The Little Wind River overflowed its banksand flooded the area being monitored by a new multi-level monitoring well (the WindRiver Casino is shown in background).

Milling operations between 1958 and 1963, in Riverton, Wyoming, left a plume of contaminated groundwater in the surficial aquifer. The deep regional aquifer was not affected by the plume. In 1989, surface remediation occurred at the Riverton site, and the uranium mill tailings were transferred to the Gas Hills East, Wyoming, Disposal Site.

In recent history, the U.S. Department of Energy (DOE) Office of Legacy Management (LM) Riverton site, which is adjacent to the Little Wind River, has been affected by two floods. One, which occurred in 2010, was considered a 75-year event. Prior to that flood, groundwater concentrations were steadily declining, as forecasted by predictive models. This supported LM’s natural flushing compliance strategy.

After the 2010 flood, routine groundwater monitoring at the site showed significant increases in uranium levels. In response, LM began an enhanced characterization study to determine how flushing from the flood mobilized uranium contamination remaining in the soil. The ongoing study seeks to understand how dilute snowmelt or rainfall that causes river flooding could result in rising groundwater-contaminant levels.

The former Riverton uranium processing site is
located in central Wyoming within the Wind River
Indian Reservation.

In 2015, soil samples collected from backhoe-excavated trenches provided soil-chemistry data and an opportunity to see the subsurface geology. This helped LM locate sites for nine new monitoring wells. The new wells are designed to collect groundwater at four horizons in both the typically dry, shallow soil, which is saturated during high water, and in the deeper, saturated sediments that typically contain groundwater.

This past May, heavy spring rainfall caused the Little Wind River to flood again. Groundwater samples were collected within days of the flood. Preliminary results indicated secondary sources of contamination coming from the typically dry, shallow soil and aquifer sediments, contributing to uranium and other elements in the groundwater plume.

LM is testing several theories on how the shallow soil near the river originally became contaminated and how to effect interaction between the shallow soil and groundwater. LM has partnered with the Northern Arapaho Environment and Natural Resources Office; DOE National Laboratories at Argonne, Los Alamos, and Savannah River; SLAC National Accelerator Laboratory; Stanford University; and the U.S. Geological Survey. LM is conducting this work under its Applied Studies and Technology program and will apply the findings to additional LM sites.

LM monitors the Riverton site, assisted by the Northern Arapaho Environment and Natural Resources Office. Monitoring includes institutional controls (e.g., alternate water supply system, excavation and well drilling restrictions, and domestic well sampling), groundwater and surface water monitoring, and extreme events, including floods.
A northern view from the Wind River Casino with the ChemTrade Logistics Inc., sulfur
refinery plant and former uranium processing site in the background. The foreground
shows the May 2016 flooding of the Little Wind River.
Flooding along Little Wind River, downstream of the Riverton site, shows USGS stream gauge and property damage.