Sensitivity of Meander Lake to Acid Deposition

Meander Lake in northeastern Minnesota was classified as “extremely sensitive” to acid deposition, due to its low alkalinity. The ILWAS model was applied to determine its actual sensitivity. The model was calibrated with observed data and then used to evaluate the response of the lake to various levels of acidic deposition. At the current deposition rate of 12 kg/ha/yr sulfate, the lake can maintain an alkalinity of $70\mu \mathrm{eq}/L$ and a pH of 6.2 indefinitely. A 50% increase in sulfate deposition would decrease the alkalinity by $13\mu \mathrm{eq}/L$ and the pH from 6.2 to 6.1 in 20 years. A 50% decrease in sulfate deposition would raise the alkalinity by $12\mu \mathrm{eq}/L$ and the pH from 6.2 to 6.3. Acidic deposition is largely neutralized by alkalinity from mineral weathering in thick mineral deposits surrounding the lakeshore. Low alkalinity is not a sufficient condition for a lake to be sensitive to acidification. Furthermore, the alkalinity of many Minnesota lakes may have been underestimated by Gran titration due to their high concentrations of organic acids.