Groundwater and surface water interactions at Georgetown Lake, Montana with emphasis on quantification of groundwater contribution

Georgetown Lake is a shallow intermontane lake located in southwestern Montana. Georgetown Lake is one of the lakes most used for recreation in the state and therefore research has been and continues to be conducted to ensure its continued quality and quantity. Knowing the contributions of the primary hydrologic parameters of a lake is vital to a lake quality-quantity analysis. Delineation of all of the hydrologic parameters can be difficult in intermontane settings. It is especially difficult to delineate groundwater flux, as groundwater is often subjected to complex flow paths due to irregular topography, and geologic folding and faulting. To account for these complexities, this study attempts to quantify groundwater contributions to the lake using an isotope mass balance. The hydrogeologic parameters of the equation were either sampled and measured or calculated for each month for a one year period and incorporated into an isotope mass balance equation. Utilization of these parameters and the corresponding isotopic compositions in the isotopic mass balance equation resulted in a groundwater inflow quantification of approximately 180 cm/yr, making groundwater flux into the lake approximately 55% of the total inflow to the lake.;This study also presents data and interpretations that give further insight to the groundwater and surface water interactions at the Georgetown Lake study site. First, to further understand groundwater movement in the area, surrounding springs, streams and domestic wells were sampled for stable isotopes in an attempt to determine their connectivity to the lake. Second, this study built upon previous radon research conducted at the lake by attempting to delineate the lateral extent of which groundwater enters the eastern flank of the lake. Third, this study looked at the stratigraphic profiles of the lake water columns during ice cover to observe the field parameter trends as depth increases.