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Title: Long-Term Stewardship at a Former Uranium Mill Tailings Site in Riverton, Wyoming WM2017-17090

Abstract

The US Department of Energy Office of Legacy Management (LM) is responsible for maintaining protective public health and environmental conditions at former uranium mill tailings sites nationwide via long-term stewardship. One of these sites, a former uranium mill near Riverton, Wyoming, is within the boundary of the Wind River Indian Reservation and operated from 1958 to 1963. Tailings and contaminated material associated with mill operations were removed and transported to an offsite disposal cell in 1989. The remedial action was completed under Title I of the Uranium Mill Tailings Radiation Control Act of 1978. Milling operations, which included an unlined tailings impoundment and an unlined evaporation pond, contaminated the shallow groundwater, resulting in a downgradient groundwater plume that discharges to the Little Wind River. A natural flushing compliance strategy was implemented in 1998. This strategy allows contaminants of concern to naturally flush from the groundwater, provided that contaminants flush below US Environmental Protection Agency maximum concentration limits within 100 years. As part of the compliance strategy, LM has implemented a groundwater monitoring program along with institutional controls that include the installation of an alternate water supply, continued sampling of private wells, and restrictions on well drilling and gravel pit construction.more » LM works closely with local stakeholders and community members to ensure that these institutional controls are in place and maintained. The Riverton site provides an interesting case study where contaminant remobilization due to river flooding prompted a reevaluation of the conceptual site model to verify if the current compliance strategy would remain protective of human health and the environment. Concentrations of groundwater contaminants, which include sulfate, molybdenum, and uranium, were transiently elevated following flooding of the Little Wind River in 2010 and 2016. These flood events provided the impetus to investigate other aspects of the hydrologic system, including the unsaturated zone, naturally reduced (sulfidic) zones, and evaporite deposits. New site conceptual models, field and laboratory studies, and numerical models are being developed to explain how biogeochemical sediment–water interactions contribute to plume persistence and flood-related increases in groundwater concentrations. Updated human health and ecological risk assessments are progressing to evaluate the risk to human health and the environment based on current site conditions. Groundwater concentrations may remain above US Environmental Protection Agency maximum concentration limits beyond the 100-year natural flushing regulatory time frame. LM in its capacity as a long-term steward continues to monitor the site to ensure protectiveness is maintained and to determine the feasibility of alternative compliance and remediation strategies.« less

Authors:
 [1];  [1];  [2];  [2]
  1. USDOE Office of Legacy Management, Washington, DC (United States)
  2. Navarro Research and Engineering, Oak Ridge, TN (United States)
Publication Date:
Research Org.:
USDOE Office of Legacy Management, Washington, DC (United States)
Sponsoring Org.:
USDOE Office of Legacy Management (LM), Office of Site Operations (LM-20)
OSTI Identifier:
1345366
DOE Contract Number:
LM0000421
Resource Type:
Conference
Resource Relation:
Conference: Waste Management Conference, Phoenix, AZ (United States), Mar 2017
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; WM2017; Riverton WY

Citation Formats

Dam, William, Gil, Dr. April, Johnson, Raymond H., and Campbell, Sam. Long-Term Stewardship at a Former Uranium Mill Tailings Site in Riverton, Wyoming WM2017-17090. United States: N. p., 2017. Web.
Dam, William, Gil, Dr. April, Johnson, Raymond H., & Campbell, Sam. Long-Term Stewardship at a Former Uranium Mill Tailings Site in Riverton, Wyoming WM2017-17090. United States.
Dam, William, Gil, Dr. April, Johnson, Raymond H., and Campbell, Sam. 2017. "Long-Term Stewardship at a Former Uranium Mill Tailings Site in Riverton, Wyoming WM2017-17090". United States. doi:. https://www.osti.gov/servlets/purl/1345366.
@article{osti_1345366,
title = {Long-Term Stewardship at a Former Uranium Mill Tailings Site in Riverton, Wyoming WM2017-17090},
author = {Dam, William and Gil, Dr. April and Johnson, Raymond H. and Campbell, Sam},
abstractNote = {The US Department of Energy Office of Legacy Management (LM) is responsible for maintaining protective public health and environmental conditions at former uranium mill tailings sites nationwide via long-term stewardship. One of these sites, a former uranium mill near Riverton, Wyoming, is within the boundary of the Wind River Indian Reservation and operated from 1958 to 1963. Tailings and contaminated material associated with mill operations were removed and transported to an offsite disposal cell in 1989. The remedial action was completed under Title I of the Uranium Mill Tailings Radiation Control Act of 1978. Milling operations, which included an unlined tailings impoundment and an unlined evaporation pond, contaminated the shallow groundwater, resulting in a downgradient groundwater plume that discharges to the Little Wind River. A natural flushing compliance strategy was implemented in 1998. This strategy allows contaminants of concern to naturally flush from the groundwater, provided that contaminants flush below US Environmental Protection Agency maximum concentration limits within 100 years. As part of the compliance strategy, LM has implemented a groundwater monitoring program along with institutional controls that include the installation of an alternate water supply, continued sampling of private wells, and restrictions on well drilling and gravel pit construction. LM works closely with local stakeholders and community members to ensure that these institutional controls are in place and maintained. The Riverton site provides an interesting case study where contaminant remobilization due to river flooding prompted a reevaluation of the conceptual site model to verify if the current compliance strategy would remain protective of human health and the environment. Concentrations of groundwater contaminants, which include sulfate, molybdenum, and uranium, were transiently elevated following flooding of the Little Wind River in 2010 and 2016. These flood events provided the impetus to investigate other aspects of the hydrologic system, including the unsaturated zone, naturally reduced (sulfidic) zones, and evaporite deposits. New site conceptual models, field and laboratory studies, and numerical models are being developed to explain how biogeochemical sediment–water interactions contribute to plume persistence and flood-related increases in groundwater concentrations. Updated human health and ecological risk assessments are progressing to evaluate the risk to human health and the environment based on current site conditions. Groundwater concentrations may remain above US Environmental Protection Agency maximum concentration limits beyond the 100-year natural flushing regulatory time frame. LM in its capacity as a long-term steward continues to monitor the site to ensure protectiveness is maintained and to determine the feasibility of alternative compliance and remediation strategies.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 2017,
month = 3
}

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  • • Evaporites occur in an unsaturated silt layer, which is underlain by a sand and gravel aquifer. • These evaporites are rich in chloride across the site. • Uranium concentrations are higher in the evaporites that overlie the uranium contaminant plume. • Flooding can solubilize the evaporites in the silt layer and release chloride, sulfate (not shown), and uranium into the underlyingsand and gravel aquifer. • The uranium-rich evaporites can delay natural flushing, creating plume persistence near the Little Wind River.
  • Mountain States Research and Development was contracted on March 1, 1981 to make an economic evaluation study at each of 12 abandoned uranium mill tailings sites in the western states. The objective of this work was to obtain the data necessary at each site to determine the possible revenue that could be derived from reprocessing the tailings. To accomplish this objective a drilling and sampling program was established for each site to determine the total amount of tailings and subbase material available for treatment and the amount of recoverable uranium, vanadium and molybdenum. These three metals were selected due tomore » their common occurrence in uranium ores and common extractability in the leaching process. Laboratory leaching was then conducted on the samples obtained to determine the extractability of each of these metals and the optimum plant process to be applied. As the metal contents were generally low and represented mineral that had not been leached during previous processing, the economic evaluation is limited to consideration of the direct capital and operating costs required in connection with processing of each respective site material. Excavating, transportation and disposal of the material from each site in an environmentally acceptable location and manner was not within the scope of this project. It will be necessary to complete a separate study of these areas in order to determine the total costs involved. This report contains the results of the investigations of the Old Rifle Site.« less
  • An engineering assessment was performed of the problems resulting from the existence of radioactive uranium mill tailings at the Spook Site, Converse County, Wyoming. Services include the performance of core drillings, soil, water and other sample analyses, radiometric measurements to determine areas with radium-contaminated materials, the evaluation of resulting radiation exposures of individuals and nearby populations, the investigation of site geology, hydrology, and meteorology and the evaluation and costing of alternative corrective actions. Radon gas release from the 187,000 tons of tailings at the Spook Site constitutes the main environmental impact, which is negligible. The two alternative actions presented aremore » better fencing of the site in its present state, and placing tailings and contaminated on-site materials and soil in the open-pit mine and covering the resulting pile with 2 ft of overburden materials. The cost estimates for the options are $81,000 and $142,000, respectively.« less
  • Ford, Bacon and Davis Utah Inc. has reevaluated the Riverton site in order to revise the December 1977 engineering assessment of the problems resulting from the existence of radioactive uranium mill tailings at Riverton, Wyoming. This engineering assessment has included the preparation of topographic maps, the performance of core drillings and radiometric measurements sufficient to determine areas and volumes of tailings and radiation exposures of individuals and nearby populations, the investigations of site hydrology and meteorology, and the evaluation and costing of alternative corrective actions. Radon gas released from the 900,000 tons of tailings materials at the Riverton site constitutesmore » the most significant environmental impact, although windblown tailings and external gamma radiation also are factors. The nine alternative actions presented in this engineering assessment range from millsite decontamination with the addition of 3 m of stabilization cover material (Option I), to removal of the tailings to remote disposal sites and decontamination of the tailings site (Options II through IX). Cost estimates for the nine options range from about $16,600,000 for stabilization in-place, to about $23,200,000 for disposal at a distance of 18 to 25 mi. Three principal alternatives for the reprocessing of the Riverton tailings were examined: heap leaching; treatment at an existing mill; and reprocessing at a new conventional mill constructed for tailings reprocessing. The cost of the uranium recovered would be about $260 and $230/lb of U/sub 3/O/sub 8/ by heap leach and conventional plant processes, respectively. The spot market price for uranium was $25/lb early in 1981. Therefore, reprocessing the tailings for uranium recovery does not appear to be economically attractive.« less
  • Ford, Bacon, and Davis Utah Inc. has reevaluated the Riverton site in order to revise the December 1977 engineering assessment of the problems resulting from the existence of radioactive uranium mill tailings at Riverton, Wyoming. This engineering assessment has included the preparation of topographic maps, the performance of core drillings and radiometric measurements sufficient to determine areas and volumes of tailings and radiation exposures of individuals and nearby populations, the investigations of site hydrology and meteorology, and the evaluation and costing of alternative corrective actions. Radon gas released from the 900,000 tons of tailings materials at the Riverton site constitutesmore » the most significant environmental impact, although windblown tailings and external gamma radiation also are factors. The nine alternative actions presented in this engineering assessment range from millsite decontamination with the addition of 3 m of stabilization cover material (Option I), to removal of the tailings to remote disposal sites and decontaminations of the tailings site (Options II through IX). Cost estimates for the nine options range from about $16,600,000 for stabilization in-place, to about $23,200,000 for disposal at a distance of 18 to 25 mi. Three principal alternatives for the reprocessing of the Riverton tailings were examined: (a) heap leaching; (b) treatment at an existing mill; and (c) reprocessing at a new conventional mill constructed for tailings reprocessing. The cost of the uranium recovered would be about $260 and $230/lb of U/sub 3/O/sub 8/ by heap leach and conventional plant processes respectively. The spot market price for uranium was $25/lb early in 1981. Therefore, reprocessing the tailings for uranium recovery does not appear to be economically attractive.« less