Year of Award
Master of Science (MS)
Other Degree Name/Area of Focus
Department or School/College
Nancy Hinman, Ben Colman
Groundwater, Hydrogeology, Age Tracers, Environmental Tracers
University of Montana
We synoptically sampled a 2 km gaining reach of the Little Wind River in central Wyoming for 222Rn, CFCs and SF6 to determine the distributed volumes and locations of discharge and estimate alluvial groundwater age characteristics. 222Rn was sampled in the stream, shallow in-stream piezometers and nearby alluvial groundwater wells to determine the volume and spatial distribution of groundwater discharge to the river. Age tracers, CFC and SF6, sampled in the stream, shallow in-stream piezometers and in alluvial groundwater wells were then used to determine whether in-stream samples are capable of constraining the residence time distribution of alluvial groundwater.
We performed synoptic surveys along a 2 km reach over three sampling sessions in 2016, and found a temporally evolving inflow profile along this reach supported by the adjacent alluvial system. Discharge in 2016 primarily occurred between 0.7 and 1.7 km down the reach, consistent with the center of a previously mapped discharging, contaminated plume. Inflow in 2017 was estimated to occur in only the upper 0.4 km portion of the reach. We compared tracer estimated inflow with flux rates obtained from point measurements of discharge using tube seepage meters and iButton vertical temperature profile loggers. CFC and SF6 concentrations measured in the stream were used to infer a mean alluvial groundwater age of 19.6 – 26.1 years depending on the assumed age distribution. This was compared to adjacent alluvial aquifer samples which showed an apparent age range of 1.1 to 49 years with a mean age of 21.7 years. The estimated recharge rate using mean age and alluvial aquifer volume is between 0.025 – 0.033 m year-1 in 2016 and a reasonable agreement with a previous estimate of 0.020 m year-1 from long term site analysis. If sampled at the proper base flow conditions, the Little Wind River could serve as an integrator of groundwater flow paths to infer age distributions of the contributing system. This method could provide convenient and efficient way to achieve a first order understanding of groundwater age characteristics in other rivers where wells or piezometers are not available.
Goble, Derek, "Estimating groundwater inflow and age characteristics in an alluvial aquifer along the Little Wind River, Wyoming" (2018). Graduate Student Theses, Dissertations, & Professional Papers. 11263.
© Copyright 2018 Derek Goble