Using Water Budgets and Isotope Analysis to Explore Water Availability in Low Flow Conditions: Hartley Pond Case Study

Abstract

Quantifying seasonal changes to the volume and timing of water flowing through a reservoir provides valuable information for responsible resource management. Hartley Pond is a reservoir in Duluth, Minnesota, that is formed by a dam on Tischer Creek. Solar forcing to the reservoir elevates the summer water temperature to a level that is unhealthy for native Brook Trout. One option to address this impairment involves decoupling the stream from the pond. Tischer Creek would flow alongside the pond and maintain a natural water temperature while Hartley Pond would be fed by groundwater and excess streamflow during storm events. For this to work, water inputs from these sources must be high enough to sustain the pond without stream inputs under low flow conditions. To answer questions about the volume and sources of water discharge into and out of Hartley Pond, I built water budgets during low flow conditions in late summer. Streamflow, groundwater, precipitation, evaporation, runoff, and pond volume data were collected to build monthly water budgets for June, July, and August 2023. Accompanying stable isotope analysis of stream, pond, and groundwater samples refined elements of the water budget. Results from the water budget show that inputs into Hartley Pond are dominated by streamflow. Isotope analysis supports the finding that evaporation outputs are greater than groundwater inputs. Therefore, the plan to disconnect the stream from the pond may not be viable because the hydrologic integrity of the pond cannot be maintained through low flow conditions without streamflow inputs. Information gained from the water budget will help local resource managers create a feasible restoration project for Hartley Pond that meets community and environmental needs.