Hydrologic and geochemical investigations of aquifer-lake interactions at Sparkling Lake, Wisconsin
Groundwater inflow and outflow rates for Sparkling Lake, Wisconsin were determined by two methods: xx0 mass balance relationships, and a three dimensional, solute transport model. Sparkling Lake is at isotopic steady state. (Formula) Groundwater up gradient from the lake is isotopically uniform, 6’ 8 0•-11 .5 Down gradient from the lake, a ·plume of isotopically heavy outflow develops, which is diluted by ambient groundwater. Average annual precipitation is 0.6°/00 heavier than groundwater. Deuterium and ••o analyses show that groundwater has not been evaporated, and that selective recharge of isotopically light spring snow melt is responsible for fractionating groundwater and precipitation. The isotope balance method estimates Outflow is calculated as the residual in the hydrologic budget, or 4.2x10° m3 /y. The solute transport model was calibrated to an observed stable isotope plume down gradient from the lake. Calibration to the isotope plume provides additional data for constraining the model, which is otherwise difficult to calibrate based on head data alone. Inflow and outflow estimates from the model are 1.6xloa m3 /y and 4.1x10 A larger discrepancy between the two inflow estimates is attributed to the use of several water table configurations from drought periods as input to the model. The estimated rates of aquifer-lake exchange were combined with chemical analyses of groundwater and lake water samples to assess the role of groundwater in the chemical evolution of the lake. Net solute loading rates calculated using the isotope balance inflow estimate and groundwater chemistry agree very closely with observed changes in lake chemistry. Net loading rates calculated using the inflow estimate from the model do not agree as well, and suggest the isotope balance estimate is closer to the real value. Chemical processes at the aquifer-lake interface remove 98\% of the nitrate and 94\% of the sulfate from inflowing groundwater, and are important for buffering against lake acidification. Sparkling Lake relies on groundwater inflow for 43\% of its ammonia. 37\% of its total N, and over 50\% of its phosphorus annually.