When contaminants in groundwater exceed the levels deemed safe for the use of a specific aquifer use the ground-water is considered polluted. There are three major sources of groundwater pollution . These include natural sources, waste disposal activities, and spills, leaks, and nonpoint source activities such as agricultural management practices.
All groundwater naturally contains some dissolved salts or minerals. These salts and minerals may be leached from the soil and from the aquifer materials themselves and can result in water that poses problems for human consumption, is considered polluted, or does not meet the secondary standards for water quality . Natural minerals or salts that may result in polluted ground water include chloride, nitrate, fluoride, iron and sulfate.
There are currently no feasible methods for the large-scale disposal of waste that do not have the potential for serious pollution of the environment , and there are a number of waste-disposal practices the specifically threaten groundwater. These include activities which range from separate sewage treatment systems for individual residences, used by 30% of the population in the United States, to the storage and disposal of industrial wastes. Many of the problems posed by industrial waste arise from the use of surface storage facilities that rely on evaporation for disposal. These facilities are also known as discharge ponds, and there are other types in which waste is treated to standards suitable for discharge to surface water. But in the use of both facilities the potential exists for the movement of contaminants into groundwater. Many of the numerous sanitary landfills in the country are in the same situation. Water moving down and away from these sites into groundwater aquifers carries with it a variety of chemicals leached from the material deposited in the landfills. The liquid that moves out of landfills is called leachate.
Agricultural practices also contribute to groundwater pollution, and there have been increases in nitrate concentrations and low-level concentrations of pesticides. For control of groundwater pollution, one of the most important agricultural practices is the management of nitrogen from all sources--fertilizer, nitrogen fixing plants, and organic waste . Once nitrogen is in the nitrate form it is subject to leaching , so it is important that the amount applied not exceed the crops' ability to use it. At the same time crops need adequate nitrogen to obtain high yields, and a good balance must be maintained. Low-level pesticide contamination occurs in areas where aquifers are sensitive to surface activity, particularly areas of shallow aquifers beneath rapidly permeable soils, and regions of "karst" topography where deep and wide range pollution can occur due to fractures in the bedrock.
Except in cases of deep-well injection waste or substances contained in sanitary landfills, most contaminants move from the land surface to aquifers. The water generally moves through an unsaturated zone, in which biological and chemical processes may act to degrade or change the contaminant. Plant uptake can also act to reduce some of the pollution. Once in the aquifer, how the contaminant moves with the water will depend the solubility of the compound, and the speed of contamination will depend on how fast water moves through the aquifer. Chemical and biological degradation of contaminants can occur in the aquifer, but usually at a slower rate than it does on the surface due to lower temperatures, less available oxygen, and reduced biological activity. In addition aquifer contaminants exist in
lower concentrations, diluted by the large water volume. Most pollution remains relatively localized in aquifers, since movement of the contaminants usually occurs in plumes that have definite boundaries and do not mix with the rest of the water. This does provide an advantage for isolation and treatment.
The types of chemicals that pollute groundwater are as varied as their sources. They range from such simple inorganic materials as nitrate from fertilizers, septic tanks, and feedlots , chloride from high salt, and heavy metals such as chromium from metal plating processes, to very complex organic chemicals used in manufacturing and household cleaners.
One of the main criteria in judging risk is public health. Acute effects from immediate exposure to a concentrated product is often well documented, but little written evidence exists to link physiological effects with long-term chronic exposure. About all that is available are epidemiological data that suggest possible effects, but are by no means conclusive. Environmental effects are even less well understood, but some have suggested that the best way to determine the potential danger is to look at how long a substance persists. Those that remain the longest are most likely to pose long-term risks.
The most efficient way to protect groundwater is to limit activities in recharge areas. For confined aquifers it may be possible to control activities that can result in pollution, but this is extremely difficult for unconfined aquifers which are essentially open systems and subject to effects from any land activity. In areas of potential salt-water intrusion excess pumping can be regulated, and this can also be done where water is being used for irrigation faster than the recharge rate, so that the water is becoming saline. Another important activity for the protection of groundwater is the proper sealing of all wells that are not currently being used.
Classification of aquifers according to their predominant use is another management tool now employed in a number of states. This establishes water-quality goals and standards for each aquifer, and means that aquifers can be regulated according to their major use. This protects the most valuable aquifers, but leaves the problem of predicting future needs. Once an aquifer is contaminated, it is very expensive if not impossible to restore, and this management tool may have serious drawbacks in the future.
In rural areas of the United States, 95% of the population draws their drinking water from the groundwater supply. With a growing population, continued industrialization, and increasing agricultural reliance on the use of chemicals, many believe it is now more important than ever to protect ground-water. Contamination problems have been encountered in every state, but prevention is far more efficient and effective than restoration after damage has been done. Prevention can be achieved through regional planning and enforcement of state and federal regulations.
See also Agricultural pollution; Aquifer restoration; Contaminated soil; Drinking-water supply; Feedlot runoff; Groundwater monitoring; Hazardous waste site remediation; Hazardous waste siting; Heavy metals and heavy metal poisoning; Waste management; Water quality; Water quality standards; Water treatment
[James L. Anderson ]
Freeze, R. A., and J. A. Cherry. Ground Water. Englewood Cliffs, NJ: Prentice-Hall, 1979.
Pye, V. I., R. Patrick, and J. Quarles. Ground Water Contamination in the United States. Philadelphia: University of Pennsylvania Press, 1983.
Hallberg, G. R. "From Hoes to Herbicides: Agriculture and Groundwater Quality." Journal of Soil and Water Conservation (November-December 1986): 358–59.