Groundwater Contamination. Advanced Hydrogeology GW Contamination H. Mohammadzadeh and MJL Robin. Contaminant hydrogeology

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1 مرکز تحقیقات آببهای زیرزمینی (متا ب) Contamination 1 III Contaminant hydrogeology Raisin River, North Branch, August 2005 Suggested reading: Freeze and Cherry, 1979., Chapitre 2 & 9 2

2 Types of problems in contaminant hydro 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. Risk assessment - Predictions - Sensitivity analysis - Political considerations - Moral considerations - Other consideration$ 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 3 Site assessment process Identify Site Define Objectives Develop / Refine Conceptual Model Gather and Assemble Available Information Identify and Prioritize Data Gaps Modeling Sensitivity Analysis Collect Data from Site Report, recommend Select Methods Implement Methods Interpret Data 4

3 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 Biological Raisin River, August Biological 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. 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

4 Radioactive 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. 7 Source: Bedient et al.,

5 Inorganic 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. 9 Nutrients: Main pollutants are products containing nitrogen and phosphorus. Nitrogen: Nitrate (NO 3 - ) - very mobile; Ammonium ion (NH 4+ ) 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. 10

6 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. 11 Source: Freeze and Cherry,

7 Organic 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; 13 Solubility, S b (mol/l), can be obtained from the octanol-water partition coefficient: X o Kow, log Kow log Sb, X X are mole fractions in octanol or water 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 m 3 )!!! 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 w 14

8 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 15 LNAPL Contamination Source: Domenico and Schwartz 16

9 DNAPL Contamination Source: Domenico and Schwartz 17 18

10 19 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 20

11 Rocks Porous Fractured (fracture distribution) Fractured-porous Other: Permafrost areas Karstic Formations Types of flow systems Saturated - Aquifer vs aquiclude vs aquitard - Artesian vs non-artesian - confined vs unconfined (phreatic) Density controlled - Instabilities caused by a density contrast - Stratification caused by a density contrast - Saltwater intrusions in coastal aquifers 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 22

12 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 nonlinear Julie Holsworth, Raisin River, July Simple-rare scenario 24

13 Medium-rare scenario 25 Well-done scenario 26

14 Burned-to-a-crisp scenario Types of sources of contamination 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,

15 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 pinpointing the source - e.g. Septic fields, Leaky Underground Storage Tanks (LUST), landfill sites, injection wells North Raisin River, August, 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 (CaCl 2 )), pipelines, sewer lines. Michel Robin, North Raisin River, August,