THIRTY-YEAR TRENDS IN ACID DEPOSITION AND MASS BALANCE IN TWO HEADWATER CATCHMENTS, NW MASSACHUSETTS

This study reports thirty-year trends in the concentration and flux of acid deposition anions (SO₄^2-, NO₃^-) and base cations (Ca²⁺, Mg²⁺, Na⁺) in two adjacent headwater catchments in the forested Taconic Range of western Massachusetts.Birch Brook, for which we have thirty years of data, is underlain by till and phyllite with local lenses of carbonate; Ford Glen (five years of data) is developed in till overlying dolomitic marble. Since the industrial revolution, acid rain has significantly impacted New England, but recent legislative controls have been aimed at curbing its influence. Composite precipitation data show that rainfall pH is fairly constant throughout the year, but nitrate and sulfate show strong seasonal trends. Since 1990, sulfate in precipitation has declined, decreasing more rapidly since the turn of the millennium and implementation of much stricter pollution legislation. Sulfate deposition from rainfall has decreased from an average of ~30 kg/ha.yr in 1983 to less than 20 kg/ha.yr in 2011. Concurrently, precipitation pH has increased from 4.4 to 4.8. Streamflow from the catchments shows a corresponding decrease in sulfate and a rise in pH over the years and a sulfate flux decrease from 53 kg/ha.yr to 38 kg/ha.yr and a pH increase from 7.2 to 7.4 between 1985 and 2012. There has also been a decrease of Ca and HCO₃^- flux from the catchments over three decades. Annual Ca loss fell from 42 kg/ha.yr to just over 30 kg/ha.yr from Birch Brook and HCO₃^- flux decreased from ~120 kg/ha.yr to 80 kg/ha.yr. Cation export from Birch Brook is lower than export from Ford Glen. In Ford Glen, for instance, Ca flux ranged from ~400 kg/ha.yr in 2008 to 150 kg/ha.yr in 2012. Overall, we observe acid deposition in northwestern Massachusetts has decreased over the past thirty years and stream chemistry is better buffered, implying that continued management of acid rain compounds can effectively combat acid deposition and its effects.