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GGroundwater roundwater RResearch esearch CCenter (enter (GRCGRC)) H. Mohammadzadeh and MJL RobinAdvanced Hydrogeology – GW Contamination

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))متآبمتآب((های زیرزمینی های زیرزمینی آبآبحقیقات حقیقات تترکز رکز ممGGroundwater roundwater RResearch esearch CCenter (enter (GRCGRC))

Groundwater Contamination


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III

Suggested reading:Freeze and Cherry, 1979. Groundwater, Chapitre 2 & 9

Raisin River, North Branch, August 2005

Contaminant hydrogeology


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Types of problems in contaminant hydroTypes of problems in contaminant hydro

• Risk assessment- Predictions - Sensitivity analysis- Political considerations- Moral considerations- Other consideration$

Four types of problems - all inter-related; e.g. we could be performing a risk assessment for litigation purposes that would lead to monitoring and remediation efforts.

• Litigation- Forecasting / “backcasting”- Past and obsolete legislation - Political considerations- Legal considerations- Orphan sites

• Monitoring and Remediation- Often an integral part of the above problems- Optimization of strategies and resources- How clean is CLEAN?

-How do we justify the limits?-Limits are black and white but problems and measurements are shades of gray!!

• Site assessment- Possible Objectives

-Identify the contaminant-Delineate the contaminant plume-Clean up (remediation) and/or keep an eye on it (monitoring) and/or examine “what-if” scenarios

- Risk assessment


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Site assessment processSite assessment processIdentify Site

Define Objectives

Develop / Refine Conceptual Model

Gather and Assemble Available Information

Identify and Prioritize Data Gaps

Collect Data from Site

Report, recommend

Implement Methods

Select Methods

Interpret Data

Modeling

Sensitivity Analysis


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Building the conceptual model (factors) Building the conceptual model (factors)

1. Types of contaminants

2. Sources of contaminants

3. Types of geological materials

4. Flow systems

5. Processes involved in contaminant transport

• Physical

• Chemical

• BiologicalRaisin River, August 2005


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1. Types of contaminants 1. Types of contaminants • Bacteria (e.g. Salmonella typhosa, E. coli, other coliforms), viruses (e.g. Hepatitis

A) and other parasites (e.g. worms).

• Cause illnesses that can be fatal: typhoid fever cholera, polio, hepatitis, poisoning.

• Principal causes: sewers, human and animal waste waters.

• Groundwater contaminated by:

• on-land disposal of waste sludges;

• leachates from “sanitary” landfill sites, or from faulty septic systems; or

• inappropriate agricultural practices.

• Contamination is generally localized.

• Contamination of wells is generally caused by short-circuiting surface waters or phreatic waters to deeper groundwater due to faulty well construction and/or proximity of well to the source of contamination.

• Recent case: Walkerton, Ontario, Spring 2000.

Biological


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Radioactive contaminantsRadioactive contaminants• Principal sources are from the nuclear power industry: Uranium mining (mining

waste), nuclear fuel production, operation of the power plants, but mostly from the nuclear waste disposal (spent fuel and other waste).

• Health hazard: cancer for the exposed person and his/her progeny.


8Source: Bedient et al., 1994.


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Inorganic contaminantsInorganic contaminants

• Major Ions:– Contribute mostly to water salinity and to the deterioration of the water

quality.

– The majority are non-toxic up to relatively high concentrations (Na+ can contribute to hypertension).

– Sources: mineral dissolution, mine or landfill leachates, industrial waste waters.


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• Nutrients:– Main pollutants are products containing nitrogen and phosphorus.

– Nitrogen:– Nitrate (NO3

-) - very mobile;– Ammonium ion (NH4

+) less mobile; – Mostly from agricultural activities (chemical fertilizers, manure

applications, pastures near or in contact with surface waters).– Effects of nitrate: methemoglobinemia in infants; cancer.– Palliative processes for nitrates: denitrification in reducing conditions.

– Phosphorus:– levels are linked to nitrate levels through common causes;– phosphorus is much less mobile than nitrate (in fact, almost immobile),

it remains attached to soil particles;– pollution due to phosphorus is mostly due to soil erosion because the

soil carries the phosphorus with it; causes an overabundance of aquatic plants in surface waters.


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• Trace metals and heavy metals:

– Common sources: mine effluents, industrial waste waters, urban runoff, urban waste, agricultural waste, chemical fertilisers and pesticides, fossil fuels

– Toxic (often lethal) at low levels because of their tendency to accumulate in biological tissues.

– Biological accumulation in the food chain can cause large scale-problems.

– Solubility and mobility depend on pH (more soluble at low pH’s) and on redox conditions.

– Metals usually have a high affinity to soil particles (adsorption and cation exchange) and are therefore relatively immobile in soils

– Transport of metals in groundwater can be accelerated considerably by complexation and chelation.


12Source: Freeze and Cherry, 1979. Groundwater


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• Type of contaminant that is pervasive, diverse and the most problematic.

• These contaminants can be found dissolved in groundwater and often as a pure immiscible form.

• Health risks are numerous but poorly understood for most organic contaminants.

• Their transport properties in groundwater depend on their physical properties:• Volatility (volatile vs non-volatile)• Viscosity (affects their transport in pure phase)• Miscibility in water (miscible vs immiscible)

“NAPL’s” Non-Aqueous Phase Liquids• Density relative to that of water (specific gravity) - depends on the size of the

molecule and the number of halogen radicals that are attached to it (they are relatively heavy): DNAPL dense NAPL

LNAPL light NAPL• Solubility in water depends on the presence of polar radicals on the

molecule;

OrganicOrganic


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• Solubility, Sb (mol/L), can be obtained from the octanol-water partition coefficient:

• Generally highly toxic at very low concentrations. Solubilities are very low but toxic concentration levels are even lower. Consequently, very small quantities of pure product can contaminate huge volumes of water.

Examples: • 1 teaspoon (5mL) of benzene, one of the components of gasoline, will

dissolve in 5.2 L of water; but it will contaminate 850,000 L of water. It would take 5 tsp to contaminate an Olympic-size swimming pool (100x25x1.75 m3)!!!

• 1 teaspoon of trichloroethylene (TEC), a dry-cleaning product, will dissolve in 6.6 L of water; but it will contaminate 1,450,000 L of water. It would take 3 tsp to contaminate the Olympic-size swimming pool.

• Cleanup is very difficult and often impossible

, log 1.0 0.7 log ,

are mole fractions in octanol or water

oow ow b

w

XK K SX

X


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• Families of organic contaminants:– Hydrocarbons

▫ aliphatics: Chains of varying length with single, double or triple bonds -found in light petroleum products

▫ aromatics: An H - ring structure (e.g. benzene), volatile, low solubility▫ polyaromatics: PAH - cluster of several benzene rings, much heavier,

less soluble, less volatile, less mobile, very toxic.

– Chlorinated hydrocarbons & pesticides▫ Chloride substitutes for some of the hydrogens▫ Examples: chlorobenzene, dichloroethane, PCB’s TCE▫ Low solubility, heavy, very toxic, persistent▫ Can be degraded in the presence of metals (iron)▫ Pesticides act in a similar way as chlorinated hydrocarbons

– Phenols▫ Benzene ring with one OH radical on it ▫ Higher solubility


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LNAPL Contamination

Source: Domenico and Schwartz


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DNAPL Contamination

Source: Domenico and Schwartz


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2. Types of geological materials 2. Types of geological materials Physical and chemical properties• Mineralogy

• Porosity

• Permeability

• Homogeneity

Unconsolidated materials• Texture (particle size, degree of sorting)

• Organic matter content

• Specific surface area (surface per unit mass)

• Cation exchange capacity

• Homogeneity


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Rocks• Porous

• Fractured (fracture distribution)

• Fractured-porous

Other:• Permafrost areas

• Karstic Formations


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• Saturated- Aquifer vs aquiclude vs

aquitard- Artesian vs non-artesian- confined vs unconfined

(phreatic)

• Unsaturated- Characteristic curves (moisture retention and

conductivity)- Thickness of the capillary fringe- Depth to the water table- Mathematical model is more complicated - the

flow equation is non-linear

3. Types of flow systems 3. Types of flow systems

• Density controlled- Instabilities caused by a density

contrast- Stratification caused by a density

contrast- Saltwater intrusions in coastal

aquifers


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• Multi-phase- Several effects to consider:

Surface tension Contact angles - wetting

surfaces Density differences

- Mathematical model is most complex - flow equations are required for each phase, the flow equations are coupled, and each equation is non-linear

Julie Holsworth, Raisin River, July 2005


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Simple-rare scenario


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Medium-rare scenario


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Well-done scenario


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Burned-to-a-crisp scenario


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Source geometry• Diffuse sources

- Contamination is spread out over a large area, often at fairly low concentrations.- e.g. Agricultural and silvicultural pollution (nitrates, pesticides, etc); acid rain;

nuclear, industrial or volcanic fallout.- Contamination is often at the soil surface (at least at first), and it is therefore

important to characterize the soils and transport in the unsaturated zone.

North Raisin River, August, 2005

4. Types of sources of contamination 4. Types of sources of contamination


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• Punctual sources- High concentrations and

small volumes- Contaminant migration is

controlled by the local hydrogeology.

- Detailed characterization of the site is required for predictions and for pin-pointing the source

- e.g. Septic fields, Leaky Underground Storage Tanks (LUST), landfill sites, injection wells

North Raisin River, August, 2005


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• Linear sources- The geometry of the source is linear but it can be of large scale.- e.g. roads (de-icing salts, heavy metals, anti-dust salts (CaCl2)), pipelines,

sewer lines.

Michel Robin, North Raisin River, August, 2005

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