Limited Field Investigation Report for Uranium Contamination in the 300 Area, 300-FF-5 Operable Unit, Hanford Site, Washington Page: 4 of 907

                                
Executive Summary

Additional data needed for development of a Comprehensive Environmental Response, Compensa-
tion, 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 accomp-
lished 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 pg/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 encountered during drilling facilitated re-interpretation
of descriptions from earlier drilling activities. Extensive analytical work was conducted on sediment
samples collected from the continuous core recovered from each borehole, and on water samples collected
from the saturated zone at depth discrete intervals during drilling. Hydrologic testing was conducted at
multiple depth levels in each borehole to provide data on the ability of the sediment to transmit water.
Geophysical logging of the entire borehole was conducted to provide additional details on stratigraphic
features, and in an attempt to identify and quantify contaminant uranium concentrations. In addition to
uranium, new information was obtained on the unexpected presence of other contaminants of concern
(i.e., volatile organic compounds) at depths below those routinely monitored by the existing well network.

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