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USGS Releases Report Showing Decadal-Scale Changes in Groundwater Quality

The news is good…and not so good. In the good news category, according to the US Geological Survey (USGS) there is no change in concentrations of chloride, dissolved solids, or nitrate in groundwater for more than 50 percent of well networks sampled in an analysis released April 30th. The less good news is although concentrations of these three constituents generally meet their respective EPA drinking water standards or guidelines, the proportion of samples nationally exceeding the limits for nitrate and dissolved solids increased significantly over the 10 year sampling period. For those networks that did have a change, seven times more networks saw increases as opposed to decreases.

USGS’ report is entitled, “Methods for Evaluating Temporal Groundwater Quality Data and Results of Decadal-Scale Changes in Chloride, Dissolved Solids, and Nitrate Concentrations in Groundwater in the United States, 1988–2010.” The analysis was done by the USGS National Water Quality Assessment Program (NAWQA) to determine if concentrations of these constituents have increased or decreased significantly from the 1990’s to the early 2000’s nationwide. The analysis consists of samples from 1,236 wells in 56 well networks, representing major aquifers and urban and agricultural land-use areas. Samples for chloride, dissolved solids, and nitrate collected from 1988-2000 were compared to corresponding samples taken from the same well between 2001 and 2010. The 22 principal aquifers represented by these 56 well networks account for nearly 80 percent of the estimated withdrawals of groundwater used for drinking-water supply in the nation.

Although chloride, dissolved solids, and nitrate concentrations were typically less than the drinking-water standards and guidelines, the proportion of samples exceeding the EPA’s Secondary Maximum Contaminant Level (MCL) for dissolved solids (500 milligrams per liter) increased significantly between the first and second full-network sampling events when evaluating all networks combined at the national level. Also, for all networks combined, the proportion of samples exceeding the MCL of 10 mg/L as N for nitrate increased significantly. One network in the Delmarva Peninsula had a significant increase in the proportion of samples exceeding the MCL for nitrate.

Other important findings include:

  • The largest increases in chloride concentrations were in urban areas in the Northeastern and Upper Midwestern United States, including suburban Boston, Chicago, Detroit and Milwaukee.
  • Dissolved solids concentrations increased throughout the nation, including areas of Florida, Illinois, and the Rio Grande region.
  • The largest increases in nitrate concentrations were in key agricultural areas, including the Great Plains, areas east of Lake Michigan, and in California.
  • The magnitudes of increases in concentrations in deeper groundwater used as a source of drinking-water supply were generally less than in shallow groundwater. However, the proportions of networks with increases for both deep and shallow groundwater were similar.

This report, as well as a series of interactive maps showing long-term groundwater trends, can be found online.