Exploring hydraulic residence in Minnesota's Sentinel Lakes: implications for management

Abstract

Lake systems present a challenge in determining how water associated solutes cycle with time. Lake hydraulic residence time is an important lake management variable dependent on several factors including: volume, watershed size, location within a watershed and climatic variability. The stable isotopes of hydrogen (Deuterium expressed as &delta;D) and oxygen (&delta;<super>18</super>O) can provide some hydrologic insight to lake water quality management. Analyzing the stable isotopic composition of lake water &delta;D and &delta;<super>18</super>O over time can aid in identifying source water input mixing and evaporative processes. Lake water &delta;D and &delta;<super>18</super>O 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). Study lakes were sampled spring, summer, and fall over a three year period. Deviations in the amplitudes of fractionated lake water compared to water vapor was modeled to predict hydraulic residence time for twenty-four lakes throughout Minnesota. Results suggest hydraulic residence time was dynamic; variations occurred with annual source water contributions, watershed size and connectivity. Ranges of annual hydraulic residence time among individual lakes were as great as 18.8 years and as small as 0.4 years. &delta;D and &delta;<super>18</super>O values were plotted in relation to the Meteoric Water Line (MWL) for all study lakes. A gradient of &delta;D and &delta;<super>18</super>O values were found in relation to latitude. Lighter values of&delta;D and &delta;<super>18</super>O were found in northern Minnesota compared to southern Minnesota. Variations in seasonal &delta;D and &delta;<super>18</super>O created annual amplitudes that provide insight into lake water budgets and residence times. The use of &delta;D and &delta;<super>18</super>O offer water quality managers a tool to better understand, protect, and remediate lakes and their watersheds.