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Groundwater Pollution

 A.     Sources and Types

     Groundwater pollution can originate from several sources: industrial, domestic, municipal, agricultural and environmental.  Environmental sources are the naturally occurring substances that are found in earth and can be present in groundwater at concentrations that exceed safe levels for human consumption  [8]. Radon gas is a breakdown product of uranium, a radionuclide that can be present in soil rocks and carried in groundwater.  Households that use water containing radon can have elevated indoor air radon levels, which poses a health risk as a carcinogen[9].    Underground rocks and soils may also contain heavy metals such as arsenic, cadmium, chromium, lead, and selenium. High levels of nitrates and nitrites and fluoride can occur naturally in some areas and can be carried in groundwater.  Nitrates and nitrates are particularly dangerous for infants when consumed in large amounts [9]. 

     Microorganisms can also serve as an environmental source of groundwater contamination.  This includes bacteria, viruses, and parasites that are sometimes found in water [9].  These pollutants can come from wildlife and soils that enter runoff, or water flowing over the land surface, which makes shallow groundwater and wells the most at risk  [9].  A variety of illnesses can occur when water containing these organisms is consumed.

     From the nonenvironmental sources, groundwater can get polluted accidentally when there are spills or leakage from storage tanks, septic systems, hazardous waste sites, and landfills (Figure 3).  These are considered point sources.  Groundwater can also get polluted when there is intentional application of road salts, fertilizers, pesticides, and chemicals (Figure 3).   These are considered non-point sources.  The NRC [10] indentified different classes of pollutants that can come from human activities.  These are separated into organic and inorganic compounds and the frequency of occurrence.

Figure 3:  Sources of groundwater pollution taken from [11]. 

     There are several inorganic compounds that can frequently pollute groundwater. These include metals, such as lead and copper, that come from mining and gasoline additives as well as nonmetals, such as arsenic and selenium, that can come from pesticides and mining [11].  Oxyanions such nitrate, perchlorate, and phosphate also frequently end up in groundwater from fertilizers, paper manufacturing, and disinfectants [11].  Inorganic compounds that can occur but are not as frequently include radionuclides that come from nuclear reactors, weaponry, and medicine [11].  

     Organic compounds can be separated into chlorinated and nonchlorinated.  For the organic compounds that are nonchlorinated, low molecular weight hydrocarbons, such as BTEX (benzene, toluene, ethylbenzene, and xylenes) and alkanes that come from crude oil refined fuels, dystuffs, and solvents, frequently pollute groundwater [11].  On the other hand, high molecular weight hydrocarbons, such as polycyclic aromatic hydrocarbons and nonvolatile aliphatic hydrocarbons are not as common [11]. Oxygenated hydrocarbons that are low in molecular weight, such as alcohols, ketones, esters, ethers, phenols, MTBE (methyl tert-butyl ether) frequently make their way into groundwater from a wide range of products: fuel oxygenates, solvents, paints, pesticides, adhesives, pharmaceuticals, fermentation products, and detergents [11].  Nitroaromatics, such as trinitrotoluene (TNT) and royal Dutch explosive (RDX) that come from explosives, are also frequent groundwater pollutants [11].

     Frequently occurring chlorinated compounds include the highly chlorinated and less chlorinated aliphatics. The highly chlorinated compounds include tetrachloroethene, trichloroethene, and trichlorethane that come from dry cleaning fluids and degreasing solvents [11].  The less chlorinated aliphatics come from biodegradation by products, plastics, solvents, and pesticides and include vinyl chloride, 1,1,1-dichloroethane, and 1,2-dichlorethene [11].  The chlorinated aromatics, highly chlorinated and less chlorinated, are common groundwater pollutants and include polychlorinated biphenyls (PCBs), and polychlorinated dioxins, and chlorinated benzenes that come from solvents and pesticides [11].   

B.     Groundwater Pollutant Mechanisms

     Rain water or surface water can carry the pollutants from the surface to the groundwater or the liquid hazardous substances themselves can soak down through the soil or rock into the groundwater [12].  An area of contaminated groundwater, called a plume, can form if there is a continuous source of contamination entering the groundwater and the contaminants can dissolve. Some hazardous substances do not mix with the groundwater but remain pooled within the soil or bedrock [12]. These are referred to as Nongaseous Phase Liquids (NAPLs) (see Figure 4).

     NAPLs can be lighter than water (LNAPL) or denser than water (DNAPL). LNAPs move through the vadose zone and can float on and depress the water table.  LNAPS consist of aromatic solvents and hydrocarbon fuels mostly from petroleum spills and leaks[13].  DNAPLs include chlorinated hydrocarbons that come from a wide range of industrial activities.  DNAPLs can sink through the water column and penetrate far below the water table.

     DNAPLs can cause more complex problems than LNAPLs.   This is because they can form pools at the bottom of an aquifer that can continue to contaminate the aquifer as they slowly dissolve and are carried away by moving groundwater [14]. Also, DNAPLs can cause residual contamination, which is tiny globs of contaminant that become trapped in the spaces between soil particles as the contaminants flow downward through the aquifer [14].

Figure 4: Schematic of goundwater contaminated by NAPL taken from [14].

     There are several mechanisms that can influence groundwater pollution.  These include advection, diffusion, dispersion, adsorption, biodegradation and chemical reactions [13].  Adsorption, biodegradation, and chemical reactions all retard the movement and alter the concentration of contamination by sorbing onto the soil matrix or conversion of the contaminant to carbon dioxide and water by biological or chemical processes  [13, 15].  Advection, diffusion, and dispersion all promote movement of the contamination [15]. Advection is the movement of contamination with the flow of groundwater.  Dispersion and diffusion cause the contamination to spread; diffusion is the result of concentration gradients and dispersion is the result of aquifer differences in hydraulic conductivity and porosity.

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