DETERMINING GROUNDWATER SOURCES AND AGES VIA ISOTOPE GEOCHEMISTRY IN BIG SKY, MONTANA

The Big Sky ski resort area in southwestern Montana is experiencing exponential growth in both development and population. Concerns have arisen over the quantity of good-quality groundwater in the region, and whether there is a large enough reserve to support the growing demand. In light of already-documented water-level decreases, and domestic wells needing to be deepened or replaced, the Montana Department of Natural Resources and Conservation (DNRC) enlisted the Montana Bureau of Mines and Geology\u27s Ground Water Investigation Program to perform an assessment of groundwater availability and quality in the region, and define groundwater supply sources. The geology in the region is complex. Faulting and folding have disturbed the once horizontal rock layers. Sub-surface rock layers that act as good aquifers (sources of groundwater) in one part of the greater Big Sky area can be dry or have poor quality water just a few miles away. This complexity adds to the challenge of performing a groundwater survey. Several water chemistry parameters were studied in hopes of answering the groundwater questions in Big Sky. Samples were collected from groundwater and surface water sites across the study area for water isotopes (oxygen-18 and deuterium), carbon isotopes, tritium, and full dissolved mineral analyses. The hope was to find a distinct chemical signature for each aquifer and/or sub-region of the study area. Surface water-groundwater interactions were also examined. The isotopic results alone were not enough to fingerprint waters in the study area, but combined with water chemistry they did. There was an abundance of evidence of mixing, both vertically between aquifers (especially between the Kootenai and Morrison Formations) and horizontally (within regional areas). The alluvial aquifer in the Meadow Village area was found to be almost completely disconnected from the underlying bedrock. Interaction between groundwater and surface water was evident in both the Meadow Village area, and in Yellowstone Club and Spanish Peaks, along the South Fork of the West Fork Gallatin River. An overall trend of gaining streams was revealed, which is a good indication that the groundwater supply is currently sufficient for the area’s needs