CHEMICAL EVOLUTION OF LAKES IN TERRAINS DOMINATED BY UNCONSOLIDATED SEDIMENTS: GEOCHEMICAL INFLUENCE OF PRECIPITATION AND BIOLOGY IN TILL-HOSTED SURFACE WATERS IN WESTERN NY

Geochemical evolution of lake water typically focuses on recording a balance between the regional hydrologic budget and the bedrock geology rather than the overlying unconsolidated sediment and the interplay of precipitation, evaporation and biology. Weathering of the till by rainwater provides a different suite of cations than local storm events and groundwater. Individual storm events carry unique signatures and might influence the chemistry of ponds if the storm track holds for a long enough period of time to override the influence of the till waters. Combining storm backtrack paths and rainwater chemistry, the following trends in cation chemistry were observed. Generally, storms tracking over Lake Erie are the dominant source of sodium during the winter whereas Lake Ontario is the dominant source during the autumn and spring. Storms tracking from the south, over Pennsylvania during the late spring, summer and early autumn can be a source of elevated potassium. Potassium was greater than sodium in storms during the drought of 2016. Overall, storm water chemistry signatures are lost through reaction of rainwaters with the till and evapo-concentration of the ponds. Dolomite and calcite in the till account for almost all of the minerals that react with rainwater (pH 4.2-5.6) thereby having a significant effect on the calcium and magnesium observed in the pond water chemistry. Higher sulfate in the till-reacted waters suggest pyrite oxidized to sulfate. Pondwater chemistries did not change significantly during evaporation as the rate of infiltration likely exceeds evaporation. Upon desiccation, residual minerals on the surfaces of mudcracks included minor to trace amounts of halite and calcite. Initial rehydration of the ponds after the drought of 2016 had the greatest dissolved loads (pH 7.8-8.6) and were subsequently diluted as water levels increased. In ponds that desiccated and then rehydrated, Chlorella were observed thriving during the initially high-dissolved chemistry. Oedogonium were not observed until water chemistries returned to pre-drought conditions. High pH values (8.9-9.8) were re-established by the alga. The initial chemical composition reflects a till- rather than bedrock-derived cation chemistry, and immediately after rehydration, likely acts as a filter for which alga are initially re-established.