SALINIZATION OF ASYLUM LAKE: EFFECTS ON TURNOVER AND LAKE GEOCHEMISTRY

Many kettle lakes in southwest Michigan are naturally dimictic. However, road salt contamination, particularly in urban lakes, may delay or prevent turnover events. Asylum Lake is a 19.8 ha kettle lake, with a maximum depth of 15 m, located in Kalamazoo, MI near three heavily salted major roadways. The objective of this study is to determine the effects of increased Cl^- levels due to road salt contamination on Asylum Lake turnover and geochemistry.

Beginning in September 2012, samples have been collected approximately monthly from the deepest point in Asylum Lake. Dissolved oxygen (DO), conductivity, pH, and temperature are measured insitu every 0.5 m. At 1 m intervals, samples are filtered into 25 mL polypropylene bottles and analyzed as soon as possible for Fe²⁺, Mn²⁺, PO₄^3-, NH₄⁺, H₂S, NO₃^-, and total alkalinity via UV/vis spectrophotometry. Within one week, Cl^- and SO₄^2- and Ca²⁺, Na⁺, K⁺, and Mg²⁺are analyzed via IC and ICP-OES. Two wells to the southeast and southwest of Asylum Lake are sampled seasonally using low flow techniques and analyzed as described for surface water samples.

Asylum Lake is eutrophic, with anoxic bottom waters persisting all year. Conductivity levels increased after spring snowmelt and show a strong correlation with chloride levels, which reach levels of up to 300 ppm. Hypolimnetic NH₄⁺, H₂S, and Mn²⁺ levels remain high all year, reaching up to 221 µM, 86 µM, and 39 µM, respectively. Although hypolimnetic DO levels were always low, DO penetrated much deeper into the lake column during spring and fall periods of expected turnover. This suggests that partial turnover may have occurred, but complete turnover was likely prevented by the elevated density associated with high Cl^- concentrations. Groundwater samples from the southwest well up gradient from the lake had high DO levels, low concentrations of reduced analytes, and low Cl^- concentrations. In contrast, the southeast well close to the lake had low DO and elevated levels of NH₄⁺, H₂S, and Cl^-.