Utilizing Isotopic Uranium Ratios in Groundwater Evaluations at NFSS
- U.S. Army Corps of Engineers, Buffalo District, 1776 Niagara Street, Buffalo, NY 14207 (United States)
- Argonne National Laboratory, Environmental Science Division, 9700 South Cass Avenue, Building 900, Argonne, IL 60439 (United States)
The U.S. Army Corps of Engineers (USACE) Buffalo District is currently evaluating environmental contamination at the Niagara Falls Storage Site (NFSS) under the Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) as part of its Formerly Utilized Sites Remedial Action Program (FUSRAP). The NFSS is located in the Town of Lewiston in western New York and has been used to store uranium-contaminated materials since 1944. Most of the radioactive materials are currently contained in an on-site structure, but past contamination remains in soil and groundwater. As a naturally occurring radionuclide, uranium is present in all groundwater. Because contamination levels at the site are quite low, it can be difficult to distinguish zones that have been impacted by the past releases from those at the high end of the natural background range. The differences in the isotopic ratio of uranium-234 (U-234) to uranium-238 (U-238) between natural groundwater systems and affected areas are being used in an innovative way to better define the nature and extent of groundwater contamination at NFSS. In natural groundwater, the ratio of U-234 to U-238 exceeds 1 due to the alpha particle recoil effect, in which U-234 is preferentially mobilized to groundwater from adjacent rock or soil. This process is very slow, and it can be hundreds to thousands of years before a measurable impact is seen in the isotopic ratio. Thus, as a result of the recoil effect, the ratio of U-234 to U-238 will be higher in natural groundwater than in contaminated groundwater. This means that if site releases were the source of the uranium being measured in groundwater at NFSS, the ratio of U-234 to U-238 would be expected to be very close to 1 (the same ratio that exists in wastes and soil at the site), because not enough time has elapsed for the alpha particle recoil effect to have significantly altered that ratio. From an evaluation of site and regional groundwater data, an isotopic ratio of 1.2 has been identified as a site-specific signature to help distinguish natural groundwater (e.g., at the high end of the background range) from zones impacted by past releases. This information is crucial for focusing the ongoing CERCLA evaluation and decision making process. This signature value is not applied as a bright line, e.g., to define samples with ratios of U-234 to U-238 above 1.2 as representing background and those with ratios below 1.2 as being affected by site releases. Rather, this ratio serves as a weight of evidence for use in conjunction with other site information, including historical activities, to form science-based decisions regarding contaminated groundwater. This novel approach for developing a groundwater signature from the isotopic uranium ratio has proven to be a very useful tool for NFSS, and it is now being considered for broader application. (authors)
- Research Organization:
- WM Symposia, Inc., PO Box 13023, Tucson, AZ, 85732-3023 (United States)
- OSTI ID:
- 21208776
- Report Number(s):
- INIS-US-09-WM-06350; TRN: US09V1087079563
- Resource Relation:
- Conference: Waste Management 2006 Symposium - WM'06 - Global Accomplishments in Environmental and Radioactive Waste Management: Education and Opportunity for the Next Generation of Waste Management Professionals, Tucson, AZ (United States), 26 Feb - 2 Mar 2006; Other Information: Country of input: France; 8 refs
- Country of Publication:
- United States
- Language:
- English
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