dry and large cracks were found in the riverbed.

Upper Nepean River The Upper Nepean River has been impacted by longwall mining at the Appin Colliery. In 2000 there was significant strengthening and repair to a freeway bridge in the area due to longwall mining subsidence, within 600 metres (Total Environment Centre, 2007). This section of the Nepean has a sandy riverbed, which makes detection of fracturing more difficult (Total Environment Centre, 2007)

Flying Fox Creek, Wongawilli Creek, and Native Dog Creek Flying Fox Creek, Wongawilli Creek, and Native Dog Creek are located in the Avon and Cordeaux Dam catchments and are impacted by the Dendrobium and Elouera Mines. Significant dewatering has occurred in these creeks due to subsidence induced cracking within the stream bed (Total Environment Centre, 2007). In some cases there is complete absence of surface flow. In the case of Wongawilli Creek, upland swamps have been drained (Total Environment Centre, 2007).

Waratah Rivulet Waratah Rivulet is located in the Southern Coalfields, and is part of the Woronora Dam Catchment, along with its tributaries, Waratah Rivulet, makes up approximately 29% of the Dam catchment (Total Environment Centre, 2007). Metropolitan Colliery removes coal from a 3.5 metre thick seam at a depth of approximately 500 metres below ground (Mills and Huuskes, 2004).

Mills and Huuskes (2004) undertook a study of the impacts of longwall mining in the Waratah Rivulet Catchment. The study focused on subsidence monitoring, flow and water quality monitoring, and the monitoring of vegetation, aquatic ecology and archaeological sites to determine the extent of mining impacts up until 2004.

The study concluded that surface subsidence due to longwall mining occurs consistently across the panels mined today, with subsidence profiles reflecting those typically associated with mining of narrow panels, relative to depth. That is, subsidence occurs in response to multiple panels acting together as one (Mills and Huuskes, 2004). Goaf edge subsidence is typically 60-70 mm and the angle of draw is about 7˚ (Mills and Huuskes, 2004).

There is extensive cracking of the riverbeds, with reduced flow and decreased water levels (Total Environment Centre, 2007). The rivulet has also tilted to the east as a result of subsidence, approximately 140-150 mm of upsidence (Total Environment Centre, 2007; Mills and Huuskes, 2004) and the likely drainage of Flat Rock Swamp at the southern recharge point of the Waratah Rivulet (Total Environment Centre, 2007). Attempts at remediation with grouting have been unsuccessful to date (Total Environment Centre, 2007).



The groundwater table is thought to be laterally connected across the site. Mills and Huuskes (2004) conclude that some natural remediation may be occurring across earlier mined longwall panels with the groundwater table appearing to respond to rainfall events to a greater extent than in the past (Mills and Huuskes, 2004). It should be noted that increased response to rainfall may not always be an indicator of the occurrence of natural remediation.

7.1.2 Western Coalfields

Farmers Creek Farmers Creek is located in the Western Coalfields of NSW, near Lithgow, and has been impacted by the Clarence Colliery. Farmers Creek supplies Lithgow with much of its drinking water, and severe cracking has been paved with cement to maintain water flow.

Cox’s River Cox’s River has been impacted by mining activities at the Springvale and Clarence Collieries in the Western Coalfields. Cox’s River, in particular, has recorded decreased flows, and hanging swamps in the area have been damaged. There is also increased salinity and alkalinity in the areas (Total Environment Centre, 2007).

7.1.3 Hunter Valley and Newcastle Coalfields

Hunter Valley Coal mining in the Hunter Valley region has been ongoing for years, and the region suffers the combined impacts of both longwall and open cut mining. In the 1980’s, Bowmans Creek (near Singleton) underwent extensive water loss due to mining induced subsidence as well as an increase in salinity in areas where flow returned (Total Environment Centre, 2007).

South Wambo Creek and Stoney Creeks are cracked due to mining at the Homestead Colliery. Coal extraction occurred at depths between 80 and 240 metres, resulting in both creeks becoming dry for the majority of the year, and the channel bed to become filled with approximately 5 metres of sand (Dawkins, 1999). At the lower mining depth of 80 metres, the surface cracks and goaf areas became connected, resulting in significant surface water flow directly into the mine and subsequent flooding. There was no surface water flow into the mine for mining depths greater than 100 metres (Dawkins, 1999).

Other impacted creeks in the Hunter areas are Glennies Creek, EuI Creek, Fishery Creek, Black Creek and Foy Brook (Total Environment Centre, 2007).

Diega Creek In Diega Creek, cracks of up to 10 cm wide formed after longwall mining under the creek was undertaken between 1999 and 2005, currently rehabilitation is being undertaken at this site (Total Environment Centre, 2007).



7.2 United States Much of the research to date has focused on sites in the United States, in particular the Appalachian and Illinois coalfields (Figure 7-2 and Figure 7-3).

7.2.1 Illinois

Figure 7-2 Illinois Coal Basin (USGS, 1996)

Jefferson County Jefferson County, in Illinois Coal Basin, has been the focus of a number of studies (Booth, 2006; Booth, 2003; Booth, 2002; Booth, 1999; Booth and Bertsch, 1999; Karaman et al., 1999; Booth et al.,1998). Booth (2006) described a mine consisting of 4, 183 metres wide by 1,530 metres long panels, 3 metres thick at a depth of 222 metres. At this site, ground subsidence rapidly reached 2 metres along the panel centreline and there was considerable strata fracturing, particularly shear fractures along the tensional margins of the subsidence trough and vertical bedding separation in the central trough area. The permeability increased by an order of magnitude and levels in the overlying sandstone aquifer declined as the mine face approached, and then partially recovered during the compressional stress phase (Booth 1999; Booth et al., 1998).

Significant changes in the groundwater chemistry were also observed, with the pre-mining water fresh to slightly brackish and sodium-bicarbonate dominant, with sulphate less than 200 mg/l (Booth and Bertsch, 1999). However, water quality in the mining recovery phase exhibited high total dissolved solid concentrations and an increase of sulphate to 1,200 mg/l. The changes were attributed to mobilization of sulphate from sulphides in the sandstone by water flowing back through the aquifer after it became unconfined (Booth and Bertsch, 1999).

Saline County Booth (2006) summarised the impacts of longwall mining at a site in Saline County, in the Southern Illinois Coal basin. At this site six adjacent longwall panels were mined in a 2metres-thick coal seam, between 97 and 122 metres deep and 204 and 287 metres wide.



The centreline subsidence was around 1.4 metres, and sandstone permeabilities over both panels were low and only minimally increased due to subsidence. Water level decrease was rapid falling from 12 metres below ground level to between 49 and 55 metres below ground level above the panel. Decreases further away from the panel were still significant, with a drop from 11 to 33 metres, 300 metres away from the panel. There was little impact on groundwater chemistry due to longwall mining (Booth and Bertsch, 1999).

At this study site full recovery after mining occurred in areas with a moderately transmissive aquifers, but there was almost no recovery in areas where the sandstone had very low transmissivity and restricted lateral pathways to sources of recharge (Karaman et al., 2001; Booth, 1999; Booth et al., 1998). Since the majority of the site contained poorly permeable sandstone, there was little recovery across the site as a whole (Booth and Bertsch, 1999).

7.2.2 Pennsylvania

Figure 7-3 Appalachian Coal Basin (USGS, 2007)

Hill and Price (1993) undertook a site specific hydrological investigation of a site overlying a longwall mining panel on the Northern Appalachian Plateau in Western Pennsylvania. Their study focused on monitoring before, after and during longwall mining. The study panel was 880 metres long, 180 metres wide and 1.4 metres thick with mining occurring at a depth of 170 metres.

The study concluded that the impact of mining on the hydrological systems was localised with regard to the mining front, with the most significant hydrological impacts occurring during the period of maximum subsidence. At this location the shallow aquifer system was isolated from the major fractured zone directly above the mine by the intermediate aquiclude zone.

Stoner (1983) formed a conceptual model of the hydrogeological impacts of longwall mining based on field results from a site in Greene County, Pennsylvania. Stoner (1983) found that as a general rule the magnitude of water level decline was expected to be inversely proportional to the thickness of bedrock between the mine and well bottom.



7.2.3 Virginia

Stout (2004) conducted the study in headwater streams in northern West Virginia, USA, to measure the extent of longwall mining impacts on headwater streams and to determine if they recover after cessation of mining. The study compared the physical (width and temperate), chemical (conductivity, dissolved oxygen, alkalinity, pH and hardness) and biological (macro invertebrate) characteristics of streams impacted by longwall mining to similar references streams, which were not impacted. Data was collected over a 2-year period, and where possible compared to historic data over a ten-year period

Significant physical differences in longwall mined versus reference streams included 0.8°C lower temperature. Lower stream temperatures appeared to be related to water loss at the surface and longer subsurface residence time. Longwall mined streams averaged 100 µmhos higher conductivity, 11% lower dissolved oxygen and 64 ppm greater alkalinity than reference streams. Over time, conductivity and alkalinity in longwall mined streams remained elevated above reference conditions.

Dissolved oxygen was lower in streams that had been impacted by longwall mines in the past compared to streams that had been longwall mined more recently. This was attributed in part due to higher chemical oxygen demand and also lower atmospheric contact in the longwall impacted streams (Stout, 2004)

The chemistry of headwater streams did not recover to reference conditions either spatially or temporarily, and appeared to get worse over time in terms of dissolved oxygen concentrations. Compared to reference conditions, lower dissolved oxygen concentrations may be in part due to higher chemical demand, and in part due to lower atmospheric contact in subsided longwall mined streams.

Longwall mine streams appeared to reach reference stream temperatures approximately one decade after mining occurred. This was thought to reflect increased surface exposure due to continual settling of the stream beds during the 10 years post mining activities.

The northern West Virginia study also found that the width of streams impacted by longwall mining was approximately 30% less than those not impacted, and there was an increased chance (18%) of longwall impacted streams being dry (Stout, 2004). The stream width did not recover to reference conditions, even 12 years after mining was completed.

Peng et al., (1996) undertook modelling of ponding in surface streams affected by longwall mining. They described a 1m deep, 4.88m wide stream in West Virginia which crossed a longwall panel at 270˚ in the direction of stream flow. The coal panel was 1,829 m long and 177 m wide and was mined at a dept of 357 m and thickness of 1.32 m. Both modelling and field observations concluded that in this situation there was negligible formation of migratory ponds.

7.2.4 Utah

Sidle et al., (2000) described a site in Utah, where longwall coal mining occurs at depths of approximately 300 metres. Siddle et al., (2000) found increases in the length of cascades and pool volumes with longwall mining. However, these increases were short lived, and the volumes and numbers of pools declined significantly. Recovery of the streams and channels was noted as occurring a year after the initial subsidence effects were noted. The study did not continue for long enough to comment on the extent of recovery.



8. Limitations of Available Information Based upon the review of available literature, PB has recognised the following areas where information is limited or not available.

• Most of the papers published have been done so by consultants to the mining companies involved. While this does not necessarily say anything about the validity of the conclusions found, it does raise the issue that few independent studies, and thus published findings, were available.

• Much of the research/literature focuses on fracturing in the immediate mine roof overburden and inflow of water into the mine through this pathway. There is limited information on groundwater changes within the upper fractured level, the extent of fracturing in this zone, and the consequent surface water losses.

• There are few relevant papers which discuss surface water impacts (flow and chemistry changes) in detail. The literature which is available discusses what the observed impacts are, but does not relate them to geological changes to any great extent.

• Information relating to changes of groundwater/surface interaction with time and as mining progresses is not published and is more than likely controlled by the mining companies. Therefore, it is not possible to independently evaluate the conclusion being drawn by the authors regarding changes in water table and groundwater flow direction, changes in hydraulic properties of an aquifer impacted by subsidence in time and space, and relation between subsidence progress, geology changes, and impacts to surface and groundwater.

• PB was unable to locate literature that discusses loss of water from the surface and groundwater systems to any other location other than the mine. In general the studies available were focused upon losses to the mine and surface water “disappearing” is assumed to reappear further down gradient, unless it leaks through to the mine.

• Most of the factors that influence the severity of impacts associated with longwall mining are dependent upon the mining activities in combination with the local settings. Thus, knowledge regarding what has happened elsewhere is not always transferable to site specific cases. Therefore, assessments of impacts, or potential for impacts, should be conducted on a case-by-case basis with detailed site and mining practice knowledge.



9. Summary of Findings Based upon the review of available literature, PB offers the following summary of findings as it pertains to the impacts of longwall mining and what investigative techniques have been previously employed to measure and predict the impacts of longwall mining.

• Subsidence and the limit of mining influence is highly site specific and dependant on the mining layout, depth of mining, thickness of coal seam, width of bord and pillar configuration, and surrounding geology and topography.

• Subsidence in NSW is likely to be between 1-2 metres, and approximately 80% occurs within 2 months of completing the longwall panel.

• When mining longwall panels as a series, the resulting subsidence profile is dependant on the size of pillars and panels in relation to the mining depth. The relationship can be qualitatively expressed as follows:

PanelWidth 1Subsidenceprofile = ×PillarWidth DepthofMining

o the deeper the mining the flatter the profile and the shallower the subsidence

o the larger the panel width the greater the subsidence and the more wavy the profile

o the greater the pillar width the flatter the profile and shallower the subsidence

• The greater the amplitude of waves in the subsidence profile the greater the likelihood of vertical fracturing and hydraulic connection.

• There are three main subsidence zones, shallow, aquiclude, and highly fractured.

• For mine depths greater than 100 metres there is unlikely to be surface water/shallow groundwater flow to the mine as this area is separated from the lower highly fractured zone by the aquiclude layer.

• Groundwater levels are unlikely to recover to pre-mining conditions. There may be some small recovery during the initial post subsidence compressional phase, but further recovery is likely to take years, if it occurs at all.

• Recovery of surface water flow conditions is dependent upon, and thus similar to, those of groundwater.

• Groundwater and surface water chemistry impacts are similar – long recovery times, if at all, and they are unlikely to recover to pre-mining conditions.

• Numerous geophysical techniques are available and proven to monitor and assess the impacts of longwall mining. Modelling techniques are also available to not only predict the subsistence effects but also the changes to hydraulic parameters.



• Once changes to hydraulic parameters are estimated, typically through geophysical and subsidence modelling and measurement, hydraulic flow models can be used to quantify the impacts to surface and groundwater systems, including water quality impacts.

• Only one study was found that used tracer experiments to investigate the impacts of longwall mining; however numerous tracer techniques have been applied for other studies which could be used for longwall mining impact assessments.

• The design of an impact study should incorporate all three types of assessment techniques, including prediction, monitoring and measurement. The specific tools to be used should be assessed based upon site specific knowledge.

• Most of the factors that influence the severity of impacts associated with longwall mining are dependent upon the mining activities in combination with the local settings. Thus, knowledge regarding what has happened elsewhere is not always transferable to site specific cases. Therefore, assessments of impacts, or potential for impacts, should be conducted on a case-by-case basis with detailed site and mining practice knowledge.



10. Acknowledgements This report is the collective efforts of PB and SCA staff. The literature search and review were conducted by the Elizabeth Cohen and Kimberly Saflian of PB. Additional support was provided by Jerzy Jankowski (SCA), Wendy McLean (PB), Mellissa Murray (PB) and Sue Price (PB).

The report was prepared by Elizabeth Cohen and Kimberly Saflian, with support from Liz Webb (PB), Wendy McLean (PB), and Brian Rask (PB). The report was reviewed and approved by Brian Rask.



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http://energy.usgs.gov/factsheets/nca/nca.html#seven

http://pubs.usgs.gov/fs/fs115-99/fig1.jpg

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