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Title: Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater-river interaction zone

Abstract

We examine subsurface uranium (U) plumes at two U.S. Department of Energy sites that are located near large river systems and that are influenced by groundwater-river hydrologic interaction. Following surface excavation of contaminated materials, both sites were projected to naturally flush remnant uranium contamination to levels below regulatory limits (e.g., 30 µg/L or 0.126 µmol/L; U.S. EPA drinking water standard), with 10 years projected for the Hanford 300 Area (Columbia River) and 12 years for the Rifle site (Colorado River). The rate of observed uranium decrease was much lower than expected at both sites. While uncertainty remains, a comparison of current understanding suggests that the two sites have common, but also different mechanisms controlling plume persistence. At the Hanford 300 A, the persistent source is adsorbed U(VI) in the vadose zone that is released to the aquifer during spring water table excursions. The release of U(VI) from the vadose zone and its transport within the oxic, coarse-textured aquifer sediments is dominated by kinetically-limited surface complexation. Modeling implies that annual plume discharge volumes to the Columbia River are small (< one pore volume). At the Rifle site, slow oxidation of naturally reduced, contaminant U(IV) in the saturated zone and a continuousmore » influx of U(VI) from natural, up-gradient sources influences plume persistence. Rate-limited mass transfer and surface complexation also control U(VI) migration velocity in the sub-oxic Rifle groundwater. Flux of U(VI) from the vadose zone at the Rifle site may be locally important, but it is not the dominant process that sustains the plume. A wide range in microbiologic functional diversity exists at both sites. Strains of Geobacter and other metal reducing bacteria are present at low natural abundance that are capable of enzymatic U(VI) reduction in localized zones of accumulated detrital organic carbon or after organic carbon amendment. Major differences between the sites include the geochemical nature of residual, contaminant U; the rates of current kinetic processes (both biotic and abiotic) influencing U(VI) solid-liquid distribution; the presence of detrital organic matter and the resulting spatial heterogeneity in microbially-driven redox properties; and the magnitude of groundwater hydrologic dynamics controlled by river-stage fluctuations, geologic structures, and aquifer hydraulic properties. The comparative analysis of these sites provides important guidance to the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by surface water interaction that are common world-wide.« less

Authors:
; ; ; ; ; ; ; ; ; ; ; ;
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
Sponsoring Org.:
USDOE
OSTI Identifier:
1080143
Report Number(s):
PNNL-SA-88773
42319; KP1702030; KP1702030
DOE Contract Number:  
AC05-76RL01830
Resource Type:
Journal Article
Journal Name:
Journal of Contaminant Hydrology, 147:45-72
Additional Journal Information:
Journal Name: Journal of Contaminant Hydrology, 147:45-72
Country of Publication:
United States
Language:
English
Subject:
groundwater-surface water interaction, surface complexation, microbial redox transformations, uranium biogeochemistry.; Environmental Molecular Sciences Laboratory

Citation Formats

Zachara, John M., Long, Philip E., Bargar, John, Davis, James A., Fox, Patricia M., Fredrickson, Jim K., Freshley, Mark D., Konopka, Allan, Liu, Chongxuan, McKinley, James P., Rockhold, Mark L., Williams, Kenneth H., and Yabusaki, Steven B. Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater-river interaction zone. United States: N. p., 2013. Web. doi:10.1016/j.jconhyd.2013.02.001.
Zachara, John M., Long, Philip E., Bargar, John, Davis, James A., Fox, Patricia M., Fredrickson, Jim K., Freshley, Mark D., Konopka, Allan, Liu, Chongxuan, McKinley, James P., Rockhold, Mark L., Williams, Kenneth H., & Yabusaki, Steven B. Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater-river interaction zone. United States. https://doi.org/10.1016/j.jconhyd.2013.02.001
Zachara, John M., Long, Philip E., Bargar, John, Davis, James A., Fox, Patricia M., Fredrickson, Jim K., Freshley, Mark D., Konopka, Allan, Liu, Chongxuan, McKinley, James P., Rockhold, Mark L., Williams, Kenneth H., and Yabusaki, Steven B. 2013. "Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater-river interaction zone". United States. https://doi.org/10.1016/j.jconhyd.2013.02.001.
@article{osti_1080143,
title = {Persistence of uranium groundwater plumes: Contrasting mechanisms at two DOE sites in the groundwater-river interaction zone},
author = {Zachara, John M. and Long, Philip E. and Bargar, John and Davis, James A. and Fox, Patricia M. and Fredrickson, Jim K. and Freshley, Mark D. and Konopka, Allan and Liu, Chongxuan and McKinley, James P. and Rockhold, Mark L. and Williams, Kenneth H. and Yabusaki, Steven B.},
abstractNote = {We examine subsurface uranium (U) plumes at two U.S. Department of Energy sites that are located near large river systems and that are influenced by groundwater-river hydrologic interaction. Following surface excavation of contaminated materials, both sites were projected to naturally flush remnant uranium contamination to levels below regulatory limits (e.g., 30 µg/L or 0.126 µmol/L; U.S. EPA drinking water standard), with 10 years projected for the Hanford 300 Area (Columbia River) and 12 years for the Rifle site (Colorado River). The rate of observed uranium decrease was much lower than expected at both sites. While uncertainty remains, a comparison of current understanding suggests that the two sites have common, but also different mechanisms controlling plume persistence. At the Hanford 300 A, the persistent source is adsorbed U(VI) in the vadose zone that is released to the aquifer during spring water table excursions. The release of U(VI) from the vadose zone and its transport within the oxic, coarse-textured aquifer sediments is dominated by kinetically-limited surface complexation. Modeling implies that annual plume discharge volumes to the Columbia River are small (< one pore volume). At the Rifle site, slow oxidation of naturally reduced, contaminant U(IV) in the saturated zone and a continuous influx of U(VI) from natural, up-gradient sources influences plume persistence. Rate-limited mass transfer and surface complexation also control U(VI) migration velocity in the sub-oxic Rifle groundwater. Flux of U(VI) from the vadose zone at the Rifle site may be locally important, but it is not the dominant process that sustains the plume. A wide range in microbiologic functional diversity exists at both sites. Strains of Geobacter and other metal reducing bacteria are present at low natural abundance that are capable of enzymatic U(VI) reduction in localized zones of accumulated detrital organic carbon or after organic carbon amendment. Major differences between the sites include the geochemical nature of residual, contaminant U; the rates of current kinetic processes (both biotic and abiotic) influencing U(VI) solid-liquid distribution; the presence of detrital organic matter and the resulting spatial heterogeneity in microbially-driven redox properties; and the magnitude of groundwater hydrologic dynamics controlled by river-stage fluctuations, geologic structures, and aquifer hydraulic properties. The comparative analysis of these sites provides important guidance to the characterization, understanding, modeling, and remediation of groundwater contaminant plumes influenced by surface water interaction that are common world-wide.},
doi = {10.1016/j.jconhyd.2013.02.001},
url = {https://www.osti.gov/biblio/1080143}, journal = {Journal of Contaminant Hydrology, 147:45-72},
number = ,
volume = ,
place = {United States},
year = {2013},
month = {4}
}