USGS: Nitrate Concentrations of Groundwater Increasing

One of the first nationwide studies of nitrate trends in groundwater of the United States was recently completed by U.S. Geological Survey (USGS) scientists, as part of the agency's federally funded National Water Quality Assessment (NAWQA) program, according to a Sept. 17 press release.

The scientists collected and analyzed monitoring data from multiple aquifers across the country to characterize near-decadal trends in groundwater nitrate concentrations between 1988 and 2004. Results from the study were published in a companion supplement to the September-October issue of the Journal of Environmental Quality.

Decadal-scale changes of nitrate concentrations were evaluated in groundwater samples from 495 wells in 24 well networks in predominantly agricultural areas. A well network is a set of about 30 randomly-selected wells designed to examine groundwater quality in a particular region. Each well network was sampled once during 1988–1995 and resampled once during 2000–2004.

Findings show statistically significant increases in concentrations of nitrate in 7 of the 24 well networks. Median nitrate concentrations of three of those seven well networks increased above the U.S. Environmental Protection Agenyc maximum contaminant level of 10 parts per million. Concentrations decreased in one network located in the Willamette Valley of Oregon. The study included estimates of the age of the groundwater (that is, time since the water recharged to the aquifer); nitrate concentrations in groundwater increased in response to the increased use of fertilizers since World War II.

"This study highlights the importance of maintaining long-term groundwater monitoring programs in the nation, because sustained monitoring provides critical information on changes of our nation's groundwater quality, and whether pollution prevention programs are effective in protecting this nation's groundwater," said Michael Rupert, a hydrologist with USGS.

The full article is available for no charge for 30 days following the date of this summary.Readers can view the abstract at

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