Exploring Hydraulic Residence in Minnesota's Sentinel Lakes: Implications for Management

Lake systems present a challenge in determining how water pollutants and associated solutes cycle over time. Lake hydraulic residence time is dependent on several factors including: volume, watershed size, location within a watershed and climatic variability. The use of the stable isotopes of hydrogen (Deuterium expressed as δD) and oxygen (δ18O) can provide some hydrologic insight and management guidance for the development of a Total Maximum Daily Load (TMDL). Analyzing the stable isotopic composition of lake water δD and δ18O over time can aid in identifying source water input mixing and evaporative processes, further explaining the relationship between hydraulic residence time and pollutant loading. δD and δ18O were compared to the isotopic composition of atmospheric water vapor which has a known isotopic concentration at specific latitudes and air temperatures (Burns and McDonnell, 1998; Dansgaard, 1964). The deviation in amplitudes of the fractionation of lake water to water vapor was modeled to predict hydraulic residence time for each lake. Twenty-four lakes throughout Minnesota were sampled over a three year period and residence times were calculated. Each of the twenty-four lakes is part of the "Sustaining Lakes in a Changing Environment (SLICE)" project, a study led by the Minnesota Department of Natural Resources.