Comment on The Role of Groundwater in Delaying Lake Acidification by M. P. Anderson and C. J. Bowser

Anderson and Bowser [1986], hereafter referred to as AB, presented a very interesting paper on the study of the role of groundwater in delaying lake acidification i Wisconsin. By considering the input/output of hydrogen ion with respect o the aquifer, including nonconservative transport through the use of the retardation factor, the effect of a delay in the arrival of acid to the lake is demonstrated. A two-dimensional, finite difference flow and random walk transport model, neglecting dispersion, was used to simulate the migration of the acid water through the groundwater system. Also, the temporal changes in lake pH were examined by modeling the lake with both a lumped parameter (well-mixed) model and an equilibrium chemistry model. Thus the lake model mixes inflow from the groundwater to the lake, precipitation on lake surface, and lake water, with the consideration of equilibrium chemical reactions. From their work they concluded that groundwater inflow can mitigate the effect of acid deposition on lake watersheds, as well as the fact that the low ionic strength of the lakes was an important buffer. The scope of this comment is confined only to the issue of modeling the buffering capability of the aquifer. The purpose is to show that an alternate and simpler formulation for the groundwater model is also appropriate for the case study of AB. Lumped parameter models for surface water systems have been widely used by engineers in the past few decades. Gelhat and Wilson [1974], hereafter referred to as GW, developed a lumped parameter model for groundwater systems to investigate the effect of deicing salt on groundwater quality. Since then the lumped parameter approach has been applied to only few groundwater elated problems [e.g., U pdegraff and Gelhar, 1977; Simonent, 1981]. Generally, the usefulness of this approach has not been fully recognized in the field of groundwater hydrology. In this comment we demonstrate the utility of a lumped parameter approach for the aquifer-lake system in Wisconsin examined by AB. We also hope, through the example, to promote the use of such an approach for other appropriate groundwater quality problems. As is pointed out by GW, a lumped parameter approach will be justified for cases of distributed contaminant sources, for cases of limited data on spatial variation of aquifer properties, or for cases in which temporal variation in mean contaminant concentration is of primary importance. In the study by AB, acid deposition is assumed a uniformly distributed contaminant source in space. Only average output concentrations of hydrogen ion entering the lake are of concern while spatial variations within the groundwater system itself are ignored. Thus a lumped parameter approach is appropriate for this study.