LAKE AND WATERSHED NEUTRALIZATION STRATEGIES D. B. Porcella 1, C. T. Driscoll 2, C. L. Schofield 3, and R. M. Newton 4 1ElectricPower Research Institute, Palo Alta CA USA; 2SyracuseUniversity, Syracuse NY USA; 3Cornell University, Ithaca NY USA;4Smith College, Northampton MA USA. Abstract. The Experimental Watershed Liming Study (EWLS)evaluated the application of CaCO3, to a forested watershed to mitigate the acidification of surface water. During October 1989, 6.9 Mg CaCO3/ha was applied by helicopter to two subcatchrnents of about 50% (102.5ha) of the Woods Lake watershed area. The EWLS team investigated the response to treatment of soils (chemistry and microbial processes), vegetation, wetland, stream and lake waters, and phytoplankton and fish, and applied the Integrated Lake Watershed Acidification (ILWAS) model in predicting a watershed treatment duration of up to 50 years. Observations showed a gradual change in pH, acid neutralizing 7+. capacity (ANC) and Ca m the water column; direct lake additions of CaCO 3 (three different times) were characterized by abrupt changes following base addition and subsequent rapid reacidification. Moreover, the watershed treatment eliminated the snowmelt acidification of the near-shore region of the lake observed during direct lake treatments. Positive ANC water in the tributary and near-shore area improved conditions for fish reproduction and for a viable fish population. Budgets for 12-month periods before and after the watershed treatment showed that the lake shifted from a source of ANC to a sink due to retention of elevated inputs of Ca2+ from the watershed CaCO3 application. Key Words: acidification, acid neutralizing capacity, calcite, fish, liming, mitigation, ecosystems. 1. Introduction Base addition ('liming') for mitigation of acidity in aquatic and terrestrial ecosystems has been practiced since Roman times (Porcella et al. 1989, Brocksen et al. 1992, Henrikson and Brodin 1995). In 1984, we began assessment of liming options in Woods Lake in the Adirondack Mountains, NY, USA, using a series of direct lake liming treatments, followed by a watershed liming. In addition, a 3- year study of acidic deposition biogeochemistry led to development of the ILWAS model (Gherini et al. 1995). These field studies enhanced understanding processes of acidification in watershed ecosystems, as well as liming and its duration. We studied watershed liming based on hydrology: precipitation onto the lake and the upland parts of the catchment, flow through soil and surface zones into Woods Lake, and outflow into the downstream drainage (Fig. 1; similarly, see e. g., Traan et al. 1995; Dalziel, et al. 1994). The Woods Lake watershed has a total area of 208 ha (Driscoll et al. 1995), with the lake at an elevation of 606 m and watershed boundaries up to 728 m. The lake is dimictic and has a mean residence time of six months, but the small (23 ha), shallow (mean depth of 3.5 m, maximum depth of 12 m) lake has extreme variation in flow-through magnified by short-circuiting due to density differences especially during spring snowmelt after the six-month winter ice-cover. Annual precipitation ranged between 110-140 cm. The watershed has 98 percent forest cover, dominated by mature, second growth of red maple, American beech, yellow birch, and red spruce after previous harvest of the forest. Soils are young (<12,000 years) and thin (in one area in the main subcatchment up to 10 m to Water, Air and Soil Pollution 85: 889-894, 1995. 9 1995 Kluwer Academic Publishers. Printed in the Netherlands.