Alkalinity and major ion budgets for a Massachusetts reservoir and watershed1

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 rctention of HNO,, and production of natural humic substances. Although HNO, deposition was completely neutralized by biological retention, measured bulk SO,“deposition could account for only about 50% of streamwater SOd2output; dry sulfur deposition estimated from throughfall enrichment accounts for an additional 400/o. Monthly budgets show a strong seasonal SO,lsink in summer, but estimation of the magnitude of net biotic SO,,*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 basisunless SOd2reduction and base cation supply increase accordingly. The effects of acidic atmospheric deposition on the aquatic chemistry of low-alkalinity softwaters of North America and Europe are receiving scientific and political attention. Although surface waters in these regions often contain zero or negative alkalinity, measured alkalinities are rarely as negative as the bulk atmospheric inputs (Likens et al. 1977; Christophcrsen and Wright 1981; Galloway et al. 1983). Numerous geochemical and biological processes are responsible for neutralizing substantial quantities of the mineral acid input as precipitation percolates upland mineral soils, passes through saturated wetland soils, and is transported via stream channels to lakes and reservoirs. In addition, biological reactions that occur in the sediments and water column of lakes may also provide significant sources of alkalinity (Schindler et al. 1980, 1986; Kelly et al. 1982). A useful experimental technique for ’ Financially supported by New England Power Scrvice Co., Northeast Utilities Service Co., American Electric Power Co., Consolidated Edison ofNcw York, Conoco Inc., the Commonwealth of Massachusetts Division of Water Pollution Control, ihc Massachusetts Water Resources Research Center, and Empire State Electric Energy Research Corp. 2 Current address: Northrop Services, Inc., Environmental Research Laboratory, 200 SW 35th St., Corvallis, Oregon 97333. quantifying the biogeochemical processes that contribute alkalinity or acidity to surface waters is a mass balance study based on the physical principle of conservation of mass and the chemical principles of electroneutrality and alkalinity (Driscoll and Likens 1982). A quantitative description of the water balance is a key clement of such an investigation. The principal objective of the Bickford Reservoir and watershed study was to determine the annual and monthly mass balances of alkalinity and major ions for a softwater reservoir and its forested watershed in central Massachusetts. By determining all advective fluxes of alkalinity to and from the reservoir, the effects of internal sources and sinks in the water column and sediments were quantified. In similar fashion, mass balances of alkalinity and major ions on the watershed provide quantitative information regarding the upland and wetland biogeochemical processes that inRucnce the acid-base status of effluent surface waters. We thank A. Provencial and L. Gaudet for ac’cess to the reservoir and to water records; we also thank D. Sachs for producing most of the figures.