Presenter Information

Melinda HorneFollow

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Presentation

Abstract

This study applied the use of environmental tracers to constrain the quantity of groundwater discharge along a 22 km reach of the Clark Fork River as it runs through Missoula, MT. The primary environmental tracer used was Radon-222 (222Rn), a radioactive isotope in the uranium decay series that is absorbed by water from subsurface media, and is thus a sensitive indicator of groundwater discharge. Dissolved 222Rn samples were taken at 2 km intervals along a reach extending from confluence of the Clark Fork and the Blackfoot River near Bonner, MT, and extending across the Missoula Valley to the confluence with the Bitterroot River. Groundwater samples were also taken from wells near Rattlesnake Creek, which represent 222Rn baseline concentrations in the Missoula aquifer. The results were compared to data in the literature from previous studies. 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 using 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 values 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 estimated of the contribution of groundwater to base-flow conditions of the Clark Fork River as it passes through the Missoula Valley.

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Apr 28th, 10:20 AM Apr 28th, 10:40 AM

Detecting Regional Groundwater Discharge to the Clark Fork River

UC 331

This study applied the use of environmental tracers to constrain the quantity of groundwater discharge along a 22 km reach of the Clark Fork River as it runs through Missoula, MT. The primary environmental tracer used was Radon-222 (222Rn), a radioactive isotope in the uranium decay series that is absorbed by water from subsurface media, and is thus a sensitive indicator of groundwater discharge. Dissolved 222Rn samples were taken at 2 km intervals along a reach extending from confluence of the Clark Fork and the Blackfoot River near Bonner, MT, and extending across the Missoula Valley to the confluence with the Bitterroot River. Groundwater samples were also taken from wells near Rattlesnake Creek, which represent 222Rn baseline concentrations in the Missoula aquifer. The results were compared to data in the literature from previous studies. 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 using 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 values 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 estimated of the contribution of groundwater to base-flow conditions of the Clark Fork River as it passes through the Missoula Valley.