Nitrate in ground water is derived from both natural and anthropogenic sources. Decomposition of naturally-occurring organic material releases nitrogen, but this is generally a small contribution to nitrate concentrations in groundwater. Sources of anthropogenic nitrate include atmospheric deposition, fertilizers (such as manure, ammonium sulfate, urea), septic tank effluent, sewage sludge, and animal wastes (from feedlots, dairies, and poultry farms). Agriculture is the most extensive source of nitrate in groundwater. The availability of nitrogen sources and regional environmental factors are associated with nitrate concentrations in groundwater in the United States. High nitrate concentrations are often found in the unconfined (surficial) aquifers; they are rarely found in the deeper confined aquifers.
High nitrate concentrations in drinking water may cause methemoglobinemia ("blue baby" syndrome) in infants. This condition is characterized by a reduced ability of the blood to transport oxygen, and can lead to severe oxygen deficiency and even death. Other adverse health effects have been associated with nitrate ingestion from drinking water; however, data from laboratory studies have been inadequate to determine if nitrate can increase the risk of cancer in humans. Nitrogen is also the limiting nutrient in many aquatic environments, and excess nitrogen in surface waters can lead to algal growth and eutrophication. For these reasons, the occurrence of nitrate and other nitrogen species has been studied extensively at local, state, and federal levels. The U.S. EPA has established an Maximum Contaminant Level of 10 mg/L for nitrate-plus-nitrite (as nitrogen) in public water supplies.