Using naturally occurring geochemical tracers to track groundwater-lake interactions at Georgetown Lake, Granite County, Montana

Regional and local heterogeneities in montane catchments including faults, folds and fractures, can cause problems when characterizing groundwater-lake interactions. In this study, geochemical tracers such as radon and stable isotopes of water (delta18O, delta 2H) were used to identify how groundwater exchanges with Georgetown Lake. Stable isotopes were used to determine groundwater inputs on a whole-lake scale, but 222Rn was used to track groundwater seepage at point locations at the lake. Sampling primarily took place during winter months when the lake was frozen to ensure that degassing of 222Rn was negligible. Samples were taken at approximately the bottom of the lake with depths ranging between 0.61 and 6.1 meters. Radon concentrations varied spatially around the lake and ranged from less than 3.5 to 194.0 pCi/L. Elevated radon was predominantly along the eastern shore, while 222Rn was typically low along the western side of Georgetown Lake. These findings suggest that groundwater primarily enters from the eastern side of the lake. Radon concentrations in groundwater are more elevated on the west side of the lake in the Precambrian Belt metasediments than on the east side which consists of Paleozoic sedimentary rocks. A solute mass balance estimated a groundwater influx of 1.31 x 106 ft3/day and outflows of 2.36 x 106 ft3/day; this yields a net groundwater loss of 1.04 x 106 ft3/day. The results of the mass balance are within the order of magnitude expected, but it is an approximation because diffusion of solutes from sediment was not considered in the mass balance. An end member mixing analysis estimated that the approximate concentration of Georgetown Lake was 27.4% groundwater, 34.8% precipitation and 38.1% water that had been evaporated. Hydrogen sulfide and ammonium were also present in several of the samples collected from the lake during winter months. Concentrations of H2S vary spatially with lower concentrations on the eastern shore of the lake ranging from non-detectible to 0.09 mg/L. Along the western shore and near the dam H2S concentrations vary from 0.1 to > 1.99 mg/L. The lower samples along the eastern shore are probably controlled by low H2S groundwater discharging to the lake. Ammonia concentrations vary around the lake with a minimum concentration of 0.01 mg/L and a maximum concentration of 4.0 mg/L, and NH4 + concentrations generally mimic H2S in the lake.;Key words: radon, geochemical tracers, groundwater-lake interactions, stable isotopes...