The role of groundwater in delaying lake acidification
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Models used to study lake acidification generally divide a watershed into several zones or compartments (e.g., soil and groundwater zones) through which water and chemicals are routed. To date most emphasis has been placed on the buffering of acid inputs by chemical reactions in the soil zone. Although buffering in the soil zone undoubtedly is important, the focus of this paper is the role of the groundwater system in delaying lake acidification. In this study, a deterministic approach was selected over the standard parametric modeling used in watershed models. Specifically, an advection-dispersion model of the groundwater system was linked to a simple groundwater-rain-lake mixing model, which in turn was linked to a geochemical speciation and reaction path model. Simulations of hypothetical groundwater-lake systems utilized chemical data from small lake systems in northern Wisconsin, an area which is currently experiencing acid rain (pH = 4.6) and is potentially sensitive to acid rain impacts owing to the absence of carbonate minerals in the underlying glacial tills and bedrock. Results of the modeling studies demonstrated that even when the effects of chemical reactions are ignored, groundwater inflow can mitigate the effects of acid deposition on lakes. When chemical reactions in the groundwater system were included in the model using a retardation factor, the acidification rate of the lake was significantly decreased. When speciation reactions between lake water and net acid additions were simulated using the geochemical model PHREEQE (Parkhurst et al., 1980), the depression of lake water pH was markedly reduced, indicating that low alkalinity lakes, like the ones in northern Wisconsin, may have appreciable buffering capacity.