Predicting episodic stream acidification in the southeastern United States: Combining a long‐term acidification model and the end‐member mixing concept

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Abstract
The model of acidification of groundwater in catchments (MAGIC) was applied to data from the Panola Mountain Research Watershed located near Atlanta, Georgia, in the southern Piedmont physiographic province of the United States. In contrast to past applications, MAGIC was calibrated directly to two soil environments using information gained from an independent mixing model rather than to the stream water. A third soil environment, the deeper soil horizons which provide base flow, was identified from the mixing model, but its chemistry was assumed to be invariant during the forecast period, and hence it was not included in MAGIC. This mixing model identified soil environments that appear to be important in determining present stream water chemistry, determined the hydrologic routing parameters necessary for the model, and permitted predictions of stream water chemistry during future storm events. Annual average stream water acid neutralizing capacity (ANC) was predicted to decrease during the next 50 years in a manner largely consistent with a regional analysis of the nearby southern Blue Ridge province performed by the U.S. Environmental Protection Agency. The two upper soil layers, however, were predicted to acidify substantially within the next 20 years so that the stream water may become unsuitable for sensitive aquatic biota for much of the year, despite a positive annual average ANC. This approach, which combines a predictive model and a mixing model, provides testable hypotheses because actual soil environments are modeled; field experiments to test the processes contained in the model are suggested.