CHARACTERIZING GROUNDWATER AND SURFACE-WATER EXCHANGES IN WHITE BEAR LAKE, MINNESOTA, USA, USING HYDROLOGIC, GEOPHYSICAL, AND WATER-QUALITY TECHNIQUES

During its 90 years of record, White Bear Lake was at a historically low level in 2013. White Bear Lake is a closed-basin lake, and one of the deepest lakes in the northeast Twin Cities Metropolitan Area. Previous periods of low water levels in White Bear Lake correlate with sustained periods of below-normal precipitation; however, recent urban expansion and increased withdrawals from underlying aquifers have raised the question of whether increased groundwater withdrawals has contributed the recent lake-water-level decline. Understanding and quantifying the amount of groundwater inflow to a lake and water discharge from a lake to aquifers is difficult but is important in the management of lake levels. A variety of hydrologic, geologic, water-quality, and geophysical techniques were applied to assess groundwater and surface-water exchanges in White Bear Lake in nearshore and deep waters.

Groundwater and surface-water exchanges for White Bear Lake occurred in shallow and deep waters, with groundwater entering the lake nearshore and lake water leaving the lake and into lake sediments in deep waters. Oxygen-18/oxygen-16 and deuterium/protium ratios for well-water, precipitation, and lake-water samples indicated that water from White Bear Lake was flowing to the underlying aquifers, eventually reaching downgradient wells that were open to the aquifers between 41 and 150 meters below land surface. Continuous seismic-reflection profiles and lake-sediment cores demonstrated that deep-water sections of White Bear Lake have surprisingly thin organic-rich sediments containing few trapped gasses. These conditions are somewhat unique to Minnesota lakes, and allow for groundwater and lake-water exchanges to occur in deep water. Water-level differences between White Bear Lake and in-lake piezometers in deep waters, and seepage-flux measurements also indicated that lake water is generally flowing into lake sediments and underlying aquifers in deep waters. Nearshore hydraulic-head measurements and seepage fluxes indicated that groundwater inflow to the lake occurred in most measured nearshore areas. White Bear Lake was found to be vulnerable to low and fluctuating lake levels because closed-basin lakes generally have limited sources of water and therefore can be vulnerable to groundwater withdrawals.