APPLICATION OF THE ILWAS MODEL TO THE NORTHERN GREAT LAKES STATES

Abstract

ABSTRACT In the northern Great Lakes States, seepage lakes are considered more vulnerable to acidification than drainage lakes. The ILWAS model, which was developed for drainage lakes in the Adirondack Mountain Region of New York, has been revised for use in seepage lake systems. Contrasts were observed between the Wisconsin seepage lakes and the drainage lakes in the Adirondack Region of New York. (1) Atmospheric loading of hydrogen and sulfate ions was 77 and 62 percent respectively, lower in Wisconsin. (2) In contrast to the low alkalinity flow-through lakes in the Adirondacks, there are no observed spring pH depressions associated with snowmelt in Wisconsin seepage lakes. (3) Because of deep glacial deposits (32–190 m) at the Wisconsin sites, a longer contact time between groundwater and weatherable minerals provides a substantial buffering capacity via silicate hydrolysis. (4) Inlake alkalinity production by bacterial sulfate reduction in the sediments of Wisconsin seepage lakes plays a significant role in ameliorating the effects of acid deposition. After the ILWAS model was revised and calibrated for Round and East Eightmile lakes, scenarios were run on two other Wisconsin seepage lakes (Crystal and Vandercook) that have alkalinities of less than 30 μeq/L. These scenarios showed that a 25 percent reduction in sulfur loading would change the chemistry of these lakes slightly. The most important processes controlling the pH and alkalinity of these lakes are groundwater input and sediment sulfate reduction. Using these results, we propose new categories of sensitivities to acidification for seepage lakes in the northern Great Lakes States. Lakes with alkalinities greater than 40 μeq/L would not be considered sensitive to acid deposition. Lakes with alkalinities between 20 and 40 μeq/L would be moderately sensitive, and lakes with alkalinities less than 40 μeq/L would be considered sensitive.