Estimating groundwater inflow and age characteristics in an alluvial aquifer along the Little Wind River, Wyoming

Author

Derek Goble, University of Montana, MissoulaFollow

Year of Award

2018

Document Type

Thesis

Degree Type

Master of Science (MS)

Degree Name

Geosciences

Other Degree Name/Area of Focus

Hydrogeology

Department or School/College

Geosciences

Committee Chair

Payton Gardner

Commitee Members

Nancy Hinman, Ben Colman

Keywords

Groundwater, Hydrogeology, Age Tracers, Environmental Tracers

Subject Categories

Hydrology

Abstract

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 SF_6, 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.

Recommended Citation

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.
https://scholarworks.umt.edu/etd/11263