As the U.S. population and related development rises, so does the impurities found in drinking water that comes from the ground.

A new study by scientists at the U.S. Geological Survey (USGS) has found an increase in nitrate, the most common chemical contaminant in the world's ground water, including in aquifers used for drinking-water supply.

Nitrate in U.S. drinking water is regulated by the Environmental Protection Agency because of concerns related to infant health and possible cancer risks. Use of man-made synthetic fertilizers has steadily increased since World War II, raising the potential for increased nitrate contamination of the nation's ground water, despite efforts in recent decades to improve land-management practices. Monitoring nitrate trends in ground water through time is important in determining how quickly ground-water systems respond to changes in chemical use and best management practices.

For the study, monitoring data collected by teams across the country in multiple aquifers were analyzed to characterize near-decadal trends in nitrate concentrations in ground water 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.

Findings show statistically significant increases in concentrations of nitrate in seven of the 24 well networks tested. Median nitrate concentrations of three of those seven well networks increased above the US Environmental Protection Agency 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 ground water (time since the water recharged to the aquifer); nitrate concentrations in ground water increased in response to the increased use of fertilizers since World War II.

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

The USGS implemented the program in 1991 to support national, regional, state, and local information needs and decisions related to water-quality management and policy. The program is designed to answer: What is the condition of our Nation's streams and ground water, and how are conditions changing over time? In the second decade of the Program (20012012), a major focus is on regional assessments of water-quality conditions and trends at sites that have been consistently monitored for more than a decade.