Regional analysis of lakes, groundwater and precipitation, northern Wisconsin: A stable isotope study
This research focuses on the use of stable isotopes and major ion solutes to expand our understanding of the hydrology and geochemistry of lakes in northern Wisconsin. Stable isotopes of oxygen and hydrogen are used to evaluate regional variation in groundwaters and the relationship to variations in atmospheric moisture. An isotope-based mass-balance approach is used to evaluate the fractional contribution of groundwater to 22 seepage lakes in northern Wisconsin with residence times longer than five years. The role of groundwater solute loading to lakes is evaluated. Groundwater inflow to seepage lakes- lakes that lack surface inflows and outflows- is of particular importance because of its role in regulating the chemistry of these lakes. Using the isotopic mass-balance method, the estimated groundwater inflow to the study lakes ranges from 0 to 37\% of total inflows, with precipitation contributing the remainder. Chemical analyses of groundwater and lake-water show that these waters are of the calcium-magnesium-bicarbonate type. Comparison of calcium to magnesium ratios in lakes and groundwater indicate that these elements are conservative in the groundwater lake system in northern Wisconsin. The presence of carbonate minerals in the glacial drift in some parts of the study area is observed in the magnitude of the Ca++fMg++ ratios. Road salt (NaCI) is detected in several groundwater and lake-water samples by high chloride and sodium concentrations with a CI-!Na+ ratio greater than one. Integration of isotopic and chemical data indicate that solute loading increases with increasing groundwater inflow and that silica may cease to be a limiting nutrient for diatom growth when groundwater inflow to a lake is high. Groundwater isotopic composition in Vilas County in northern Wisconsin has been observed to be very close to average annual precipitation, with groundwater 0.6 per mil lighter than precipitation (Krabbenhoft et al., 1990a). This difference is attributed to selective recharge of isotopically light spring snowmelt. Isotopic analyses of groundwater from 52 locations in northern Wisconsin, indicate that areas that receive more snow due to the lake-effect snow from Lake Superior have a lighter o 18Q composition. Improved correlation of groundwater isotopic composition with precipitation could simplify application of the stable isotopic mass-balance method in northern Wisconsin in future studies.