Detecting Groundwater Discharge in the Clark Fork River near Stone Container Using Spectral Alpha Decay Detection for Dissolved Radon in Surface Water Samples

Radon 222 (222 Rn) was measured along 8.7 kilometers of the Clark Fork River, between Harper’s Bridge and Frenchtown, MT. 12 water samples were taken along the stretch. Samples 1 through 4 and 10 through 12 were collected on a 1 km interval, samples 5 through 9 were taken on a 500 meter interval. Samples were analysed for dissolved 222 Rn using a RAD7 spectral alpha decay detector. Instream 222Rn was modeled to quantify groundwater discharge to the river. Well logs from the Montana Groundwater Information Center and literature on the Missoula Valley aquifer were analyzed. Analysis of well logs adjacent to the river reveals an alluvial aquifer system to the east consisting of interbedded gravel, sand, silt, and clay. To the west, bedrock rises steeply from underneath the river to crop out at the surface. Analysis of the samples reveals that there are measurable quantities of 222 Rn through the entire stretch sampled, starting at 395 mBq/L near Harper’s bridge, with peaks of 950 mBq/L at 2 km and 632 mBq/L at 6.5 km. Lowest concentrations were 395 mBq/L at the start of sampling, 355 mBq/L at 4.3 km, and 336 mBq/L at 8.7 km. Modeling results averaged to 5.5*105 m3/day of groundwater entering the river, with a standard deviation of 1.2*105 m3/day, occuring in areas of high 222Rn activity. This work identifies and quantifies the spatial distribution of groundwater discharge at the west end of the Missoula Valley postulated by previous works.