Groundwater is an important source of drinking water for more than half of the nation's population and nearly all its rural population. In recent years, widespread reports of bacteria, nitrate, synthetic organic chemicals and other pollutants in groundwater have increased public concern about the quality of groundwater. What do we know - and what don't we know - about groundwater quality? What causes groundwater contamination, and to what extent are the nation's groundwater supplies at risk?
How Much Do We Depend on Groundwater?
According to 1985 U.S. figures, groundwater provides an estimated: * 22 percent of all freshwater withdrawals
* 53 percent of drinking water for the total population and 97 percent of drinking water for the rural population * 40 percent of public water supply withdrawals
* 46 percent of domestic and commercial use
* 24 percent of industrial and mining use
* 34 percent of agricultural use (mostly for irrigation) How Susceptible Groundwater to Contamination
About one-fourth of the average 4.2 trillion gallons of precipitation that falls each day on the conterminous United States infiltrates the soil and recharges local aquifers, the sediments and roeks that store and transport groundwater. In general, shallow, permeable water table aquifers are the most susceptible to contamination, but susceptibility of all aquifers to contamination is determined largely by such site-specific characteristics as: * distance from the contamination source to the aquifer and residence time of the water in the unsaturated zone; * presence of clay and organic matter in the unsaturated zone materials; * potential of a particular contaminant to biodegrade and decompose; * amount of precipitation, which affects recharge and the rate at whieh contaminants move downward; * evapotranspiration, which in recharge areas may decrease the amount of water that moves downward to the aquifer. What Causes Groundwater Contamination
Groundwater contamination can occur in many ways and from many sources, both natural- and human-induced. Groundwater commonly contains one or more naturally ocourring chemicals, leached from soil or rocks by percolating water, in concentrations that exceed federal or state drinking water standards or otherwise impair its use. Dissolved Solids and Chloride
One of the most common water quality concerns is the presence of dissolved solids and chloride in concentrations that exceed the recommended maximum limits in federal secondary drinking water standards: 500 mg/L (milligrams per liter or approximately equivalent to parts per million) for dissolved solids and 250 mg/L for chloride. Such concentrations are found at the seaward ends of all coastal aquifers and are quite common in aquifers at depths greater than a few hundred feet below the land surface in many parts of the United States. Iron and Manganese
Although not partieularly toxic, iron and manganese in concentrations greater than the limits for federal secondary drinking water standards (0.3 mg/L for iron and 0.05 mg/L for manganese) can impair the taste of water; stain plumbing fixtures, glassware and laundry; and form encrustrations on well screens, thereby reducing well-pumping efficiency. Nitrate-Nitrogen
Most groundwater not affected by human activity contains less than 10 mg/L nitrate-nitrogen, the maximum concentration allowed by federal primary drinking water standards. Nationwide, nitrate-nitrogen concentrations of less than 0.2 mg/L generally represent natural conditions, whereas values greater than 3 mg/L may indicate the effects of human activities. Although relatively nontoxic, nitrate may be reduced by bacteria to nitrite in the intestines of newbom infants and cause the disease methemoglobinemia. Nitrate also can react with amines in the human body to form N-nitrosamines, careinogenic chemicals known to induce tumors in laboratory animals and thought to be...
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