Alkalinity and major ion budgets for a Massachusetts reservoir and watershed1

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Abstract
A detailed field study of the major ion and alkalinity budgets of Bickford Reservoir and its forested watershed in Massachusetts was conducted to quantify the hydrochemical response of the ecosystem to acid deposition. Three years of alkalinity data from this oligotrophic reservoir exhibit repeating seasonal variability but no clear trend. The annual acid‐base status of the system is predominantly determined by the following terrestrial biogeochemical processes: wet and dry acidic deposition, production of base cations by mineral weathering or cation exchange, biological retention of HNO3, and production of natural humic substances. Although HNO3 deposition was completely neutralized by biological retention, measured bulk SO42− deposition could account for only about 50% of streamwater SO42− output; dry sulfur deposition estimated from throughfall enrichment accounts for an additional 40%. Monthly budgets show a strong seasonal SO42− sink in summer, but estimation of the magnitude of net biotic SO42− removal is confounded by evidence that the weathering of sulfur‐containing minerals is significant. Watershed budgets suggest that increased sulfur deposition may cause a decrease in surface water alkalinity—possibly on an equivalent for equivalent basis—unless SO42− reduction and base cation supply increase accordingly.