GROUNDWATER CONTRIBUTION TO THE WATER AND CHEMICAL BUDGETS OF WILLIAMS LAKE, MINNESOTA, 1980-1991

Groundwater seepage was the largest annual flux of water into (58-76%) and out of (73-83%) Williams Lake during a 12-year study, during whic h the entire volume of the lake was replaced four times. The only othe r water fluxes to and from the lake, which has no surface-water inlet or outlet, were atmospheric precipitation and evaporation. Nearly all of the annual input of calcium, magnesium, sodium, potassium, chloride , sulfate, and silica was provided by groundwater. Although much of th e calcium and most of the silica input was retained in the lake, this retention did not result in increased chemical mass in the lake water mass because biologically mediated removal of calcium and silica to th e sediments equaled or exceeded loss by lake seepage to groundwater. G roundwater represented as much as one-half the annual hydrological inp ut of phosphorus and nitrogen; the remainder was supplied by atmospher ic precipitation. From about 70 to 90% of the annual input of phosphor us and nitrogen was retained in the lake. Although water and chemical fluxes varied from year to year, interaction of the lake with groundwa ter determined the hydrological and chemical characteristics of Willia ms Lake.