A new publication by the U.S. Geological Survey (USGS) locates where groundwater pollution from a former uranium mill site impacts a stream’s ecosystem on the Wind River Indian Reservation. Eleven coauthors from USGS and two universities collaborated on the study with me when I worked with the U.S. Department of Energy’s Office of Legacy Management (DOE). My co-authored 2015 DOE investigation was limited to looking at soils and groundwater while USGS-university expertise examined the land, surface water and groundwater, sediments and aquatic biota.
My recent interview in the ProPublica news article discusses similar uranium mill sites where DOE is failing to contain groundwater contamination hoping that ‘dilution is the solution to pollution.’ However, the latest USGS report identifies continued impacts to the river environment at Riverton even though the mill stopped operating in 1963, surface contamination was removed by 1990, and remaining contaminant concentrations are now significantly lower! Current EPA regulations allow DOE site managers to wait and see for 100 years after the U.S. Nuclear Regulatory Commission approval using the “natural flushing” compliance strategy.
Confrontation and not collaboration between agency representatives initially occurred after a rain on snow event in 2010 flooded rivers on both sides of the site which caused increases, and not decreases, in groundwater contamination. Tribal officials wrote letters to the Wyoming Governor, Secretary of Energy, and other elected officials tying to get DOE to explain the surprising results. The Wind River Environmental Quality Commission (WREQC) hired USGS to assess the effectiveness of the existing DOE monitoring network at the Riverton, Wyoming, Uranium Mill Tailings Remedial Action (UMTRA) site which produced this initial USGS publication. WREQC consisted of representatives from the Northern Arapaho and Eastern Shoshone tribes representing the community who understandably held deep grudges for multi-generational human rights abuses including the uranium mill contaminating their property. Many of the tribal members and families have suffered from cancer and other illnesses that they believe came from living next to the uranium mill site.
In 2012, I was working for a different USGS office than the authors working with WREQC. I knew the DOE manager on the Riverton project when we worked together on the defunct Yucca Mountain high-level waste repository project. She and her management asked me to review the groundwater monitoring strategy at several UMTRA sites. Soon after I started, DOE held a town-hall meeting in Riverton airing public concerns for contamination impacting the health of the community.
After six months of the USGS detail assignment, I transferred to the better-funded DOE and looked for ways to improve collaboration such as by attending tribal council meetings, involving the tribal hydrogeology consultant in field investigations and communicating with the larger community our proactive investigation by conducting interviews through the news media.
Earth scientists traditionally study college subjects including biology, chemistry, geology, hydrology, and physics taught as separate classes and discrete major disciplines. Due to nature’s complexity, professionals are collaborating and integrating scientific knowledge by merging disciplines and combining research such as geophysics, biogeochemistry, and hydrogeology. This USGS report applies numerous state-of-the-art tools that are improving our understanding of the environment.
It’s common practice driven by regulations to monitor groundwater pollution in wells and randomly grab river samples upstream and downstream of contaminated sites. However, the small volume of groundwater discharging somewhere adjacent to and beneath a river is quickly diluted in the stream so determining the impacts to biological organisms like algae and crayfish is not possible. To get a more accurate understanding of the groundwater-surface water interactions, the USGS scientists and professors used innovative approaches to locate groundwater discharge using several comparable approaches including fiber optic cables that measure warmer groundwater entering the cooler river. The authors also quantified contaminants sorbed onto river sediments and accumulating in biological samples.
Several of the authors previously retired, including the lead author Dave Naftz who dedicated his 36-year career to these types of investigations, yet continued to persevere through the arduous and lengthy peer-review publication process as a volunteer in the USGS emeritus program. Many thanks to all these dedicated public servants for advancing environmental science by producing outstanding reports!
Here are details of Scientific Investigations Report 2022–5089:
Interaction of a Legacy Groundwater Contaminant Plume with the Little Wind River from 2015 Through 2017, Riverton Processing Site, Wyoming
Abstract
The Riverton Processing site was a uranium mill 4 kilometers southwest of Riverton, Wyoming, that prepared uranium ore for nuclear reactors and weapons from 1958 to 1963. The U.S. Department of Energy completed surface remediation of the uranium tailings in 1989; however, groundwater below and downgradient from the tailings site and nearby Little Wind River was not remediated. Beginning in 2010, a series of floods along the Little Wind River began to mobilize contaminants in the unsaturated zone, resulting in substantial increases of uranium and other contaminants of concern in monitoring wells completed inside the contaminant plume. In 2011, the U.S. Department of Energy started a series of university and Government agency retrospective and field investigations to understand the processes controlling contaminant increases in the groundwater plume. The goals of the field investigations described in this report were to (1) identify and quantify the contaminant flux and potential associated biological effects from groundwater associated with the legacy plume as it enters a perennial stream reach, and (2) assess chemical exposure and potential effects to biological receptors from the interaction of the contaminant plume and the river.
Field investigations along the Little Wind River were completed by the U.S. Geological Survey during 2015–17 in cooperation with the U.S. Department of Energy Office of Legacy Management to characterize: (1) seepage areas and seepage rates; (2) pore-water and bed sediment chemistry and hyporheic exchange and reactive loss; and (3) exposure pathways and biological receptors. All data collected during the study are contained in two U.S. Geological Survey data releases, available at https://doi.org/10.5066/F7BR8QX4 and https://doi.org/10.5066/P9J9VJBR. A variety of tools and methods were used during the field characterizations. Streambed temperature mapping, electrical resistivity tomography, electromagnetic induction, fiber-optic distributed temperature sensing, tube seepage meters, vertical thermal sensor arrays, and an environmental tracer (radon) were used to identify areas of groundwater seepage and associated seepage rates along specific sections of the study reach of the river. Drive points, minipiezometers, diffusive equilibrium in thin-film/diffusive gradients in thin-film probes, bed-sediment samples, and equal discharge increment sampling methods were used to characterize pore-water chemistry, estimate hyporheic exchange and reactive loss of selected chemical constituents, and quantify contaminant loadings entering the study reach. Sampling and analysis of surface sediments, filamentous algae, periphytic algae, and macroinvertebrates were used to characterize biological exposure pathways, metal uptake, and receptors.
Areas of focused groundwater discharge identified by the fiber-optic distributed temperature sensing surveys corresponded closely with areas of elevated electrical conductivity identified by the electromagnetic induction survey results in the top 5 meters of sediment. During three monitoring periods in 2016, the mean vertical seepage rate measured with tube seepage meters was 0.45 meter per day, ranging from −0.02 to 1.55 meters per day. Five of the 11 locations where vertical thermal profile data were collected along the study reach during August 2017 indicated mean upwelling values ranging from 0.11 to 0.23 meter per day. Radon data collected from the Little Wind River during June, July, and August 2016 indicated a consistent inflow of groundwater to the central part of the study reach, in the area congruous with the center of the previously mapped groundwater plume discharge zone. During August 2017, the greatest attenuation of uranium from reactive loss in pore-water samples was observed at three locations along the study reach, at depths between 6 and 15 centimeters, and similar trends in molybdenum attenuation were also observed. Bed-sediment concentration profiles collected during 2017 also indicated attenuation of uranium and molybdenum from groundwater during hyporheic mixing of surface water with the legacy plume during groundwater upwelling into the river. Streamflow measurements combined with equal discharge increment water sampling along the study reach indicated an increase in dissolved uranium concentrations in the downstream direction during 2016 and 2017. Net uranium load entering the Little Wind River study reach was about 290 and 435 grams per day during 2016 and 2017, respectively. Biological samples indicated that low levels of uranium and molybdenum exposure were confined to the benthos in the Little Wind River within and immediately downstream from the perimeter of the groundwater plume. Concentrations of molybdenum and uranium in filamentous algae were consistently low at all sites in the study reach with no indication of increased exposure of dissolved bioavailable molybdenum or uranium at sites next to or downstream from the groundwater plume.
Comparison of the August 2017 results from electromagnetic induction, tube seepage meters, vertical thermal profiling, and pore-water chemistry surveys were in general agreement in identifying areas with upwelling groundwater conditions along the study reach. However, the electroconductivity values measured with electromagnetic induction in the top 100 centimeters of sediment did not agree with sodium concentrations measured in pore-water samples collected at similar streambed depths. Differences and similarities between multiple methods can result in additional insights into hydrologic and biogeochemical processes that may be occurring along a reach of a river system interacting with shallow groundwater inputs. It may be advantageous to apply a variety of geophysical, geochemical, hydrologic, and biological tools at other Uranium Mill Tailings Remedial Action (https://www.energy.gov/sites/prod/files/2014/10/f19/UMTRCA.pdf) sites during the investigation of legacy contaminant plume interactions with surface-water systems.
Suggested citation:
Naftz, D.L., Fuller, C.C., Runkel, R.L., Solder, J., Gardner, W.P., Terry, N., Briggs, M.A., Short, T.M., Cain, D.J., Dam, W.L., Byrne, P.A., and Campbell, J.R., 2023, Interaction of a legacy groundwater contaminant plume with the Little Wind River from 2015 through 2017, Riverton Processing site, Wyoming: U.S. Geological Survey Scientific Investigations Report 2022–5089, 66 p., https://doi.org/10.3133/sir20225089.