TRANSIENT MASS BALANCE APPROACH TO ESTIMATION OF GROUNDWATER INFLOW TO A POLYMICTIC LAKE

Abstract:

Transient mass balances were performed using electrical conductivity (EC) and stable isotopes (δ¹⁸O and δ²H) as tracers to estimate the groundwater inflow to a polymictic lake. Lake input and output fluxes, EC, δ¹⁸O and δ²H for lake water, streams, springwaters, marsh water, and precipitation were measured over a 20-month period. LMWL-LEL plots revealed that amount‐weighted precipitation was the primary source of lake input and was slightly distinct from groundwater isotopic composition. Estimates generated a different temporal pattern of groundwater inflow for each tracer, but the median annual values were similar at 0.03 ±0.005 m³/sec, 0.05 ±0.01 m³/sec, and 0.03 ±0.01 m³/sec for EC, δ18O, and δ2H, respectively. These groundwater inflows represent 15%, 25%, and 18% of the total lake inflow, respectively, while the groundwater outflow represents approximately 32% of the total lake outflow. Thus, the annual lake water budget is dominated by surface water flow, which is reasonable given the low lake-watershed ratio, and by evaporation. The mean residence time of the lake water is 3.07 ±0.47 years, 2.7 ±0.45 years, and 2.95 ±0.47 years based on EC, δ¹⁸O, and δ²H, respectively. Overall, the results illustrate the usefulness of multiple solutes in tracer mass balances for understanding the role of groundwater flow in the lake system.