Unit 12 : Advanced Hydrogeology

Contaminant Hydrogeology

Contamination Problems

• So far we have concentrated on physical and chemical processes of mixing dissolved species in groundwater systems.

• Most of the discussion has largely avoid issues of organic non-aqueous phase liquids (NAPLs) and multiphase flow.

• In this section we will discuss practical problems of groundwater systems where a vapour phase and more than one liquid phase may be present.

NAPLs

• In general, the complexity of contamination problems increases when NAPL’s are present.

• Instead of one, there are up to three plumes to track:– vapour phase plume– immiscible liquid phase plume– dissolved phase plume

DNAPLs and LNAPL’s

• NAPL’s are classified according to their density relative to water:– LNAPL’s (light non-aqueous phase liquids) are

lighter than water and tend to “float” at the water-table and give rise to associated vapour phase and dissolved phase plumes.

– DNAPL’s (dense non-aqueous phase liquids) are heavier than water and tend to “sink” to the bottom of aquifers where they can also give rise to dissolved phase plumes.

Mainly LNAPL Vapour

Complex Plumes

Clay Lenses

Primary LNAPLSecondary LNAPL

Secondary DNAPL

Mainly DNAPL Residual

Flow

Primary DNAPL

Contamination Attributes

– degree of localization• point or local• non-point or diffuse

– loading history• pulse• continuous

– contaminant type• radionuclides• trace metals• nutrients• other inorganics• organics• biological

• Three important attributes distinguish sources of groundwater contamination:

Degree of Localization of Source

• A point source is characterized by the presence of an identifiable small-scale source such as a leaking tank, a spill, a small pond or a landfill.

• A diffuse source refers to a source emanating from many poorly defined locations. Pesticides, fertilizers, acid rain and highway salt are typical non-point diffuse sources.

Loading History• Loading history describes how the source

concentration varies as a function of time.

Time

Co

nce

ntr

atio

n

Time

Co

nce

ntr

atio

n

Time

Co

nce

ntr

atio

n

Time

Co

nce

ntr

atio

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Pulse Continuous Constant

Continuous Variable Continuous Decaying

Sources of Contamination• A very long list of domestic, agricultural and industrial

activities have the potential to result in contamination.• Category I: Designed Discharges

– Septic tanks, injection wells, land application of wastes, etc

• Category II: Designed Storage Facilities– landfills, dumps, tailings piles, ponds, underground tanks, etc

• Category III: Designed Transportation Systems– pipelines, transport routes, transfer stations, etc

• Category V: Other Planned Discharges– pesticide/fertilizer application, deicing salts, mine drainage, etc

• Category V: Potential Conduits– production wells, monitoring wells, construction excavations, etc

• Category VI: Naturally Occurring Sources– natural leaching, saltwater intrusion, saline water upconing, etc

U.S. Office of Technology Assessment, 1984

Types of Contaminants• The list of potential contaminants number in the tens

of thousands and organization of this list is a major problem.

• Six major categories: (1) radionuclides (2) trace metals (3) nutrients (4) other inorganics (5) organics and (6) biological provide a framework for discussion.

• All the contaminants have the potential to produce health problems. Too much of anything is a potential health hazard but tolerable thresholds are finite.

• For some contaminants, particularly radionuclides, the threshold level is such that anything above natural background is of concern.

Radionuclides• The nuclear fuel industry is the main source of

radioactive contaminants.• Potential sources occur throughout the nuclear fuel

cycle.• During mining when raw ore is processed 238U, 229Th,

226Ra, and 222Rn are potential contaminants.• Fuel fabrication, fuel reprocessing and power-

generating facilities are other potential sources.• Colorado, New Mexico, Utah, Wyoming,

Saskatchewan and Ontario are the U producing regions of North America.

• The nuclear industry is very closely controlled and monitored by federal, state and provincial authorities.

Trace Metals• Trace metals are a natural component of all

groundwaters and represent the largest group of elements in the periodic table.

• Additional sources include (1) mining effluents, (2) industrial wastewaters, (3) urban runoff, (4) agricultural wastes and fertilizers, and (5) fossil fuels.

• Some trace metals (B, Cu, Fe, Zn) are essential for health but others have a tendency to accumulate in the body (or bioaccumulate in organisms low in the food chain) from sources at relatively low concentrations.

• The 13 trace metals that make the EPA list of 129 priority pollutants are: Ag, As, Be, Cd, Cr, Cu, Hg, Ni, Pb, Sb, Se, Tl, Zn.

Nutrients• Nutrients are ions and compounds containing

nitrogen and phosphorus.• The dominant nitrogen species in groundwater in

NO3- and to a lesser extent NH4

+.

• Phosphorus is less important as a contaminant because of its low solubility and tendency to readily sorb onto solids.

• Sources of N and P are largely agricultural including the use of fertilizers and the cultivation of virgin soils (when large quantities of N are released.)

• Sewage (N) and municipal wastewaters (P) are also sources of nutrients.

Other Inorganic Species• This group of “contaminants” includes the major ions usually

present in groundwater.• Extremely high concentrations render water unsuitable for

human consumption, animal watering and many industrial uses.

• Health related concerns are low for this group of contaminants but high concentrations of Na+ can disrupt blood chemistry and lower Na+ concentrations may lead to hypertension.

• Fluoride a good example of a trace non-metal contaminant. At low concentrations F- has the beneficial effect of reducing tooth decay. At higher concentrations (only x5 higher) F- can lead to serious health problems including goitre and fluoridosis.

Organics• Contamination of groundwater by organic compounds

is a consequence of the large number of petroleum products and man-made organics in common use.

• The EPA list of 131 priority pollutants contains 116 organic compounds, 13 trace metals, asbestos and cyanide. Over 90% are organics.

• The EPA divides the organics into four groups based on the methods used for analysis but ultimately the analytical tool is gas chromatography-mass spectrometry (GC-MS):– base-neutral extractables (47)– acid extractables (12)– volatiles (32)– pesticides and PCBs (25)

Petroleum Products• Organic compounds in crude oil and crude oil

products fall into three main groups:– Alkanes (n-butane, n-hexane, cyclohexane, etc)

– Alkenes (ethene, propene, etc)

– Aromatics (BTEX group: benzene, toluene, ethylbenzene and xylene; PAH group: anthracenes, naphthalenes, etc)

• Distillation separates crude oil into fractions:– Gasolines (C4 to C12 alkanes, C4 to C7 alkenes, BTEX group)

– Middle Distillates (C10-C24 alkanes, some BTEX, some PAHs)

– Residual Products (C20-C78 alkanes, aromatics mainly PAHs)

Halogenated Compounds

• Aliphatics (chain molecules with H replaced by F, Cl, Br) including PCE (CCl2CCl2), TCE (C2HCl3) and CT (CCl4).

• Aromatics (benzene rings with substituted halogens) including chlorobenzene (C6H5Cl) and dichlorobenzene (C6H4Cl2)

• Both groups of halogenated compounds tend to have densities greater than water and usually fall in the DNAPL group.

PCBs• Polychlorinated biphenyls (PCBs) .Due to their non-

flammability, chemical stability, high boiling point and electrical insulating properties, PCBs were used in hundreds of industrial applications prior to 1977 (when N.American production was discontinued).

• They consist of joined, chlorine substituted benzene rings.

• PCBs are relatively insoluble in water but are extremely persistent in the environment breaking down very slowly.

• PCBs are highly toxic and highly soluble in biological lipids, and have been found to accumulate in animal and human tissues.

Organics in Groundwater

Petroleum DerivativesFuels benzene, xylene, hexane, octane

PAHs benzopyrene, naphthalene

Alcohols methanol, glycerol

Creosote m-cresol, o-cresol

Ketones acetone

Halogenated CompoundsAliphatics PCE, TCE, CT

Aromatics chlorobenzene, dichlorobenzene

Polychlorinated Biphenyls 2,4’-PCB, 4-4’-PCB

water soluble DNAPL LNAPL

Biological Contaminants• The important biological contaminants are:

– pathogenic bacteria (Fecal streptococci, Fecal colliforms, Escherichia coli, Shigella dysenteriae, Salmonella typhi, Vibrio cholerae)

– viruses (enteroviruses, hepatitis A virus, polio virus and rotavirus )

– parasites (Giardia, Entamoeba, Cryptosporidium )• The main sources are (1) land disposal of sewage

and septic tanks, (2) leachates from sanitary landfills and (3) agricultural wastes.

• The contaminants are particulate rather than dissolved, transport distances are limited and problems tend to be related to localized sources.

Plume Morphology Basics

• Mass transport processes control:– maximum plume extent– geometric distribution of concentrations

• Advection is the dominant control on plume shape.

• Hydrodynamic dispersion is usually a secondary factor.

• Chemical, nuclear and biologic processes generally attenuate spreading.

Controls on Advection

• Magnitude and direction of advective transport is controlled by:– hydraulic conductivity field– potentiometric head distribution– distribution of sources and sinks– shape of the flow domain

• All these factors influence groundwater flow velocity, which drives advective transport.

Hydraulic Conductivity FieldH

L

Hydraulic Conductivity

• Plume advects faster with increasing K.

• Plume migrates in highest K horizon

• Low K near surface delays plume. development.

• Plumes tend not to invade low K units.

Dispersion

L=0T=0

L>0LT

L>0LT

L>>0LT

Dispersion

• Increased dispersion gives broader plume.

• Concentration pattern is spread and high concentration zone is reduced with increasing dispersivity.

• Increasing T results in increased vertical spreading.

• High L and high T gives most mixing.

Reaction Rate Half-Life

t1/2=30

t1/2=3

t1/2=1

t1/2=10

Reaction Rate Half-Life

• Reaction removes contaminant mass.• As half life reduces, more material has

reacted and the plume shrinks.• After about 5 half-lives the plume is

indistinguishable from background.• Both biological degradation and

radioactive decay result in plume shrinkage.

Sorption

Kd=0.01

Kd=10

Kd=1

Kd=0.1

Sorption

• Sorption is another attenuation process.

• As Kd increases the plume becomes smaller as sorption retards and attenuates the mass in solution.

• Sorption is a potent mechanism for reducing the extent of contaminant plumes.

Pulse Loading

Co=1t=5

Co=1/2t=10

Co=1/4t=20

Co=1/8t=40

t =50

Pulse Loading

• The same mass is added over an increasing time period from 5 to 40 years.

• Peak concentrations are reduced by spreading and the entire mass is advected towards the discharge end of the flow system.

• For lower source concentrations over longer periods the plume is closer to the source and less compact.

QA/QC in Water Sampling

• Characterization of the distribution of contaminants in the subsurface is an integral part of any field study.

• The collection of these data is fraught with difficulties and “bad data is worse than no data at all.”

• Design of collection systems to ensure that data is both reliable and representative is a considerable challenge.

Screen Length

• When piezometers are designed to measure hydraulic head, screen length is not a critical concern.

• For chemical sampling, screen length can lead to widely variable results.

1 mg/L

1 mg/L

10 mg/L

Point Sampling

• Point sampling implies a short screen section and small diameter access tube to minimize fluid volumes are avoid mixing and dilution.

• Narrow tubes are not ideal for head measurements (where access for a tape is required).

Chemical Monitoring Well

Piezometer

Sampling Points

• Sampling point locations need take account of the complexity of the flow system.

• Plumes often have a restricted vertical extent and may not resolve a plume.

• Multiport monitoring wells solve this problem (at a price).

standpipepiezometers

multilevelsampler

QA/QC for Chemical Data

• There are four potential problems that can have a serious impact on chemical data:

1) Contamination of samples with drilling fluids.

2) Changes in water quality as a result of well construction.

3) Sample deterioration prior to analysis.4) Careless field and laboratory practices.

Sample Contamination

• Occasionally, samples become contaminated with gas, oil, or water in combination with foams, emulsions and muds used in the drilling process.

• These contaminants are difficult to remove once the well has been completed.

• Contamination can be avoided by using a readily detected tracer in the drilling fluid and developing the well until the tracer can no longer be detected

Well Construction

• Cement and other soluble materials used in well construction can influence groundwater chemistry.

• High pH (>9) is often characteristic of continuing interaction between cement and the water being sampled.

• Samples can also be contaminated with trace metals from steel casing and adhesives from plastic casing joints.

• Selection of threaded plastic casing avoids these problems.

• Purging wells by removing at least 2 or 3 well volumes before sampling is also recommended.

Sample Deterioration

• Temperature, pressure and gas content changes in samples following collection can seriously impact chemistry.

• Exposure to atmospheric oxygen can lead to rapid Eh changes and metals precipitation from anoxic waters.

• Loss of CO2 can raise pH and decrease bicarbonate concentrations (and perhaps precipitate CaCO3.

• These problems can be addressed by specialized sampling equipment for high P-T environments, the use of flow-through cells and various sample treatments to minimize deterioration.

Field Sample Handling

• Field handling of samples requires standardized bottle washing, filtering and use of chemically pure preservatives.

• Sorption of organics onto containers or filters and loss of volatiles during samples are potential problem areas.

• Running “blanks” of ultra-pure distilled water through the field sampling procedure allows for rapid detection of problems.

Laboratory Sample Handling

• Ongoing QA/QC checks for analytical laboratories are a necessity. Common approaches include:– Submitting “spiked” samples of known composition

for analysis.– Submitting duplicate samples to different

laboratories.– Submitting replicate samples of the same water.

Sampling Systems

• Sampling systems capable of providing point samples from the saturated zone include:

1) Nests of standpipe piezometers

2) Multi-level (or multiport) samplers installed in a single boreholes

3) Packers systems that can isolate various levels in a single borehole.

Nested Standpipes

• Single wells are easier to complete and seal:– Drill– Sand pack– Bentonite seal– Backfill– Surface cement seal

• Installation are durable and reliable.

• Main disadvantage is high cost of drilling.

Cement Seal

Bentonite Seal

Fill

Sand Pack Slotted Screen

Multi-level Sampling

• Multi-level samplers involve placing several sample points in a single hole:– seals are required between sample points– filter sand may or may not be required– sand packs are difficult to install– sand packs are often impossible to develop– small internal volume– reduced drilling costs

Multi-level Samplers

• A wide variety of multi-level samplers are available: – multiple standpipes in single hole– bundled standpipes– valved suction couplings – retrievable downhole tools with packers

and suction ports– gas-drive samplers

Multiple Standpipes

Sand pack

Bentonite Seal

Screen

Bundled

Suction Ports

PVC Pipe

Stainless Steel Screen

Plastic Tube

Rubber Seal

Installation involves multiple ports and does not require sand pack or seals. Subject to clogging. Works best in cohesive materials.

Westbay-Type Samplers

• Downhole sampling tool.

• Casing has sampling ports

• Packers inflated to isolate particular port

• Suction recovers small sample

• Transducers measure pore-pressure

• Effective but expensive system

Unsaturated Zone Samplers

• Most systems involve vacuum suction through porous ceramic cup.

• Small sample chamber is filled.• Sample moved to the surface by suction

or gas displacement.• Problems include:

– reaction of pore fluids with ceramic– loss of gases as a result of vacuum suction

Solids Samplers• Another approach to fluids sampling is to take an entire soil

sample and extract the pore fluid.• This method is often used for LNAPL and DNAPL

sampling.• There are a variety of sampling devices:

• split spoons • thin-walled tubes• piston samplers

• Advantages include relating contamination directly to lithology and minimal cross-contamination.

• Disadvantages include cost and difficulties with resampling.

Soil-Gas Characterization

• Soil gas sampling has become the industry standard for sampling organic volatiles in groundwater.

• A metal probe with a perforated tip is driven into the ground.

• Small gas sample is extracted by pumping.

• Alternatively static collectors can be buried and recovered for analysis.

• The collectors sorb volatiles onto activated charcoal.

• Samples are analyzed by GC.

• Gas sampling is a simple, rapid and cost-effective screening tool.

Geophysical Methods

• Geophysical methods have been used for plume delineation for decades.– Surface Resistivity and EM conductivity methods

have met with most success.– Ground Penetrating Radar (GPR) has become a

useful tool for shallow investigations.– Magnetometry is very useful to locate buried

pipes, metal objects and containers.– Shallow Seismic survey have met with limited

success in true groundwater applicationsand are relatively high-cost.

ERT Measurements

• ERT (electrical resistance tomography) is a DC resistivity technique.

• Electric potentials generated by a current source are measured by a receiver array.

• Measurements may be made on the earth’s surface or in boreholes.

ERT Data Processing• Measured potentials are sensitive to the bulk electrical

properties, which are diagnostic of porosity, connectivity of pore fluid, and pore fluid chemistry.

• The ERT image creation process involves solving both forward and inverse problems on an FE mesh.

• Image resolution is a complicated function of many factors, including reconstruction pixel size, data signal-to-noise ratio, electrode and borehole separation, the subsurface resistivity distribution, and the degree to which the resistivity matches the two-dimensional model used for the forward calculations.

• Resolution can be no better than one pixel; typical pixel size is 1 to 3 meters and improves as borehole spacing decreases.

ERT Images

%Change in ResistivityAir Sparging Experiment, Florence, Oregon

ERT Section

EM Basics• The electromagnetic (EM) geophysical method

determines electrical properties of earth materials by inducing electromagnetic currents in the ground and measuring the secondary magnetic field produced by these currents.

• An alternating current is generated in a wire loop or coil above the ground's surface. Both the primary magnetic field (produced by the transmitter coil in the instrument) and the secondary field (produced by currents in the earth) induce a corresponding alternating current in the receiver coil of the instrument.

• The coils are kept at a fixed distance and orientation relative to the ground to simplify data analysis.

EM Data Processing

• After compensating for the primary field, both the magnitude and relative phase (in-phase and quadrature-phase) of the secondary field are measured.

• The quadrature-phase component is converted to a value of apparent soil electrical conductivity (EC).

• This value represents an estimate of the local average soil EC.

• The depth of measurement is dependent on the instrument's coil spacing, orientation, and operating frequency, and the actual subsurface EC variations.

EM Interpretation

• Averaging limits discrimination of thin, high concentration brine intrusions from broader, more diffuse plumes.

• Multiple profiles using differing coil spacing can be performed to bracket approximate depths of brine affected groundwater.

• Data quality may be degraded by cultural interference as caused by utility lines, steel fences or other large metallic objects.

EM Instrumentation

EM31 – 1 to 6 m EM34 – up to 60 m

EM38 – 75 cm to 1.5 m EM39 – downhole to 500 m

EM31

• The EM31 maps geological variations, groundwater contaminants or any subsurface feature associated with changes in the ground conductivity using an electromagnetic inductive technique that makes the measurements without electrodes or ground contact.

• With this inductive method, surveys can be carried out under most geological conditions including those of high surface resistivity such as sand, gravel and asphalt.

• The effective depth of exploration is about six meters, making it ideal for many geotechnical and groundwater contaminant surveys.

EM31 Advantages

• Important advantages of the EM31 over conventional resistivity methods include:– the speed with which surveys can be conducted. – the precision with which small changes in conductivity can

be measured. – the continuous readout and data collection (logging) while

traversing a survey area.

• The in-phase component is especially useful for detecting shallow ore bodies and buried metal hazardous waste.

EM34

• The EM34-3 has been particularly successful for mapping deeper groundwater contaminant plumes and for groundwater exploration.

• Using the same patented inductive method as the EM31, the EM34-3 uses 3 intercoil spacings to give variable depths of exploration down to 60 meters.

• With the 3 spacings and 2 dipole modes vertical electrical soundings can be obtained.

EM38

• Designed to be particularly useful for agricultural surveys measuring soil salinity, the EM38 can cover large areas quickly without ground electrodes.

• Based on the same patented induction principle as the EM31, the EM38 provides depths of exploration of 1.5 meters and 0.75 meters in the vertical and horizontal dipole modes respectively.

• The lightweight,one meter long, EM38 provides rapid surveys with excellent lateral resolution.

• Measurement is normally made by placing this instrument on the ground and recording the meter reading.

EM39

• The EM39 provides measurement of the electrical conductivity of the soil and rock surrounding a borehole or monitoring well, using the inductive electromagnetic technique.

• The unit employs coaxial coil geometry with an intercoil spacing of 50 cm to provide a substantial radius of exploration into the formation while maintaining excellent vertical resolution.

• Measurement is unaffected by a conductive borehole fluid or the presence of plastic casing. The instrument operates to a depth of 500 metres.

EM Survey Results

Magnetometry

• Magnetomety detects changes in the earth’s magnetic field created by buried ferromagnetic objects.

•  A magnetometer measures the intensity of the earth’s magnetic field. The presence of ferrous metals creates variations in the local strength of that field, permitting their detection.

• A magnetometer’s response is proportional to the mass of the ferrous target. Typically a single drum can be detected at distances up to 6 meters, while massive piles of drums can be detected at distances up to 20 meters or more.

Magnetometers

• Modern magnetometers permit the acquisition of continuous data as the magnetometer is moved across the site.

• This continuous coverage is needed for high resolution requirements and the mapping of extensive areas.

• The effectiveness of a magnetometer can be reduced or totally inhibited by noise or interference from time-variable changes in the earth’s field and spatial variations caused by magnetic minerals in the soil, or iron and steel debris, ferrous pipes, fences, buildings, and vehicles.

Applications of Magnetometry

• Applications of magnetometry include:– Location buried steel containers, such as 55–

gallon drums.– Definition of boundaries of trenches filled with

ferrous containers.– Location ferrous underground utilities, such as iron

pipes or tanks, and the permeable pathways often associated with them.

– Selection of drilling locations that are clear of buried drums, underground utilities, and other obstructions.

Magnetometer Survey

GPR

• Ground penetrating radar (GPR) has been used for over twenty years at chemical and nuclear waste disposal sites as a non-invasive technique for site characterization.

• Standard GPR surveys are conducted from the surface of the ground providing geotechnical information from the surface to depths of 2 to 15 m, depending on GPR frequency of operation and soil conductivity.

• Commercially available GPR systems operate over the frequency range 50 MHz to 1000 MHz.

GPR Details

• Lower frequencies provide better penetration but poor resolution, while the higher frequencies give poor penetration but good resolution.

• There are many critical environmental monitoring situations where surface GPR does not provide the depth of penetration or necessary resolution.

• Borehole radar can place the sensor closer to the region of interest, overcoming the high signal attenuation in the near-surface soils.

GPR Images

GPR Borehole Tomography of Water Injection Well

Shallow Seismic

• Initiated by rapid technical developments over the past two decades or so, multi-channel engineering seismographs with wide dynamic range, powerful computer facilities, and sophisticated seismic software have become available to a broad community of environmental and geotechnical users at reasonable prices.

• Based on these instrumental improvements, acquisition and processing techniques established for deep seismic surveying in the hydrocarbon industry have been successfully adapted and applied in shallow seismic studies.

Difficulties with Shallow Surveys

• The most important difficulty to be overcome in near-surface seismics is the inevitable interference of strong source-generated noise with reflections from the shallow subsurface.

• To extract shallow reflection signals from data contaminated by source-generated noise, careful choices of recording parameters (e.g., source type, source and receiver intervals, minimum and maximum source-receiver offsets) and processing strategies are even more important than in deep seismic studies.

Resolution of Shallow Seismic Data• Important considerations when designing shallow

seismic reflection investigations are the spatial and temporal resolution.

• Mapping of small-scale features in the near surface requires much higher resolution than is demanded of conventional seismic surveys.

• Improvement of horizontal resolution may be achieved by simply decreasing source and receiver intervals.

• Higher vertical resolution requires broader bandwidth signals. Sources must be capable of generating a wide band of high frequencies (>100 Hz), and receivers must be capable of faithfully recording them.

Shallow Seismic Reflection Applications

• Shallow reflection methods have proved to be reliable and valuable tools for non-destructive mapping of the shallow subsurface associated with: – near-surface structural investigations – mapping the depth to bedrock – delineating shallow faults – locating subsurface cavities and tunnels– mapping suitable formations for potential construction and

waste disposal sites – water table location and other groundwater studies– mineral exploration and exploitation– detecting fracture zones in crystalline rocks

3D Reflection Survey

3D Reflection Survey: P-wave seismic reflectors