GROUNDWATER AND SURFACE WATER INTERACTIONS AT GEORGETOWN LAKE, MT

Georgetown Lake is a highly recreated shallow lake located in southwestern Montana. The Georgetown Lake area has experienced population and recreation increases over the past few decades and therefore interest in quantity and quality of the Lake have arisen. This study presents a geochemical investigation that builds on our previous conceptual understanding of how groundwater interacts with the lake. The Georgetown thrust fault splits the lake with west dipping Paleozoic carbonates to the east and west dipping Precambrian metasedimentary rocks to the west. Previous studies show that groundwater enters the lake through the carbonates near the perimeter of the lake. However, these studies 1) do not investigate groundwater inflows to the lake interior, 2) show no indication of groundwater outflows from the lake, and 3) could not quantify groundwater inflow or outflow rates to and from the lake. To address these three concerns we sampled lake, surface water, and groundwater for major ions, radon, and water isotopes (δD and δ¹⁸O). We used radon to map groundwater inflows to the interior of the lake. Results show inflows occur from the eastern boundary to about halfway to the thrust fault. Stable isotopes of lake water and groundwater were used to investigate groundwater recharge from lake seepage through bedrock. Groundwater wells around the lake perimeter show that the water has undergone little to no evaporation, while Georgetown Lake water shows significant evaporation. However, springs discharging down gradient to the west of the lake show a similar evaporation signal to the lake samples, suggesting that the lake recharges groundwater through the westward dipping bedding planes and fractures. Monthly surface water inflows and outflows, lake stage, evaporation, and precipitation were used in a physical mass balance to determine the net gain or loss of groundwater from the lake. Stable isotopes and major ions were collected monthly from all major inflows and outflows to use a chemical mass balance to separate and quantify groundwater inflows and outflows.