Estimation of nutrient loading by groundwater into a lake using stable isotopes

Fulda lake is a small eutrophic lake located in southwestern Minnesota. It has a single main inlet and outlet stream, and is contained in ground moraine (Hobbs and Goebel, 1982). This study compared the contributions made by surface and subsurface flow to the eutrophication of the lake system by determining their loadings of nutrients (Nitrogen and Phosphorus). To estimate groundwater's contribution to the lake's water budget, a mass balance was conducted using the stable isotopes of hydrogen and oxygen. Groundwater was sampled from the lake's littoral zone where positive potentiometric heads were measured. The origin of groundwater was confirmed using a local meteoric water line. The samples were analyzed for TKN, N-NH¬3, N-NO3-NO2, TP, and P-SRP. The resulting concentrations and inflow rates yielded nutrient loadings. The relative contributions of groundwater, precipitation, and the inlet stream to Fulda's water budget were 28.06$, 11.58%, and 60.36% respectively. Loadings from groundwater in metric tons per year were 7.08 for TKN, 5.86 for N-NH3, 0.07 for N-NO3-NO2, 0.77 for TP, and 0.33 for P-SRP. Groundwater's relative contributions to the lake system's nutrient budget, when compared to the inlet stream, were 68.57% for TP, 61.68% for P-SRP, 68.01% for TKN, 98.62% for N-NH3, and 0.15% for N-NO3-NO2. Loadings from the measured groundwater alone could lead to the hyper-eutrophication of the Fulda Lakes system.