WATER QUALITY, MIXING, AND GROUNDWATER AGE IN AN URBANIZING AGRICULTURAL INTERMOUNTAIN BASIN, GALLATIN VALLEY, MONTANA

Intermountain basins in the western US rely on mountain snowmelt for groundwater recharge, which in turn maintains the long-term aquifer storage that provides a critical source for dry-season river flows. Aquifer recharge and the timing of instream flows have potential to be transformed by climate change, irrigation diversions with consumptive use and leaky ditches, or reduction in permeable landcover by urbanization. The Gallatin Valley is an intermountain basin in the headwaters of the Missouri River that is subject to all of these potential mechanisms of changes in water resource availability, either due to issues with water quantity or quality. Nutrient impairment affects over 200 miles of streams in the Gallatin River Watershed. The rapidly growing city of Bozeman at the heart of municipal development in the valley is anticipating water shortages within a decade, if consumption increases at predicted rates. This projected shortage has led to exploration of new water resources including pipelines from resources outside the local watershed and well fields extracting from the intermountain basin aquifer.

Here, we present work seeking the understanding of groundwater age, flow path dynamics, and surface water-groundwater connection in the Gallatin Valley alluvial aquifer that is necessary for characterizing this aquifer’s role in the availability of surface and subsurface water resources for humans and ecosystems. Between 2017 and 2023, we performed longitudinal stream surveys and groundwater sampling across seasons and in variable drought conditions. Dissolved ⁸⁷Sr/⁸⁶Sr isotopic ratios and ²³⁴U /²³⁸U activity ratios ([²³⁴U/²³⁸U]) in groundwaters and surface waters suggest source and mixing relationships. The first full suite noble gas measurements in the Gallatin Valley provide groundwater age and recharge conditions throughout the aquifer. The presence of excess ⁴He in the alluvial aquifer at both the rangefront and in the lower valley indicates connection to premodern waters. Groundwater age at one point in the underlying Tertiary aquifer appears to be in the range of many hundreds of thousands of years. Major and trace ions indicate contaminant sourcing and geochemical evolution of waters. These geochemical methods illuminate the connections between hydrogeologic processes and nutrient contamination of groundwater and streams in the Gallatin Valley.