CHAPTER 6: PART B

Introduction of Hydrogeology Nature of Groundwater and its Occurrence

OUTLINE

What is groundwater?

How much of the water (hydrosphere) is groundwater?

How water gets underground?

Where it store?

How it moves while underground?

How we look for it

Water is one of the most common place compounds on earth.

The largest proportion Is in the oceans which roughly holds

1370 millions km3 of salt water.

The largest fresh water stored is in the glaciers and icecaps

which is about 30 millions km3.

Rivers, lakes, soils and the atmosphere contributes 200, 000

km3 of freshwater which is less than 1 fiftieth of 1 % of the

worlds total water supply.

Storage of water include atmospheric, river/streams, lake,

ocean, groundwater and ice.

INTRODUCTION

(%)

Oceans 96.5

Glaciers and other ice 1.76

Groundwater 1.70

Fresh 0.76

Saline 0.94

Lakes 0.013

Fresh 0.007

Saline 0.006

Soil moisture 0.001

Atmosphere 0.001

Rivers 0.0002

Distribution of Water in the Hydrosphere (%)

Reproduced by Plummer et al.,(2013) Physical Geology, 14 Edition. McGraw Hill

What is GROUNDWATER ?

The water that lies beneath the ground surface, filling pores in

sediments and sedimentary rocks and cracks in other rock types

Represents 0.6% of the hydrosphere (35x the water in all lakes

and rivers combined)

• resupplied by slow infiltration of precipitation

• generally cleaner than surface water

• accessed by wells

Schematic Representation of The Hydrologic Cycle

Schematic Representation of The Hydrologic Cycle

Groundwater is one of the fundamental of the earth materials and the process of groundwater flow is one of the principal geologic process operating within the earth.

It is a major economic resource, particularly in the dry western area, where surface water is scarce.

Particularly in parts of the developing world, groundwater is probably the best solution (if not the only) for drinking water supply and irrigation because commonly it less contaminated and more economical to use than surface water.

In the last 30 or so years the specialist area of hydrogeology has advanced by developing methods and techniques of sedimentology, structural geology, hydraulics, civil engineering and drilling technology.

Porosity & Permeability

Porosity - the percentage of rock or sediment that consists of voids

or openings

• measurement of a rock’s ability to hold water

• loose sand has ~30-50% porosity

• compacted sandstone may have only 10-20% porosity

Permeability - the capacity of a rock to transmit fluid through pores

and fractures

• measures the relative ease of water flow

• interconnectedness of pore spaces

• most sandstones and conglomerates are porous and permeable

• granites, schists, unfractured limestones are impermeable (does not

allow water to flow through it easily)

Types of Voids

a) Well-sorted sedimentary deposits having high porosity.

b) Poorly-sorted sedimentary deposits having low porosity.

c) Well-sorted sedimentary deposits consisting of pebbles that are porous, therefore has a very high porosity.

d) Well-sorted sedimentary deposits whose porosity has diminished by the deposition of mineral matter in the interstices.

e) Rock rendered porous by solution.

f) Rock rendered porous by fracturing.

Table 6.1: Porosity & Permeability of Sediments

and Rocks

Table 6.2:Range of Values of Hydraulic Conductivity

and Permeability

The Water Table Saturated zone – subsurface zone in which

all rock openings are filled with water

Water table – top of the saturated zone • water level at surface of most lakes and rivers

corresponds to local water table

Unsaturated zone – zone above the water

table where not all sediment or rock openings are

filled with water. • Within the unsaturated zone, surface tension

causes water to be held above the water

table.

Capillary fringe – a transition zone just

above the water table

Perched water table – top of a body

of groundwater and separated from main

water table by an unsaturated zone • commonly produced by thin lenses of

impermeable rock (e.g., shales or clays)

within permeable ones

The Water Table

The Movement of Groundwater Movement of ground water through pores

and fractures is relatively slow (cm to

meters/day) compared to flow of water in

surface streams

• Groundwater moves in response to differences

in water pressure and elevation causing water

within the upper part of the saturated zone to

move downward following the slope of water

table.

• flow velocities in cavernous limestones can be

much higher (kms/day)

Flow velocity depends upon:

• slope of the water table

• permeability of the rock or sediment

(Henry Darcy, 1856)

Darcy’s Law and Fluid Potential

The hydraulic head = elevation + pressure

Figure A

The points A and B – on the water table, so

pressure = zero (there is no water to create

pressure).

Point A is at higher elevation than B, so A has

a higher hydraulic head than B

So, water will move from point A to point B – water moves from a region

of high hydraulic head to a region of low hydraulic head

The difference in elevation (hA-hB) = difference in head (h)

The distance the water moves from A to B is labeled (L)

The hydraulic gradient = difference in head = Δh

distance L

Darcy’s Law and Fluid Potential

The hydraulic head = elevation + pressure

Figure B

The points C and D – underneath the water

table, have same elevation.

But pressure on point C > point D – more

water to create pressure above point C than D

So, the hydraulic head point C > higher than

point D

Water will moves from C to D

Figure C

Point F has lower elevation than point G

But pressure on point F > point G – more

water to create pressure above point C than D

The difference in pressure is greater than the

difference in elevation.

So, the hydraulic head point F > higher than

point G

Water will moves from F to G

Darcy’s Law and Fluid Potential

Groundwater moves from

regions of high head to

region of low head

Aquifers & Aquitards Aquifer - body of saturated rock or sediment through

which water can move easily - Highly permeable & saturated

Good aquifers include:

• sandstone

• conglomerate

• well-jointed limestone

• sand and gravel

• highly fractured volcanic rock

Aquitard - rock/sediment that slow down ground water

flow due to low porosity and/or permeability – porosity < 1%

• shale, clay, unfractured crystalline rocks

Aquiclude - no water passes through the rock/sediment. • K < 10-9 m/s • e.g. clay soils, unfractured low porosity rocks, highly anisotropic

rocks.

Aquifer Definitions with Sample K values

UnConfined vs Confined Aquifers Unconfined Aquifer – has a water

table, and is only partly filled with water

• Exposed to the surface

• rapidly recharged by precipitation

infiltrating down to the saturated zone

• Rising & falling water table during wet

and dry season – water drains out of the

saturated zones into rivers.

• Rapid movement of groundwater through

it.

Wet season : water table and rivers are high Dry season : water table and rivers are low

Confined Aquifer – completely

filled with water under pressure

(hydrostatic head), separated from

surface by impermeable confining

layer/aquitard (overlain by an

impervious layer)

• very slowly recharged

• No response at all to wet & dry

seasons

• Also called artesian aquifers

UnConfined vs Confined Aquifers

Sedimentary Strata

Arenaceous rocks – the classification of materials into consolidated and unconsolidated due to high permeability and excellent storage potential e.g. loose dune sand, gravels and alluvium are unconsolidated materials whereas sandstone and conglomerates are consolidated (cemented) materials.

Carbonate rocks – the chemical composition makes it amenable to solution by groundwater that are not fully saturated with calcium carbonate e.g. limestones due to its fissured nature, dolostones which contain decomposed portions with absence of extensive water table and chalk provides excellent aquifers and good storage potential if less clay content.

GEOLOGICAL FORMATIONS AS AQUIFERS

Argillaceous rocks – are relatively impermeable, therefore functions as aquicludes e.g. fine-grained clays, marls, shales and mudstones.

Igneous Rocks

Plutonic rocks are generally not good aquifers e.g. granites, granodiorites, diorites and gabbros.

Extrusive igneous rocks are commonly represented by those of volcanic origin and show considerable variation in aquifer potential.

Intrusive igneous rocks can be restricted to sills and dykes where injection into impermeable formations could be concentrated by groundwater.

Metamorphic Rocks

Gneisses are not good aquifers.

Schists and slates resembles shale in their water-bearing properties and may be aquicludes when their planes of schistosity or of cleavage are horizontal.

The weathered zones, particularly if the dip is high, may be tapped for water supplies down to the limit of decomposition.

Aquifer Examples

Aquifer Examples

Wells

Well - a deep hole dug or drilled into the

ground to obtain water from an aquifer

• for wells in unconfined aquifers, water

level before pumping is the water table

• water enters well from pore spaces within

the surrounding aquifer

• water table can be lowered by pumping, a

process known as drawdown

• water may rise to a level above the top of

a confined aquifer, producing an artesian

well

Groundwater Fluctuation

Water levels fluctuate commonly under both confined and unconfined conditions, the causes which give rise to such fluctuations are not always identical and may be natural or artificial.

The most significant fluctuations in an unconfined aquifer are those resulting from;

seasonal infiltration where the effects of pumping on changes of groundwater levels may be important locally.

Under confined conditions,

• the effect of transpiration and where adjacent river and sea may respond to changes in river stage and tidal level can cause minor fluctuations.

Similarly, wells in confined conditions exhibit fluctuations that are related to loading such as brought by atmospheric pressure, tides, earthquakes and landslips.

Groundwater is never found in the chemically pure form of H2O because prior to becoming groundwater, it has been involved in chemical reactions with the materials comprising the atmosphere and zone of aeration.

As a result of contact with a variety of gases in the air and a variety of minerals beneath the surface, water is complex dilute solution by the time it reaches the zone of saturation.

The dominant factor governing the change is the rate of groundwater movement.

Chemical processes that are responsible for the actual changes are:

i) Solution

ii) Precipitation

iii) Reduction

iv) Concentration

v) Absorption

vi) Ion exchange

GROUNDWATER CHEMISTRY

Effects of Chemical Composition The constituents of groundwater derived directly from;

• solution of minerals in rocks are the cations and silica,

• while most of the anions are derived from other sources.

The selection of lining materials, pipe work or amount of open-area available for flow will be influenced by;

• the corrosive or encrusting nature respectively of the groundwater.

A pH value of 7.0 - denotes a neutral reaction where lesser values will result in acidic water which tends to be corrosive

pH higher than 7.0 - will be alkaline and tends to be encrusting.

Alkalinity - is a property determined by the amount of carbonate and bicarbonate.

Hardness - is the property of water dominantly due to the presence of calcium and magnesium compounds.

Purpose and Approach The purpose of exploration is to enable the hydrogeological

conditions to be investigated in as detail a fashion as is necessary to meet the requirement of the project.

The following stages are planned before data collection and interpretation is conducted are:

i) Desk study – collation and analyses of existing data. ii) Feasibility study – Collection and analyses of additional

data. iii) Pilot study – site investigation and subsequent action. iv) Development.

GROUNDWATER EXPLORATION

Field Reconnaissance

1. Topography – A base map showing topographic features and elevation contours relative to some datum is a necessary prerequisite to field reconnaissance.

2. Geology – Photo geological interpretation and field control allow preparation of a geological map that is representative of surface conditions.

3. Hydrogeology – A 3-dimensional representation of the sub-surface hydro geological regime would be greatly assisted by field measurement and determination of the following:

i) Precipitation and evaporation.

ii) Location, elevation and discharge of springs and seepages.

iii) Stream discharges.

iv) Evidence of saline and alkaline soils.

v) Distribution of vegetation types.

vi) Location of wells and measurement of water levels and abstraction.

Groundwater Exploration Techniques

Geological exploration which consist of:

i) Regional geology maps.

ii) Local geology maps/field mapping.

iii) Geomorphology/Topography.

iv) Air photos/satellite imagery.

v) Hydrology and hydro geological data.

Geophysical exploration which consist of:

i) Seismic.

ii) Electrical resistivity.

iii) Electromagnetic methods.

iv) Gravity.

v) Drilling.

vi) Borehole logging.

Types of Geological Exploration

Geological Maps Field Mapping

Geomorphology

Hydrogeological Data Aerial Photographs

Types of Geophysical Exploration

Seismic Methods Electrical Resistivity

Gravity Methods

Borehole Logging

Electromagnetic Methods

Drilling

EXERCISE

1) Explain with the aid of diagram the following

term of hydrogeology;

- Saturated zone

- Water table

- Unsaturated zone

- Aquifer

- Aquicludes

- Aquitards

1) Identify the types of rocks that make good

aquifers.

2) Identify the types of rocks that called

aquitards

1) Describe the hydrological cycle by using an

illustration.

2) With the aid of diagram, distinguish

between:

- Confined aquifer

- Unconfined aquifer

1) Define the following term of hydrogeology:

- Permeability

- Porosity

- Hydraulic gradient

- Impermeable rock