USGS Home
Contact USGS
Search USGS

 Go to:      NAWQA Home     National-Synthesis Assessments

Nutrients National Synthesis Project

Nutrients in Ground Waters of the Conterminous United States, 1992-1995

By Bernard T. Nolan and Jeffrey D. Stoner

Abstract
Results of a national water quality assessment indicate that nitrate is detected in 71 percent of ground-water samples, more than 13 times as often as ammonia, nitrite, organic nitrogen, and orthophosphate, based on a common detection threshold of 0.2 mg/L. Shallow ground water (typically 5 m deep or less) beneath agricultural land has the highest median nitrate concentration (3.4 mg/L), followed by shallow ground water beneath urban land (1.6 mg/L) and deeper ground water in major aquifers (0.48 mg/L). Nitrate exceeds the maximum contaminant level, 10 mg/L as nitrogen, in more than 15 percent of ground-water samples from four of 33 major aquifers commonly used as a source of drinking water. Nitrate concentration in ground water is variable and depends on interactions among several factors, including nitrogen loading, soil type, aquifer permeability, recharge rate, and climate. For a given nitrogen loading, factors that generally increase nitrate concentration in ground water include well-drained soils, fractured bedrock, and irrigation. Factors that mitigate nitrate contamination of ground water include poorly drained soils, greater depth to ground water, artificial drainage systems, intervening layers of unfractured bedrock, a low rate of ground-water recharge, and anaerobic conditions in aquifers.

• Report (0.4 mb PDF)