HYDROGEOLOGY OF A KETTLE LAKE, MADISON, NEW YORK

Madison Lake is a 32-acre kettle lake located in central New York in a glaciofluvial aquifer deposited south of the Valley Heads Moraine. The lake is located very close to a surface water divide and has no outlets. The one inlet, an ephemeral stream, has not flowed during our study. Residents near the lake have noted that in the past seven years the clarity of the lake has decreased and the amount of plant life, including the invasive seaweed Myriophyllum spicatum (Eurasian milfoil), has increased. We are studying the lake to determine the causes of changes of the water quality. We analyzed lake water, water from shallow piezometers installed below the lake, and water from nearby domestic wells using ion chromatography. We used head data from the piezometers and the results of seepage meters to determine the direction of groundwater flow near the lake. We are modeling the aquifer with the analytic element model GFLOW to determine the contribution zone of the lake and to test hypotheses about groundwater flow directions.

Lake water quality data was collected sporadically from 1988 to 2011 through the Citizens Statewide Lake Assessment Program (CSLAP). These data show a decrease in water clarity and increasing concentrations of total phosphorus and ammonium starting around 2005. Our water analyses show very high concentrations of nitrate in domestic wells, particularly on the south shore of the lake. Nitrate concentrations in the lake and minipiezometers are consistent and are much lower than domestic well concentrations. Phosphate is present in all the water sources; concentrations show very little spatial or source variation. The numerical model indicates that water flows through the lake from the southwest to northeast, which is consistent with the orientation of the valley aquifer and the direction of surface water flow toward the Mohawk River to the north. All piezometers show a downward head gradient (i.e. a losing lake). We believe that groundwater inflow occurs in deeper parts of the lake, but we have not detected it to date. The CSLAP studies found higher concentrations of phosphorus and ammonium in deep parts of the lake, which may support our hypothesis. The source of the increased nutrients is unknown, but may be related to changes in manure storage and spreading practices within the watershed.