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Ground Water Contamination

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Ground Water Contamination Ground Water Contamination round water contamination is nearly ical or chemical properties, do not always follow GG always the result of human activity. In ground water flow.) It is possible to predict, to areas where population density is high and human some degree, the transport within an aquifer of use of the land is intensive, ground water is espe- those substances that move along with ground cially vulnerable. Virtually any activity whereby water flow. For example, both water and certain chemicals or wastes may be released to the envi- contaminants flow in the direction of the topogra- ronment, either intentionally or accidentally, has phy from recharge areas to discharge areas. Soils the potential to pollute ground water. When that are porous and permeable tend to transmit ground water becomes contaminated, it is difficult water and certain types of contaminants with rela- and expensive to clean up. tive ease to an aquifer below. To begin to address pollution prevention or reme- Just as ground water generally moves slowly, so diation, we must understand how surface waters do contaminants in ground water. Because of this and ground waters interrelate. Ground water and slow movement, contaminants tend to remain surface water are interconnected and can be fully concentrated in the form of a plume (see Figure 1) understood and intelligently managed only when that flows along the same path as the ground that fact is acknowledged. If there is a water sup- water. The size and speed of the plume depend on ply well near a source of contamination, that well the amount and type of contaminant, its solubility runs the risk of becoming contaminated. If there is and density, and the velocity of the surrounding a nearby river or stream, that water body may ground water. also become polluted by the ground water. Figure 1 CONTAMINANT PLUME HOW DOES GROUND WATER BECOME CONTAMINATED? Depending on its physical, chemical, and biological prop- erties, a contaminant that has been released Water Table into the environment may move within an aquifer in the same manner that ground Direction of Ground Water Flow water moves. (Some contaminants, because of their phys- C•1• ➤ Getting Up to Speed: ground water contamination Ground water and contaminants can move rapidly contaminants that reach ground water directly, through fractures in rocks. Fractured rock pre- without passing through the unsaturated zone, sents a unique problem in locating and controlling can become less concentrated by dilution (mixing) contaminants because the fractures are generally with the ground water. However, because ground randomly spaced and do not follow the contours water usually moves slowly, contaminants general- of the land surface or the hydraulic gradient. ly undergo less dilution than when in surface Contaminants can also move into the ground water. water system through macropores—root systems, animal burrows, abandoned wells, and other sys- SOURCES OF GROUND WATER tems of holes and cracks that supply pathways for CONTAMINATION contaminants. Ground water can become contaminated from In areas surrounding pumping wells, the potential natural sources or numerous types of human for contamination increases because water from activities. (See Tables 1 and 2 and Figure 1.) the zone of contribution, a land area larger than Residential, municipal, commercial, industrial, the original recharge area, is drawn into the well and agricultural activities can all affect ground and the surrounding aquifer. Some drinking water water quality. Contaminants may reach ground wells actually draw water from nearby streams, water from activities on the land surface, such as lakes, or rivers. Contaminants present in these releases or spills from stored industrial wastes; surface waters can contribute contamination to from sources below the land surface but above the the ground water system. Some wells rely on arti- water table, such as septic systems or leaking ficial recharge to increase the amount of water underground petroleum storage systems; from infiltrating an aquifer, often using water from structures beneath the water table, such as wells; storm runoff, irrigation, industrial processes, or or from contaminated recharge water. treated sewage. In several cases, this practice has resulted in increased concentrations of nitrates, ■ Natural Sources metals, microbes, or synthetic chemicals in the Some substances found naturally in rocks or soils, water. such as iron, manganese, arsenic, chlorides, fluo- rides, sulfates, or radionuclides, can become dis- Under certain conditions, pumping can also cause solved in ground water. Other naturally occurring the ground water (and associated contaminants) substances, such as decaying organic matter, can from another aquifer to enter the one being move in ground water as particles. Whether any pumped. This phenomenon is called interaquifer of these substances appears in ground water leakage. Thus, properly identifying and protecting depends on local conditions. Some substances may the areas affected by well pumping is important to pose a health threat if consumed in excessive maintain ground water quality. quantities; others may produce an undesirable odor, taste, or color. Ground water that contains Generally, the greater the distance between a unacceptable concentrations of these substances is source of contamination and a ground water not used for drinking water or other domestic source, the more likely that natural processes will water uses unless it is treated to remove these con- reduce the impacts of contamination. Processes taminants. such as oxidation, biological degradation (which sometimes renders contaminants less toxic), and ■ Septic Systems adsorption (binding of materials to soil particles) One of the main causes of ground water contami- may take place in the soil layers of the unsaturat- nation in the United States is the effluent (out- ed zone and reduce the concentration of a con- flow) from septic tanks, cesspools, and privies. taminant before it reaches ground water. Even C•2• ➤ Getting Up to Speed: ground water contamination Table 1 TYPICAL SOURCES OF POTENTIAL GROUND WATER CONTAMINATION BY LAND USE CATEGORY Approximately one-fourth of all homes in the thetic organic chemicals (such as 1,1,1- United States rely on septic systems to dispose of trichloroethane or methylene chloride). These their human wastes. Although each individual sys- cleaners can contaminate water supply wells and tem releases a relatively small amount of waste interfere with natural decomposition processes in into the ground, the large number and widespread septic systems. use of these systems makes them a serious conta- mination source. Septic systems that are improper- Most, if not all, state and local regulations require ly sited, designed, constructed, or maintained can specific separation distances between septic sys- contaminate ground water with bacteria, viruses, tems and drinking water wells. In addition, com- nitrates, detergents, oils, and chemicals. Along puter models have been developed to calculate with these contaminants are the commercially suitable distances and densities. available septic system cleaners containing syn- C•3• ➤ Getting Up to Speed: ground water contamination ■ Improper Disposal of Hazardous Waste Improper chemical storage, sloppy materials han- Hazardous waste should always be disposed of dling, and poor-quality containers can be major properly, that is to say, by a licensed hazardous threats to ground water. Tanker trucks and train waste handler or through municipal hazardous cars pose another chemical storage hazard. Each waste collection days. Many chemicals should not year, approximately 16,000 chemical spills occur be disposed of in household septic systems, from trucks, trains, and storage tanks, often when including oils (e.g., cooking, motor), lawn and materials are being transferred. At the site of an garden chemicals, paints and paint thinners, disin- accidental spill, the chemicals are often diluted fectants, medicines, photographic chemicals, and with water and then washed into the soil, increas- swimming pool chemicals. Similarly, many sub- ing the possibility of ground water contamination. stances used in industrial processes should not be disposed of in drains at the workplace because ■ Landfills they could contaminate a drinking water source. Solid waste is disposed of in thousands of munici- Companies should train employees in the proper pal and industrial landfills throughout the coun- use and disposal of all chemicals used on site. The try. Chemicals that should be disposed of in haz- many different types and the large quantities of ardous waste landfills sometimes end up in munic- chemicals used at industrial locations make proper ipal landfills. In addition, the disposal of many disposal of wastes especially important for ground household wastes is not regulated. water protection. Once in the landfill, chemicals can leach into the ■ Releases and Spills from Stored ground water by means of precipitation and sur- Chemicals and Petroleum Products face runoff. New landfills are required to have Underground and aboveground storage tanks are clay or synthetic liners and leachate (liquid from a commonly used to store petroleum products and landfill containing contaminants) collection sys- other chemical substances. For example, many tems to protect ground water. Most older land- homes have underground heating oil tanks. Many fills, however, do not have these safeguards. Older businesses and municipal
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