Refining the site conceptual model at a former uranium mill site in Riverton, Wyoming, USA

Environmental Earth Sciences, Jul 2015

Milling activities at a former uranium mill site near Riverton, Wyoming, USA, contaminated the shallow groundwater beneath and downgradient of the site. Although the mill operated for <6 years (1958–1963), its impact remains an environmental liability. Groundwater modeling predicted that natural flushing would achieve compliance with applicable groundwater protection standards by the year 2098. A decade of groundwater monitoring indicated that contaminant concentrations were declining steadily, which confirmed the conceptual site model (CSM). However, local flooding in 2010 mobilized contaminants that migrated downgradient from the Riverton site and resulted in a dramatic increase in groundwater contaminant concentrations. This observation indicated that the original CSM was inadequate to explain site conditions and needed to be refined. In response to the new observations after the flood, a collaborative investigation to better understand site conditions and processes commenced. This investigation included installing 103 boreholes to collect soil and groundwater samples, sampling and analysis of evaporite minerals along the bank of the Little Wind River, an analysis of evapotranspiration in the shallow aquifer, and sampling naturally organic-rich sediments near groundwater discharge areas. The enhanced characterization revealed that the existing CSM did not account for high uranium concentrations in groundwater remaining on the former mill site and groundwater plume stagnation near the Little Wind River. Observations from the flood and subsequent investigations indicate that additional characterization is still needed to continue refining the CSM and determine the viability of the natural flushing compliance strategy. Additional sampling, analysis, and testing of soil and groundwater are necessary to investigate secondary contaminant sources, mobilization of contaminants during floods, geochemical processes, contaminant plume stagnation, distribution of evaporite minerals and organic-rich sediments, and mechanisms and rates of contaminant transfer from soil to groundwater. Future data collection will be used to continually revise the CSM and evaluate the compliance strategy at the site.

A PDF file should load here. If you do not see its contents the file may be temporarily unavailable at the journal website or you do not have a PDF plug-in installed and enabled in your browser.

Alternatively, you can download the file locally and open with any standalone PDF reader:

https://link.springer.com/content/pdf/10.1007%2Fs12665-015-4706-y.pdf

Refining the site conceptual model at a former uranium mill site in Riverton, Wyoming, USA

Refining the site conceptual model at a former uranium mill site in Riverton, Wyoming, USA William L. Dam 0 1 2 3 4 Sam Campbell 0 1 2 3 4 Raymond H. Johnson 0 1 2 3 4 Brian B. Looney 0 1 2 3 4 Miles E. Denham 0 1 2 3 4 Carol A. Eddy-Dilek 0 1 2 3 4 Steven J. Babits 0 1 2 3 4 0 Stoller Newport News Nuclear, Inc., a Wholly Owned Subsidiary of Huntington Ingalls Industries, Inc. , 2597 Legacy Way, Grand Junction, CO 81503 , USA 1 US Department of Energy Office of Legacy Management , 2597 Legacy Way, Grand Junction, CO 81503 , USA 2 & William L. Dam 3 Shoshone and Arapaho Tribes , 57 Deer Valley Drive, Lander, WY 82520 , USA 4 Savannah River National Laboratory, US Department of Energy Office of Environmental Management, Center for Sustainable Groundwater and Soil Solutions , Aiken, SC 29808 , USA Milling activities at a former uranium mill site near Riverton, Wyoming, USA, contaminated the shallow groundwater beneath and downgradient of the site. Although the mill operated for \6 years (1958-1963), its impact remains an environmental liability. Groundwater modeling predicted that natural flushing would achieve compliance with applicable groundwater protection standards by the year 2098. A decade of groundwater monitoring indicated that contaminant concentrations were declining steadily, which confirmed the conceptual site model (CSM). However, local flooding in 2010 mobilized contaminants that migrated downgradient from the Riverton site and resulted in a dramatic increase in groundwater contaminant concentrations. This observation indicated that the original CSM was inadequate to explain site conditions and needed to be refined. In response to the new observations after the flood, a collaborative investigation to better understand site conditions and processes commenced. This investigation included installing 103 boreholes to collect soil and groundwater samples, sampling and analysis of evaporite minerals along the bank of the Little Wind River, an analysis of evapotranspiration in the shallow aquifer, and sampling naturally organic-rich sediments near groundwater discharge areas. The enhanced characterization revealed that the existing CSM did not account for high uranium concentrations in groundwater remaining on the former mill site and groundwater plume stagnation near the Little Wind River. Observations from the flood and subsequent investigations indicate that additional characterization is still needed to continue refining the CSM and determine the viability of the natural flushing compliance strategy. Additional sampling, analysis, and testing of soil and groundwater are necessary to investigate secondary contaminant sources, mobilization of contaminants during floods, geochemical processes, contaminant plume stagnation, distribution of evaporite minerals and organic-rich sediments, and mechanisms and rates of contaminant transfer from soil to groundwater. Future data collection will be used to continually revise the CSM and evaluate the compliance strategy at the site. Conceptual site models; Uranium; Unsaturated zone; Groundwater contamination; flooding; Evaporite mineral deposits - The former uranium mill site (Riverton site) is located 3.9 km southwest of the town of Riverton, Wyoming, USA, on the Wind River Indian Reservation (Fig. 1). The mill operated from 1958 to 1963 and processed a total of 816,470 tonnes of uranium ore that was mined in the Gas Hills mining district in Wyoming (Merritt 1971). In the uranium milling process, ore was crushed and ground, and water was added to create a slurry. Mill tailings that remained after Fig. 1 Former uranium mill site is located southwest of the town of Riverton in central Wyoming on the Wind River Indian Reservation. The Little Wind River, Oxbow Lake, and wells 707, 788, and 789 are frequently discussed in the text. The institutional control boundary extraction of uranium consisted of waste solutions and sand solids, which were conveyed by slurry to a 29-hectare unlined tailings impoundment. The tailings slurry contained high concentrations of calcium, sodium, magnesium, sulfate, chloride, iron, and manganese in addition to trace elements found in the ore, including molybdenum, radium, thorium, vanadium, and residual uranium. The tailings slurry was the primary source that contaminated the shallow groundwater beneath and downgradient of the site (White et al. 1984; Narasimhan et al. 1986; DOE 1998b). Hydrogeologic setting The Riverton site lies in the Wind River Basin at an elevation of 1506 m above mean sea level and 80 km east of the Wind River Mountains, which reach elevations exceeding 3960 m. The site is located on an alluvial terrace between the Wind River 1220 m north of the site and the Little Wind River 915 m to the southeast. Figure 2 illustrates groundwater occurs in three aquifers beneath the site: (1) a shallow, unconfined aquifer, (2) a middle, semiconfined aquifer, and (3) a deeper, confined aquifer (DOE 1998b). The shallow a (...truncated)


This is a preview of a remote PDF: https://link.springer.com/content/pdf/10.1007%2Fs12665-015-4706-y.pdf

William L. Dam, Sam Campbell, Raymond H. Johnson, Brian B. Looney, Miles E. Denham, Carol A. Eddy-Dilek, Steven J. Babits. Refining the site conceptual model at a former uranium mill site in Riverton, Wyoming, USA, Environmental Earth Sciences, 2015, pp. 7255-7265, Volume 74, Issue 10, DOI: 10.1007/s12665-015-4706-y