Cadwell Creek, in central Massachusetts, is a small stream that undergoes episodes of lowered pH (acidification) during spring runoff. Mundberry Brook, a stream not far away, shows no such effects. The general hydrogeochemistry of the streams is also distinctly different, although the bedrock and surficial geologic characteristics of the two water sheds are broadly similar. This study examines the weathering processes in the unconfined aquifer, the hydrogeology of the watersheds, and the export of dissolved materials in the stream water to evaluate the reasons for these geochemical differences. The composition of ground water is regulated by processes of silicate weathering and cation exchange. The relative importance of these two mechanisms is approximately the same in both watersheds, as determined from the solution of simultaneous equations based upon likely weathering reactions. However, dissolved silica in ground water shows a better correlation with cations and acid-neutralizing capacity in Cadwell (R2 > 0.6) than in Mundberry (R2 <0.3). Mass-balance calculations based on stream water composition show that cation denudation is about 50% higher in the Mundberry watershed than in the Cadwell watershed. Hydrogeological surveys document that ground-water base flow from well-buffered deep sources, which have elevated dissolved solids, is much greater in Mundberry Brook, and this contributes to the cation flux from this watershed and the pH buffering ability of the stream. In contrast, much of this deep ground water in the Cadwell Creek watershed does not feed the stream. Silica denudation compares favorably between the watersheds and indicates that, despite differences in the pathways of cations in the watersheds, the ultimate process of silicate weathering is proceeding at a similar rate.

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