Septic Systems. Septic systems are the largest source by volume of waste discharged to the land. These systems are sources of bacteria, viruses, nitrate, phosphorus, chloride and organic substances, including organic solvents such as trichloroethylene that are sold commercially to "clean" the systems.

In 1980, about 22 million domestic disposal systems were in operation, and about one-half million new systems are installed each year. It is estimated that from one-third to one-half of existing systems could be operating improperly because of poor location, design, construction or maintenance practices. Even when operating properly, systems can be spaced so densely that their discharge exceeds the capacity of the local soil to assimilate the pollutant loads. Because the 10-to 15-year design life of many septic systems built during the 1960s and 1970s is now exceeded, groundwater contamination caused by septic system failure probably will increase in the future.

Landfills. About 150 million tons of municipal solid waste and 240 million tons of industrial solid waste are deposited in 16,400 landfills each year. Some hazardous waste material may be deposited in municipal landfills and underlying groundwater may become contaminated. Wastes deposited at industrial landfills include a large assortment of trace metals, acids, volatile organic compounds and pesticides, which may cause significant local contamination.

Surface impoundments. Surface impoundments are used to store, treat or dispose of oil and gas brines, acidic mine wastes, industrial wastes (mainly liquids, animal wastes, municipal treatment plant sludges and cooling water. For the most part, these impoundments contain non-hazardous wastes: however, hazardous wastes are known to be treated, stored and disposed of by 400 facilities involving about 3,200 impoundments. Some of these impoundments have significant potential for contaminating groundwater.

Injection Wells. ln some parts of the country, injection wells dispose of liquid wastes under-ground. Of particular concern is the widespread use of drainage wells to dispose of urban stormwater runoff and irrigation drainage. Contaminants associated with drainage wells include suspended sediments; dissolved solids; bacteria; sodium; chloride; nitrate; phosphate; lead; and organic compounds, including pesticides.

Land Application of Wastes. In many places, solid and liquid wastes are placed or sprayed on the land, commonly after treatment and stabilization. The U.S. Environmental Protection Agency (EPA) has estimated that more than 7 million dry tons of sludge from at least 2,463 publicly owned waste treatment plants are applied to about 11,900 parcels of land each year. Contamination can occur from improper land-disposal techniques.

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