Long-term trends in streamwater and precipitation chemistry at five headwater basins in the northeastern United States

Stream water data from five headwater basins in the northeastern United Sta tes covering water years 1968-1996 and precipitation data from eight nearby precipitation monitoring sites covering water years 1984-1996 were analyze d for temporal trends in chemistry using the nonparametric seasonal Kendall test. Concentrations of SO4 declined at three of five streams during 1968- 1996 (p < 0.1), and all of the streams exhibited downward trends in SO4 ove r the second half of the period (1984-1996). Concentrations of SO4 in preci pitation declined at seven of eight sites from 1984 to 1996, and the magnit udes of the declines (-0.7 to -2.0 mu eq L-1 yr(-1)) generally were similar to those of stream water SO4. These results indicate that changes in preci pitation SO4 were of sufficient magnitude to account for changes in stream water SO4. Concentrations of Ca + Mg declined at three of five streams and five of eight precipitation sites from 1984 to 1996. Precipitation acidity decreased at five of eight sites during the same period, but alkalinity inc reased in only one stream. In most cages the decreases in stream water SO4 were similar in magnitude to declines in stream water Ca + Mg, which is con sistent with the theory of leaching by mobile acid anions in soils. In prec ipitation the magnitudes of SO4 declines were similar to those of hydrogen, and declines in Ca + Mg were much smaller. This indicates that recent decr eases in SO4 deposition are now being reflected in reduced precipitation ac idity. The lack of widespread increases in stream water alkalinity, despite the prevalence of downward trends in stream water SO4 suggests that at mos t sites, increases in stream water pH and acid-neutralizing capacity may be delayed until higher soil base-saturation levels are achieved.