INVESTIGATION OF GROUNDWATER GEOCHEMISTRY AND SEEPAGE RATES TO DETERMINE DOMESTIC NUTRIENT INPUT INTO EUTROPHIC LAKE ALTOONA, WISCONSIN

Lake Altoona, located in West Central Wisconsin, is an impounded lake created by the damming of the Eau Claire River. The lake is surrounded by several hundred homes with private septic systems. Lake Altoona has exhibited eutrophication and sedimentation problems largely attributed to agrochemicals transported via inflow from the Eau Claire River. Research conducted at Lake Altoona has revealed several areas of groundwater influx along the lakeshore that may serve as possible sites of domestic nutrient influence into Lake Altoona. This study aims to determine the chemical composition of both ground and surface waters and the degree of groundwater influx into the lake. Chemical analyses were used to determine the significance of domestic derived nutrients on lake water chemistry and seepage rate measurements were performed to delineate possible regions of groundwater influx.

During summer and fall months, surface water samples were collected weekly to biweekly where the Eau Claire River flows in and out of the lake. Groundwater samples were collected at a number of spring locations along the lakeshore with minipiezometers. Measurements of electrical conductivity, dissolved oxygen, and pH where performed in the field, and NH₄⁺, NO₃^-, total and reactive PO₄^3-, and major and minor ion concentrations were determined in a laboratory. Groundwater influx was quantified with seepage meters placed in multiple locations along the lakeshore.

Results of this study thus far show high variability in both groundwater discharge rates and in the chemical composition of ground and surface waters. Groundwater seepage rates along the lakeshore range from 0.6-71.0 m/s. Electrical conductivity for groundwater samples ranged from 84-344 µS/cm and from 85-133 µS/cm for surface water samples. These data indicate that nutrient influx from domestic sources may be higher than previously anticipated. Chemical analyses of surface water samples show that there are high levels of photosynthetic activity during low flow in the fall, indicating an abundant availability of nutrients. Domestic derived nutrients are expected to be proportionally higher during low flow events than at other times, suggesting that domestic input is a contributing factor to large algae blooms experienced during these low flow events.