Abstract

Lake hydraulic residence time is can be an important lake management variable that is dependent on several factors including: lake volume, watershed size, location within a watershed and climatic variability. The use of the heavy stable isotopes of hydrogen (Deuterium expressed as δD) and oxygen (δ18O) can provide some hydrologic insight for the development of a Total Maximum Daily Load (TMDL). Constructing a detailed lake water budget can be expensive and challenging. We show that a limited number of stable isotopic lake composition water δD and δ18O samples over time can aid in inference of source water input mixing and evaporative processes. Lake water 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.1,2 The deviation in amplitudes of the fractionation of lake water to water vapor was modeled to predict hydraulic residence time for twenty-four lakes throughout Minnesota. Lakes were sampled spring, summer, and fall over a three-year period. Results showed a wide range of hydraulic residence time; variations occurred in annual source water contributions, along with watershed size and connectivity. Ranges of estimated annual hydraulic residence time among individual lakes were as great as 18.8years and as small as 0.4years. δD and δ18O ratios were plotted in relation to the Meteoric Water Line (MWL) for all study lakes. Annual ?D and δ18O amplitudes provide directional insight into individual lake water residence times by documenting seasonal variations in a lake’s δD and δ18O compositions. The use of δD and δ18O can offer water quality managers an inexpensive tool to better understand, protect, and remediate lakes and their watersheds.

Keywords

isotopes, hydraulic residence time, deuterium, ?18O, ?D