Long‐term trends in stream water and precipitation chemistry at five headwater basins in the northeastern United States

Stream water data from five headwater basins in the northeastern United States covering water years 1968–1996 and precipitation data from eight nearby precipitation monitoring sites covering water years 1984‐1996 were analyzed for temporal trends in chemistry using the nonparametric seasonal Kendall test. Concentrations of SO₄declined at three of five streams during 1968–1996 (p < 0.1), and all of the streams exhibited downward trends in SO₄ over the second half of the period (1984–1996). Concentrations of SO₄ in precipitation declined at seven of eight sites from 1984 to 1996, and the magnitudes of the declines (−0.7 to −2.0 µeq L⁻¹ yr⁻¹) generally were similar to those of stream water SO₄. These results indicate that changes in precipitation 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 increased in only one stream. In most cases the decreases in stream water SO₄ were similar in magnitude to declines in stream water Ca + Mg, which is consistent with the theory of leaching by mobile acid anions in soils. In precipitation the magnitudes of SO₄ declines were similar to those of hydrogen, and declines in Ca +Mg were much smaller. This indicates that recent decreases in SO₄ deposition are now being reflected in reduced precipitation acidity. The lack of widespread increases in stream water alkalinity, despite the prevalence of downward trends in stream water SO₄, suggests that at most sites, increases in stream water pH and acid‐neutralizing capacity may be delayed until higher soil base‐saturation levels are achieved.