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THIS ARTICLE IS A CASE STUDY OF HISTORICAL PROJECTS HYDROSOLUTIONS HAVE WORKED ON.
Contaminant Hydrogeology
Groundwater is the water contained within porous or fractured media below ground. Hydrogeology may be defined as the study of groundwater or water accumulation and flow beneath the ground surface. Contaminant hydrogeology may be defined as the study of the occurrence, impact and management of groundwater contamination.
The quality of groundwater is dependent on the dissolved or phase separated chemistry, biological composition, and also the physical state of the water. Groundwater quality varies naturally, dependent on its recharge water, interaction with surface waters (via seepage recharge or groundwater discharge), the dissolution of chemicals within the rock matrix along it’s flow path, membrane filtration effects, mixing of different water types and other natural chemical or physical processes etc. Groundwater contaminant hydrogeological quality can also be profoundly impacted by anthropogenic activities, including et al:
• Abstraction: e.g. drawdown altering the redox chemistry/ de-saturating overlying layers allowing oxidation of previously reduced minerals/ saline intrusion etc
• Spillages/ leakages of stored chemicals (e.g. underground storage tanks/ pipelines conveying for instance hydrocarbons/ in-organic chemicals
• Recharge operations: infiltration basins from road runoff, seepages from stormwater compensation basins/ aquifer storage and recovery operations etc
• Waste disposal operations: e.g. seepage of leachate from landfills, deliberate deep-(liquid) waste injection
• Heat exchange operations (e.g. reinjection of boiler blow-down water etc)
.A contaminant hydrogeology assessment will need to consider all potential influences which may affect groundwater quality
Contaminant Hydrogeology: Groundwater Contaminants
We will consider the common contaminant groups which are normally considered in contaminant hydrogeology studies. Whether groundwater is contaminated is often dependent on the environmental value or beneficial use of the resource, either at the point of monitoring or at the point of discharge (e.g. wetland/ lake/ surface water body or abstraction bore). Australian Federal and State Government agencies have set specifications regarding current best environmental practices to protect environmental values and these are known as environmental guidelines. Contaminant hydrogeology assessments involve identifying the presence of chemicals at sufficient concentration which may render the groundwater unfit for use for a specific purpose; this may happen naturally (e.g. Totals Dissolved Solids > 1,000mg/L renders water unfit for human drinking, although sheep may tolerate up to 13,000mg/L). For human drinking water, the Australian Drinking Water Guidelines specifies c103 physical and chemical characteristics with health-based or aesthetic guideline values which are maximum permitted concentrations, with additional guideline values for pesticides, bacteria and viruses. The Australian & New Zealand Guidelines for Fresh and Marine Water Quality includes trigger values for toxicants for ecosystem protection, irrigation, stock rearing, aquaculture, recreational water (e.g. swimming), industrial use (e.g. boiler supplies), and also for sediment quality.
In contaminant hydrogeology, groundwater is examined for contaminants which may include the following common species et al:
• Aromatic hydrocarbons (e.g. benzene, toluene, ethylbenzene and xylenes (‘BTEX’ components of petroleum et al), Polycyclic aromatic hydrocarbons (PAHs), e.g. benzo(a)pyrene/ naphthalene
• Oxygenated hydrocarbons (e.g. acetic acid, methanol, phenol, MTBE)
• Chlorinated or halogenated hydrocarbons (e.g. 1,2 DCE, 1,1,2 TCE (degreasers/ solvents)
• Polychlorinated biphenyls (PCBs)
• Other hydrocarbons including alkanes (e.g. paraffin’s) & Total Petroleum Hydrocarbons (TPH) from fuels
• Organochlorine/phosphate (OCP/OPP) pesticides & herbicides (e.g. Atrazine, dieldrin etc.)
• Metals: e.g. Al, As, Cd, Cu, Fe, Pb, Mg, Mn, Ni, Se, Zn etc.)
• Nutrients: e.g. Total nitrogen, nitrate (NO3), nitrite (NO2), total Kjeldahl nitrogen (TKN), ammonia (NH3), Total phosphorous, filterable reactive phosphorous, soluble reactive phosphorous
• Volatile Organic Compounds (VOCs), e.g. PCE, VC.
• Biological contaminants – E coli, coliform etc resulting from septic tanks, raw sewerage overflow et al
• Radioactivity
• Emerging contaminants
Contaminant Hydrogeology: Contaminant Behaviour
In studies of contaminant hydrogeology we observe contaminants behaving differently within groundwater systems based on their physical or chemical nature. For instance hydrocarbons may be hydrophobic, or water ‘hating’, and may be immiscible or insoluble in water, forming separate phases with water (non-aqueous phase liquids or NAPLs). Poorly soluble chemicals may form emulsions, while others may be taken partially into solution as dissolved phases dependent on their solubility. Chemicals which are less dense than water (‘LNAPLs’, e.g. hydrocarbons) tend to float on the water table, forming a distinct layer which represents a continuous source of dissolved phase contaminant due to groundwater underflow). Chemicals denser than water (‘DNAPLs’, e.g. 1,2 DCE) tend to sink through the water column, and may pool at the base of aquifers upon lower permeability layers; these may also form continuous sources of dissolved contamination.
Inorganic chemicals may be taken into solution within groundwater, dependent on their solubility. In studies of contaminant hydrogeology we also observe ‘conservative’ chemicals (e.g. chloride) which do not react with the aquifer environment and may migrate at the same rate as the groundwater flow. Reactive chemicals may be sorbed onto the aquifer matrix materials, or be desorbed when the concentration in the influent groundwater is less than the sorbed concentration, and hence will migrate more slowly (i.e. be ‘retarded’) relative to groundwater flow. Other physico-chemical reactions can also retard (slow) the solute movement relative to the groundwater flow rate including advective dispersion (tortuosity of the flow path causing lateral and transverse spreading), cation exchange between solute and matrix, precipitation out of solution as particulate matter which may be liable to filtration effects, and sorbsion onto ferrous iron (FeII) and organic carbon fractions et al.
Contaminant Hydrogeology: Aquifer Environment
Contaminant hydrogeology behaviour is also affected by the aquifer environment for e.g.;
• Porous media: occurs within granular media such as sands, resulting in dispersion as the solute travels through the media, and the potential for increased sorption onto the high surface area of the material. Capillary pressure within the vadose (unsaturated) zone may lead to the retention of contaminants within soils which may continue to leach to the underlying groundwater.
• Limestone – likely to possess secondary porosity due to the formation of solution voids by dissolution of the rock matrix during mildly acidic groundwater flow.
• Sandstone: – may have both high granular porosity and also flow through fractures of secondary features such as bedding planes.
• Other competent (strong) rocks form fractures during deformation, such as basalt, although it may possess porosity as a result of vesicles or gas pockets formed during cooling.
• Massive hard rocks (such as granite) may also possess fractures which may dominate the groundwater flow mechanism, although again granites may possess granular porosity due to weathering of feldspars etc.
In contaminant hydrogeology studies it is also necessary to check for the presence of fractures which provide high permeability conduits through which groundwater may flow preferentially, often at very high flow rates in comparison with intergrannular flow. Conversely, partially or non-interconnected fractures may provide points of contaminant retention due to high capillary pressures.
Once contaminants are present in groundwater, they may persist for long periods due to favourable conditions for their preservation, such limited or no evaporation, no photodegradation, little or no oxygen, limited microbial activity due to lack of nutrients/ oxygen or sorbsion onto aquifer materials etc. Conversely, some contaminants are susceptible to natural attenuation processes.
Contaminant Hydrogeology: Hydrocarbons
Hydrocarbons are widely used as fuels (e.g. petroleum, diesel, paraffin, heavy fuel oil, aviation fuels), as lubricants (oils), as chemical intermediaries, and coal tars for e.g. bitumen. Waste oil is generated from mechanical servicing. In studies of contaminant hydrogeology we observe that hydrocarbons typically enter the groundwater environment from leaking storage tanks or pipelines, or spillages. Most hydrocarbons may be biodegraded by naturally occurring bacteria which consume the carbon within the contaminant as an energy source in the presence of oxygen as an electron-acceptor. BTEX compounds within petroleum are highly susceptible to this process, although BTEX constitutes less than 25% of diesel, with the remaining predominant alkanes having little susceptibility and therefore persisting within the aquifer. When oxygen has been consumed, biodegradation processes can still occur using other chemicals present as electron acceptors, such as in order) nitrate (nitrification), manganese reduction (MnIV), ferric iron reduction (FeIII), sulfate reduction (SO4) and methanogenesis (acetate). As contaminant Hydrogeologists we find that Monitored Natural Attenuation (MNA) may be an important remedial management technique in these circumstances.
Contaminant Hydrogeology: Chlorinated Solvents
In contaminant hydrogeology assessments chlorinated solvents (CS) may also be naturally biodegraded primarily as a growth substrate but only under specific redox conditions, as either electron donors (contaminant is oxidised either aerobically or anaerobically) or acceptors (contaminant is reduced via halorespiration (a biological reductive dechlorination process where the CS is consumed by bacteria as a substrate, with hydrogen (present from fermentation of organic compounds) as the electron donor). An example is the reductive dechlorination of PCE to TCE to DCE to VC. Daughter products of biological degradation (e.g. VC, from the parent PCE) are often however toxic to the bacteria, while VC is a known carcinogen to humans (i.e. it is more toxic than the parent). Cometabolism, where bacteria gain carbon & energy from a primary substrate produces enzymes which are also important degradation agents for CS, e.g. cometabolic oxidation of TCE under aerobic conditions, with oxygen as the electron acceptor and methane as the electron donor, with TCE degraded via enzymes present for methane oxidation. However, methane and oxygen are not common in large quantities and hence cometabolic oxidation is not an important degradation process in many CS plumes. Reductive dechlorination may also occur anaerobically via cometabolism, although this is often a slow and incomplete process.
Contaminant Hydrogeology: Organo-Chlorine and Organo-Phosphate Pesticides (OCP/OPP) and Herbicides
In contaminant hydrogeology assessments, examples of OCPs include
• Cyclodienes: e.g. aldrin, chlordane, dieldrin, endrin, heptachlor
• Chlorinated benzenes: e.g. hexachlorobenzene (HCB)
• Cyclohexanes: e.g. hexachlorocyclohexane (HCH)
• Dichlorodiphenylethanes: e.g. DDT/DDD/DDE (i.e. metabolic products of DDT)
• Trichlorophenols (2,4,5 & 2,4,6).
Their use includes fungicides and bactericides. Many have been banned (e.g. DDT etc) due to concerns over their persistence (resistance to degradation) and toxicity. They may enter groundwater from run-off, as leachate from disposal to landfills or incinerators, or from manufacturing sites, which need to be assessed in contaminant hydrogeology studies.
Many are SVOCs and may partition into the soil gas or sorb onto soils. They may bioaccumulate (i.e. accumulate within an organism) in fish and animals, and since they are soluble in fat, may accumulate in foods and entre the food chain in humans via milk, fish, meat, potable (drinking) water and via inhalation of soil gases.
In contaminant hydrogeology assessments, common examples of OPPs include: parathion, malathion, chlorpyrifos, diazinon, dichlorvos, and azinophos methyl. OPPs are esters of phosphoric acid. In addition to their use as herbicides/ pesticides, OPPs are used as solvents, plasticisers, extreme pressure lubricants & as nerve gases. OPPs degrade rapidly (in comparison to OCPs) in the environment via hydrolysis on exposure to sunlight and air in soils, but have a greater acute (i.e. short-term) toxicity (e.g. neurotoxins), and their daughter products may be more toxic than the parent compounds.
Contaminant Hydrogeology: Metals and Non-metallic Elements
In contaminant hydrogeology assessments, metal concentrations and their stable form within groundwater are predominantly controlled by the prevailing redox (Eh) and acidity/alkalinity (pH) conditions. As an example, the stable form of iron in oxidised groundwater (Eh>~1mV) is Fe3+ (ferric) at low (acidic) pH, while at higher (alkaline) conditions across most EH ranges insoluble ferric hydroxide forms and will tend to precipitate out of solution. For mildly oxidising to mildly reducing conditions Fe2+ (ferrous) iron is stable at lower pHs, while Iron sulfide (pyrite) FeS2 and FeS are stable at neutral to alkaline conditions. In reduced groundwater conditions (Eh<=-0.6mV) the stable form is Fe0. Therefore a change in Eh/pH conditions, such as the introduction of oxygenated groundwater to reduced water in the presence of pyrite, or where ferric hydroxide comes into contact with reducing substances, will cause iron be taken into solution (i.e. dissolved), or vice versa cause iron to precipitate out of solution.
An example of a non-metallic element where Eh/Ph conditions also determine the stable form in contaminant hydrogeology is arsenic. The stable forms in groundwater are arsenate (As5+) in oxygenated (Eh>0mV) water, generally strongly absorbed and Arsenite (As3+) in reduced water (Eh<0mV). Dissolved arsenic can be absorbed by ferric hydroxides which are stable over a wide range of Eh/pH conditions and hence arsenic mobility is limited, although reducing conditions tend to increase mobility due to ferric hydroxide being taken into solution, with maximum mobility in mildly reducing conditions in the absence of hydrogen sulfide. Similar controls exist for common metal contaminants, including Al, Cr, Cu, Mn, and Pb
Contaminant Hydrogeology: Volatile Organic Compounds (VOCs)
In contaminant hydrogeology assessments, VOCs do not have any single definition, but can include any organic (carbon containing) compound that is volatile or tends to vaporise under normal conditions, or has a boiling point (BP) between 50-250oC (Health Canada) or have a BP<250oC and can do damage to visual or audible senses (European Union), or any carbon compound that undergoes atmospheric photochemical reactions (USEPA) or has a Henry’s Law Constant >= 1×10-5 atm-m3/mol (USEPA, OSWER 2002). They are commonly used as fuels, solvents, scents, chemical precursors or markers, propellants, refrigerants, drugs, pesticides. Examples which we look for in contaminant hydrogeology studies include:
• Trihalomethanes (THMs); e.g. chloroform (solvent/refrigerant) & brominated/chlorinated methane (formed as by-products of water disinfection using chlorine in the presence of naturally occurring organic material
• Monocyclic Aromatic Hydrocarbons (MAHs); e.g. BTEX, in petroleum and c25% in diesel, also widely used as solvents
• Volatile Chlorinated Hydrocarbons (VCH); e.g.: 11DCE, chloroform, 111TCA, TCE, carbon tetrachloride (or tetrachloromethane, dry cleaning fluid), PCE (i.e. typically industrial solvents), 135 Trimethylbenzene (diesel component), vinyl chloride (VC; industrial chemical in PVC, plastics, rubber et al)
• Methyl Tert Butyl Ether (MTBE); formerly used as a petroleum additive (‘anti-knock’)
In contaminant hydrogeology assessments VOCs will exist in multiple phases, dissolved within groundwater, in the vapour phase as interstitial soil gases, and be adsorbed within the soil. The accumulation of soil vapours in the vadose (unsaturated) zone beneath buildings may lead to potentially toxic or explosive concentrations in conditions of poor ventilation. Remedial measures often seek to volatilise dissolved VOCs and then extract the soil gas under vacuum. Dissolved phase contaminants may sorb onto the surfaces of aquifer materials, particularly those with high surface areas such as fine sands/silts and clays, and tend to have an affinity for organic carbon and also ferrous iron (FeII). This tends to limit the mobility of the contaminant in comparison with groundwater, which behaviour may be expressed partially in the octanol/water partitioning coefficient (Kow) of the compound; contaminants with very high Kow, e.g. the PAH dibenz(a,h)anthracene (1,668,800, an SVOC) will be practically immobile, while low Kow compounds, e.g. benzene (97) will be relatively mobile. VOCs will be variably susceptible to other degradation process, including photodegradation and hydrolysis (the splitting of water molecules into hydrogen & hydroxide ions).
Contaminant Hydrogeology: Semi-Volatile Compounds (SVOCs)
In contaminant hydrogeology studies SVOCs are defined as a subset of VOCs with a boiling point (BP)>100oC (USEPA), or may simply be defined as having a relatively lower volatility than VOCs. Examples which we look for in contaminant hydrogeology assessments include:
• Carbamates: insecticides (e.g. propoxur (‘Baygon’), bendiocarb, methomyl)
• Chloral hydrate: medicine, manufacture of DDT
• Common herbicides (e.g. aldrin, atrazine, azinophos-methyl, BHC, carbaryl, chlordane, chlorpyrifos, DDT/E/D, endosulfan, endrin, fenitrothion, methoxychlor, permethrin, simazine et al)
• Chlorinated benzenes & chlorinated naphthalenes
• Chloroethanes (e.g. ethyl chloride, chemical manufacture, refrigerant, insecticide)
• Methyl ethyl ketone (MEK); industrial solvent for lacquers, adhesives, inks, typically present in industrial wastewater
• Perchloric acid (PCA): explosive manufacture
• Polychlorinated biphenyls (PCBs); e.g. arachlor 1242 et al, formerly used in transformer fluids, highly toxic & persistent (i.e. resistant to degradation) in the environment
• Phthalates: esters of phthalic acid, e.g. di-2 ethyl P (DHEP), diisodecyl P (DIDP), diisonyl P (DINP) & Benylbutyl P (BBP)et al, used as plasticizers for PVC, also as stabilisers, lubricants, emulsifying agents et al. They are reported endocrine disruptors, and may be linked to obesity in humans, although they are non-human carcinogens They undergo photo & bio-degradation in air and also anaerobically
• Polycyclic Aromatic Hydrocarbons (PAHs), e.g. anthracene, benzo(a)pyrene, fluoranthene, naphthalene, phenanthrene et al. These occur naturally in oils/coal/tar, but are also produced during incomplete fuel combustion. They may be present in solution in groundwater, absorbed in soils, and also present in soil gases/vapours. They are persistent (resistant to degradation) but often retarded or immobile, & may be retained within soils. They are one of the most toxic components of fuels/ oils at low concentrations, with some species known carcinogens, mutagens (genetic mutations) and teratogens (foetus malformation). .
• Synthetic pyrethroids: insecticides, e.g. bifenthrin, permethrin et al. Persistent and toxic to fish.
In contaminant hydrogeology studies, SVOCs will exhibit similar behaviour within groundwater to VOCs, although being less volatile will have a lower tendency to partition into soil vapours.
Contaminant Hydrogeology: Nutrients
Nutrients analysed in groundwater in contaminant hydrogeology assessments include:
• Nitrogen: Total nitrogen, nitrate (NO3), nitrite (NO2), total Kjeldahl nitrogen (TKN), ammonia (NH3)
• Phosphorous: Total phosphorous, filterable reactive phosphorous (FRP), soluble reactive phosphorous (SRP)
Nitrogen is present naturally in groundwater due to atmospheric exchange with recharge water, biological processed involving bacteria, algae and nitrogen fixing plants (legumes); nitrogen oxide (NOX) is present in the atmosphere from the burning of coal and petrol. Its presence in elevated concentrations in groundwater is typically from fertiliser applications in agriculture, via nitrification into NO3 and NO2. Nitrite, ammonia and Kjeldahl nitrogen (associated with organic nitrogen/ blood/ animal products etc) are unstable in aerated groundwater and are normally considered to indicate pollution associated with e.g. wastewater effluent, septic tanks, animal wastes etc. Typical background (i.e. unpolluted) concentrations of NO3 in streams and groundwater would be around <5-10mg/L. The consumption of excess nitrate in drinking water is associated with methaemoglobin (‘blue-baby’) in infants, with the ADWG set at 50mg/L as NO3 for children< 3-months old; older children and adults may tolerate up to 100mg/L as NO3.
In contaminant hydrogeology assessments phosphorous may be present naturally in groundwater through the dissolution of phosphorous containing minerals (e.g. apatite, fluorite). Anthropogenic sources include run-off from mining of phosphorate deposits and calcium phosphate fertiliser applications in agriculture, and it’s used in detergents. It is a common within sewage as an animal metabolite. It is responsible for causing algal blooms in rivers and the eutrophication (oxygen depletion). It is present in both dissolved and particulate form in groundwater, with its stable form controlled by pH; in acidic waters as phosphoric acid, and in alkaline waters as orthophosphate (PO43-), with the concentration controlled by adsorption and co-precipitation. Phosphorous is a strong complexing agent for some metals which are then retained in solution. Typical background (i.e. unpolluted) concentrations of PO4 in streams and groundwater would be around <1mg/L.
Contaminant Hydrogeology: Biological Contaminants in Groundwater
In contaminant hydrogeology studies, human and animal wastes enter groundwater through breakdowns in sewerage and animal waste disposal and should be considered in a contaminant hydrogeology assessment. They may be present in domestic water sourced as raw water from bores (not recommended as a source of potable water by DoW), or from improper or ineffective treatment of raw water sources prior to entering supply. Bacterial infections in humans from Escherichia (or E.) Coli and coliform bacteria usually require very high numbers of organisms to be present in the system. Bacteria can be largely removed by water treatment and disinfection, usually via chlorination or less frequently ozonation. The ADWG for E. Coli and Thermotolerant bacteria is zero. Within the groundwater environment, bacteria usually only survive for <=c50-days, and this has lead to the concept of ‘sanitary zones’ around wellheads, equivalent to the 50-day groundwater time of travel distance, within which landuses which may generate bacterial wastes, such as animal husbandry are not permitted.
Contaminant Hydrogeology: Radioactivity
In contaminant hydrogeology studies, radioactivity (the release or energy and energetic particles) may be naturally occurring in groundwater through the dissolution of radioactive minerals, including Cobalt-60, Iodine-131, Potassium-40, Radium, Radon, Rubidium-87, Strontium-89 , 90, Thorium-232, Uranium-238 (main source of groundwater radioactivity), and U235 (actinium series). Decay products go via several daughter products with various half-lives (T1/2, the time taken to decay to half of the previous concentration) of days to millions of years to e.g. stable lead. Uranium is soluble in groundwater as the uranyl ion UO22+ at high (alkaline) conditions, its stable form and concentration is dependent on the Eh/pH conditions similar to metals. Radon 222, with T1/2 of 3.8 days is a radioactive gas produced from the decay of radium, which is produced naturally in granitic terrains and may be present is he dissolved phase in groundwater and also within soil gases, which may accumulate in poorly ventilated buildings and may constitute a significant human health hazard.
In contaminant hydrogeology assessments, specific radionuclides may serve useful purposes. Tritium is naturally occurring in small quantities; however it was produced in large quantities by atmospheric nuclear explosions. It has a T1/2 of 12.3 years, which makes it ideal as a ‘tracer’ for the movement of recharge water through the vadose zone to groundwater. Carbon-14 is a radioactive isotope produced by cosmic rays with a T1/2 of 5,730 years, which also allows its use in groundwater recharge studies (e.g. BHPB PROJECT). Other ‘dating’ radionuclides in groundwater studies include Chlorine-36, with T1/2 of 308,000 years.
Contaminant Hydrogeology: Emerging Contaminants
Recent USGS studies in 2009 identified the following compounds associated with pharmaceuticals, personal care products & human & animal wastes at low concentrations in a variety of groundwater bores & streams which make them emerging contaminants in the field of contaminant hydrogeology:
• Insecticides: e.g. N,N-diethyltoluamide (insect repellent)
• Industrial chemicals: e.g. 1,4-dioxane (industrial solvent stabilizer), phthalates (plasticizers)
• Fuel additives: e.g. MTBE & TBA (oxygenates)
• Disinfection by-products: e.g. N-Nitrosodimethylamine (NDMA), also rocket propellant, solvent, rubber accelerator
• Plastics manufacture: e.g. bisphenol A (plastic- and epoxy-manufacturing ingredient)
• Fire retardants: e.g. polybrominated diphenyl ethers (PBDEs), e.g. dibromophenyl ether, and tri(2-chloroethyl) phosphate
• Pharmaceuticals: (antibiotics/ drugs) e.g. sulfamethoxazole (veterinary and human antibiotic), carbamazepine
• Personal care products: e.g. (polycyclic musks),
• pesticides/herbicides: e.g. 1,2,3-trichloropropane
• Detergents: e.g. 4-octylphenol monoethoxylate (detergent metabolite).
• Metabolites (produced by living organisms): e.g. cotinine (from nicotine), and 1,7-dimethylxanthine (from caffeine)
The US Department of Defence MERIT (Materials of Evolving Regulatory Interest Team) recognises the following emerging contaminants relevant to contaminant hydrogeology:
• ‘Watch list’: NDMA, 1,4-Dioxane (solvent, textiles, dye, detergent, fumigant) 1,2,3-TCP (trichloropropane, a persistent mobile contaminant in groundwater, used as a chemical intermediate, solvent, degreaser), nanomaterials, dintrotoluene (DNT), used in organic synthesis, dyes, explosives, and PBDEs
• ‘Action list’: perchlorate (rocket propellant), RDX (cyclonite, explosive), and TCE (degreasing solvent)
Other emerging contaminants may include human hormones within raw and treated water used for potable purposes.
Contaminant Hydrogeology: Remediation
Groundwater remediation in contaminant hydrogeology assessments is an enormous topic in its own right. However, the mere presence of potentially hazardous ‘contaminants’ in groundwater may not pose an unacceptable risk where there is no complete linkage or pathway (e.g. via migration, abstraction, volatilisation (to soil gases) or discharge to an exposed population, which may include human consumption, recreational use, other usage (e.g. irrigation, industrial use, animal stock) or to the ecological environment via discharge to water bodies (e.g. wetlands, rivers, lakes or the marine environment). If a risk assessment process determines that there is an unacceptable risk, then preventative management will be required.
In contaminant hydrogeology studies, remediation may be passive (e.g. monitored natural attenuation) or active (e.g. ‘pump and treat’, vacuum extraction et al), in-situ (e.g. air sparging, permeable reactive barriers et al). The appropriate remedial strategy will depend on the contaminants present, the risk that they represent, the population to be protected, the extent of protection required, the hydrogeological environment and its sensitivity, and the acceptable timescale for clean-up. Our current series of newsletters are reviewing appropriate remedial strategies for a variety of contaminants and applicable remedial technologies.
Sources:
Fetter, C.W. Contaminant Hydrogeology
Groundwater Association of California
Hawley’s Condensed Chemical Dictionary
Hem, J.D. Study and Interpretation of the Chemical Characteristics of Natural Water.
Lloyd, J.W. and Heathcote, J.A. Natural inorganic hydrochemistry in relation to groundwater
National Health and Medical Research Council, Natural Resource Management Ministerial Council. Australian Drinking Water Guidelines (ADWG)
Shineldecker, C.L. Handbook of environmental contaminants: a guide for site assessment
United States Geological Survey
Wiedemeier, T.H. ed. Natural attenuation of fuels and chlorinated solvents in the subsurface.
Stuart Jeffries and Lekha Siraz, Hydrosolutions Pty Ltd.
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