Table 2.1. Stratigraphy of the Vadose Zone Beneath the SX Tank Farm. Stratigraphic Symbol (a) Formation Facies/Subunit Description Genesis Holocene/Fill NA Backfill Poorly sorted gravel to medium sands and silt derived from the Hanford formation (Price and Fecht, 1976a) Anthropogenic Unit H1a-gravelly sand Upper coarse-grained sequence equilvalent to Johnson et al.'s (1999) "Hanford Gravel Unit B" and Sobczyk's (2000) "Hanford Unit B" H1a Unit H1a-slightly silty sand Upper fine sand and silt sequence. Equivalent to "Hanford silty sand" of Sobcyzk (2000) H1 Unit H1 Lower coarse-grained sequence equivalent to "Gravel Unit A" described by Johnson et al. (1999) and "Hanford Unit A" described by Sobcyzk (2000).

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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 ...

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Microbial diversity in subsurface sediments at the Hanford Site 300 Area near Richland, Washington state (USA) was investigated by analysing 21 samples recovered from depths of 9-52 m. Approximately 8000 near full-length 16S rRNA gene sequences were analysed across geological strata that include a natural redox transition zone. These strata included the oxic coarse-grained Hanford formation, fine-grained oxic and anoxic Ringold Formation sediments, and the weathered basalt group. We detected 1233 and 120 unique bacterial and archaeal OTUs (operational taxonomic units at the 97% identity level) respectively. Microbial community structure and richness varied substantially across the different geological strata. Bacterial OTU richness (Chao1 estimator) was highest (> 700) in the upper Hanford formation, and declined to about 120 at the bottom of the Hanford formation. Just above the Ringold oxic-anoxic interface, richness was about 325 and declined to less than 50 in the deeper reduced zones. The deeper Ringold strata were characterized by a preponderance (c. 90%) of Proteobacteria. The bacterial community in the oxic sediments contained not only members of nine well-recognized phyla but also an unusually high proportion of three candidate divisions (GAL15, NC10 and SPAM). Additionally, 13 novel phylogenetic orders were identified within the Deltaproteobacteria, a clade rich in microbes that carry out redox transformations of metals that are important contaminants on the Hanford Site.