An analysis of more than 4000 samples of precipitation that were collected weekly from 1978 through 1983 at 19 sites in 13 states showed no clear pattern of changes in overall acidity, according to a recent report by the U.S. Geological Survey (USGS) of the Department of the Interior. The analysis did, however, reveal significant declines in sulfate and nitrate concentrations during that 5-year period.
USGS hydrologists said that sulfate and nitrate, sometimes used as indirect indicators of the acidity of precipitation, showed widespread and substantial downward trends in the eastern United States. (All but three of the 19 stations are located east of the Mississippi River.) At the five stations in Ohio, Pennsylvania, and West Virginia, for example, sulfate concentrations declined 25% to 40% during the 5-year period. Only two of those five stations showed statistically significant trends in pH; these were decreases in acidity comparable to the decreases in sulfate.