THIRTY-YEAR TRENDS IN ACID DEPOSITION AND NEUTRALIZATION IN TWO HEADWATER CATCHMENTS, NORTHWESTERN MASSACHUSETTS, USA

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. Using Hopkins Memorial Forest (HMF) as our primary research site, we report 30-yr geochemical trends from the Birch Brook catchment and annual and seasonal variations from Birch Brook and the adjacent Ford Glen, which drain phyllitic and carbonate bedrock. Median pH of bulk precipitation and throughfall in HMF has increased irregularly since 1983 and 1986 respectively, whereas SO₄ concentrations have decreased, consistent with regional trends. Increases in precipitation pH and decreases in SO₄ concentration mirror increasing stream pH and decreasing stream SO₄ concentration and flux over at least the past 30 years. Birch Brook median Ca concentrations and acid-neutralizing capacity (ANC) decreased from 1984 to 2005, then began to increase, whereas Mg concentrations began to decrease in 2005. Birch Brook and Ford Glen display contrasting chemistries that likely reflect the influence of rock materials and flow pathways. Sampled at a point and over a range of flows, Birch Brook chemistry is a mixture of upstream source waters (throughfall; soil water; ground water) with different flow characteristics. Acidic precipitation and shallow soil water affect stream chemistry 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 magnesian calcite at most flows. Long-term trends measured at Birch Brook and other streams in western New England show that stream geochemistry mirrors decreases in acidic deposition in upland catchments, even where ecosystem chemistry is buffered by bedrock composition.