Executive SummaryAdditional data needed for development of a Comprehensive Environmental Response, Compensation, and Liability Act (CERCLA) Phase III Feasibility Study to address a persistent uranium plume in 300 Area groundwater provided the stimulus for the limited field investigation (LFI) described in this report. The focus of the LFI was to determine the location and geochemical nature of the source for the uranium plume. These objectives were accomplished by drilling four new groundwater monitoring wells in the 300-FF-5 Operable Unit (OU) in fiscal year 2006 as defined in the Operable Unit Limited Field Investigation Plan (DOE 2006a). Wells 399-3-18 (C4999), 399-3-19 (C5001), 399-3-20 (C5002), and 399-1-23 (C5000) were drilled to characterize the uranium distribution in sediments in the vadose zone and the unconfined aquifer. In addition to uranium, the presence of other contaminants of concern were also evaluated.Uranium contamination in groundwater beneath the Hanford Site's 300 Area has persisted longer than predicted by modeling that was conducted during the 1990s as part of the initial remedial investigation for the 300-FF-5 Operable Unit. Even though discharge of uranium-bearing effluent to infiltration ponds and trenches ended by the mid-1980s, and removal of contaminated soil from former waste sites was accomplished in the late 1990s, the groundwater plume today continues to occupy a relatively constant area, with concentrations remaining within a fairly fixed range. Because portions of the plume exceed the drinking water standard for uranium (30 µg/L), the U.S. Department of Energy is supporting renewed remedial investigation activities and remedial action feasibility studies. The goal of this renewed effort is to find a remedy that will reduce uranium concentrations in the aquifer such that the aquifer is restored to its maximum beneficial use, i.e., as a potential supplier of drinking water.To provide the information necessary to proceed with the remedial action feasibility study and possible field treatability tests, a limited field investigation (LFI) has been conducted. The focus of the LFI was to determine the location and mobility characteristics for contaminant uranium that continues to re-supply the groundwater plume. Presumed sources include uranium remaining in the vadose zone and/or sequestered in the aquifer sediments, which interact with the fluctuating groundwater-river water. This information is fundamental for evaluating remedial action alternatives to reduce the concentration of uranium in groundwater to meet regulatory standards. New results provided by the LFI will be used in developing computer simulations of groundwater flow and uranium transport, in designing treatability field tests, and when implementing remedial action decisions.The four LFI borehole locations were chosen to represent various combinations of proximity to former waste disposal sites, proximity to the Columbia River, and wide ranging hydrogeologic features. Highly detailed descriptions of geologic features ...

A structured performance assessment approach is useful to evaluate pump-and-treat (P&T) groundwater remediation, which has been applied at numerous sites. Consistent with the U.S. Environmental Protection Agency's Groundwater Road Map, performance assessment during remedy implementation may be needed, and should consider remedy optimization, transition to alternative remedies, or remedy closure. In addition, a recent National Research Council study examined groundwater remediation at complex contaminated sites and concluded that it may be beneficial to evaluate remedy performance and the potential need for transition to alternative approaches at these sites. The intent of this document is to provide a structured approach for assessing P&T performance to support a decision to optimize, transition, or close a P&T remedy. The process presented in this document for gathering information and performing evaluations to support P&T remedy decisions includes use of decision elements to distinguish between potential outcomes of a remedy decision. Case studies are used to augment descriptions of decision elements and to illustrate each type of outcome identified in the performance assessment approach. The document provides references to resources for tools and other guidance relevant to conducting the P&T assessment. v SummaryA structured performance assessment approach is useful to evaluate pump-and-treat (P&T) groundwater remediation, which has been applied at numerous sites. Consistent with the U.S. Environmental Protection Agency's Groundwater Road Map, performance assessment during remedy implementation may be needed, and should consider remedy optimization, transition to alternative remedies, or remedy closure. In addition, a recent National Research Council study examined groundwater remediation at complex contaminated sites and concluded that it may be beneficial to evaluate remedy performance and the potential need for transition to alternative approaches at these sites. The intent of this document is to provide a structured approach for assessing P&T performance to support a decision to optimize, transition, or close a P&T remedy.The performance assessment approach described in this document is organized to use a set of decision elements to help decision makers distinguish between several categories of decision outcomes associated with transition, optimization, or closure of P&T systems. The decision elements selected for use in the P&T performance assessment include information about:

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