Thirty‐year trends in acid deposition and neutralization in two headwater catchments, northwestern Massachusetts, USA

Abstract

Long-term decreases in acidic precursors have changed the chemistry of precipitation and streamflow in two moderately to well-buffered, forested headwater catchments in the Taconic Range of western New England, USA. We report 30-yr geochemical trends from Birch Brook, and annual and seasonal variations from Birch Brook and adjacent Ford Glen, which drain phyllitic and carbonate bedrock. Median pH of precipitation has increased irregularly since 1983, consistent with regional trends. Increases in precipitation pH and decreases in sulfate (SO₄²⁻) correspond with increasing stream pH and decreasing SO₄²⁻ concentration and flux over at least the past 30 years. Calcium (Ca²⁺) concentration and acid-neutralizing capacity (ANC) in Birch Brook began to increase about 2005, in contrast to geochemical indicators from some poorly buffered catchments in the NE USA. Point and longitudinal sampling over a range of flows show that Birch Brook chemistry is a mixture of upstream source waters (throughfall, soil water, ground water) with different chemistries. Acidic precipitation and shallow soil water affect stream chemistry directly only in the upper reaches of Birch Brook and during periods of extremely high flow, such as snowmelt. Ford Glen has high ANC and is near saturation with calcite at most flows. Long-term trends measured at Birch Brook and at other streams in western Massachusetts show that stream chemistry reflects changes in acidic deposition in upland catchments, even where ecosystem geochemistry is well buffered by bedrock composition.