Sr and U isotopes reveal the influence of lithologic structure on groundwater contributions along a mountain headwater catchment (Hyalite Canyon, MT)

Abstract

Changing precipitation patterns are projected to reduce snowpack storage and late summer stream flows in mountain headwaters of the western United States. Ecosystems, agriculture, and municipalities depend on late summer flow in streams supplying water to the mountain front. Therefore, improved understanding of groundwater storage contributions to mountain streams is increasingly important. In this work, we use 87Sr/86Sr ratios and 234U/238U activity ratios ([234U/238U]) as indicators of water-rock interaction contributing to runoff, groundwater, and surface-subsurface hydrologic exchanges along Hyalite Creek, a mountain headwater tributary within the upper Missouri River basin. Main stem and tributary flow was sampled longitudinally from 2016 to 2018, focusing on presumed baseflow conditions in February and August. Changing stream chemistry of the main stem is associated with groundwater inflows from local springs discharging from Madison Group limestones, and inputs of water from the Archean gneiss exposed along the mountain front. We use mixing models to estimate that locally in the stream reach traversing a spring outlet, distinct inflows from the Madison aquifer contribute ~4% of streamflow during baseflow conditions. Contributions to streamflow by local inflows from Archean gneiss varied seasonally, with increases in 87Sr/86Sr values in Hyalite Creek that suggest contributions of ~2% in August to ~8% in February; at the same time, decreasing [234U/238U] values indicate an otherwise undetected additional endmember, and 222Rn assays show that total inflows are likely more extensive. Our results reveal distinct groundwater contributions to streamflow from specific rock units within this mountain headwater system, and elucidate pathways of mountain stream flow generation in lithologically diverse watersheds.