Detecting Regional Groundwater Discharge to the Clark Fork River

In this study, Radon-222 (222Rn) measured in stream water and groundwater was used to constrain the quantity of groundwater discharge along a 22 km reach of the Clark Fork River as it runs through Missoula, MT. Dissolved 222Rn samples were taken at 2 km intervals along a reach extending from the confluence of the Clark Fork and the Blackfoot River near Bonner, MT to the confluence with the Bitterroot River on the northwestern edge of the Missoula Valley. Groundwater samples were taken from wells in an alluvial aquifer near Rattlesnake Creek, and combined with previous data from the Missoula aquifer to characterize 222Rn groundwater concentrations. All samples were analyzed for dissolved radon concentration using a spectral alpha-decay detector. Observed 222Rn concentrations in the stream and groundwater were then used to quantify the groundwater discharge along the reach with a stream transport model which includes groundwater discharge. 222Rn concentration was observed to increase to 553 mBq/L just downstream of the confluence of the Blackfoot and Clark Fork, drop below detection limits through most of the Missoula Valley, and rise to 995 mBq/L at Kelly Island just before the confluence with the Bitterroot. Estimated discharge ranged from 10 m3/day/m near the Blackfoot to 40 m3/day/m around the Bitterroot. Groundwater discharge from unconfined aquifers to adjacent streams is an important factor in watershed resiliency to climate change and can vary dramatically along the river due to unseen changes in subsurface properties. Our results provide spatially distributed estimates of the contribution of groundwater to base-flow conditions of the Clark Fork River as it passes through the Missoula Valley