LONG TERM TRENDS AND PATTERNS IN SULFATE BUDGETS AT TWO CATCHMENTS IN THE NORTHEASTERN US

Analysis of trends and patterns in stream sulfate (SO₄^2-) concentrations and catchment SO₄^2- budgets provides a means to evaluate the effectiveness of air quality policies that affect sulfur dioxide emissions and resulting atmospheric SO₄^2- deposition. In the northeastern US, soils that generally developed in recent glacial till, show little tendency for strong SO₄^2- adsorption by mineral soils; therefore, stream SO₄^2- patterns in this region generally reflect changes in atmospheric SO₄^2- deposition that result from air emissions policies. However, in some catchments, SO₄^2- also originates from the weathering of sulfide minerals, and this geological source must be considered when evaluating SO₄^2- budgets at such sites. Sulfur dioxide emissions in the US, mainly from coal-fired power plants, peaked in the 1970s, and have been declining since, as the Clean Air Act of 1970 and subsequent amendments in 1990 have resulted in sharp declines of >65% since that time. These emissions declines have resulted in sharp decreases in atmospheric SO₄^2- deposition, and parallel decreases in SO₄^2- concentrations in monitored surface waters in eastern North America. A recent analysis of catchment SO₄^2- budgets in this region highlights these decreases in loads, and suggest that excess SO₄^2- previously stored in soil organic matter is currently being mineralized and exported. This change in net SO₄^2- export has resulted in a transition from catchments primarily reflecting changes in emissions and deposition of S, to a greater influence of climatic variation in more recent years. This greater climatic sensitivity of SO₄^2- export will be highlighted in this presentation by comparing and contrasting SO₄^2- budgets at Biscuit Brook in the Catskill Mountains of New York, a site with little geological S, with those of Sleepers River in northeastern Vermont, a site with significant geological S.