Site observational work plan for the UMTRA project site at Grand Junction, Colorado

Abstract

The U.S. Department of Energy (DOE) has prepared this initial site observational work plan (SOWP) for the Uranium Mill Tailings Remedial Action (UMTRA) Project site in Grand Junction, Colorado. This SOWP is one of the first UMTRA Ground Water Project documents developed to select a compliance strategy that meets the UMTRA ground water standards (40 CFR Part 192, as amended by 60 FR 2854) for the Grand Junction site. This SOWP applies information about the Grand Junction site to the compliance strategy selection framework developed in the UMTRA Ground Water Project draft programmatic environmental impact statement (PEIS) (DOE, 1995a). This risk-based, decision-making framework identifies the decision logic for selecting compliance strategies that could be used to meet the ground water standards. The DOE goal is to implement a cost-effective site strategy that complies with the ground water standards and protects human health and the environment. Based on an evaluation of the site characterization and risk assessment data available for the preparation of this SOWP, DOE proposes that the most likely compliance strategy for the Grand Junction site is no remediation with the application of supplemental standards. This proposed strategy is based on a conceptual site model that indicates site-related contamination is confined to a limited-use aquifer as defined in the ground water standards. The DOE developed the conceptual site model by evaluating available site-specific and regional data. There are two aquifers beneath the site: a surficial, unconfined alluvial aquifer underlain by a shale formation, which acts in part as an aquitard, and a deeper confined sandstone aquifer underlying the shale. There is an upward vertical gradient from the confined aquifer to the unconfined alluvial aquifer. The conceptual model demonstrates that the uranium processing-related contamination at the site has affected the unconfined alluvial aquifer, but not the deeper confined aquifer. The contamination in the alluvial aquifer appears to be migrating west and southwest of the site until the ground water eventually discharges to the Colorado River. Evaluation of DOE data and studies published by others indicate that ground water in the alluvial aquifer is of limited use with the total dissolved solids in the alluvial aquifer most likely exceeding 10,000 milligrams per liter and widespread, ambient contamination that did not result from activities involving residual radioactive materials. There is no current known use of ground water crossgradient or downgradient of the site in the area impacted by uranium processing activities. Assessments of human health and ecological risk indicate that use of background ground water could have adverse effects. Additional data still are needed to confirm that the alluvial aquifer qualifies for supplemental standards and that supplemental standards will be protective of human health and the environment. These data will help define the regional background ground water quality, the impacts of recharge fr...