Investigation of the Hyporheic Zone at the 300 Area,Hanford Site

Abstract

SummaryAt the Hanford Site in southeastern Washington State, contaminated groundwater discharges to the Columbia River after passing through a zone of groundwater/river water interaction at the shoreline (i.e., the hyporheic zone). In the hyporheic zone, river water may infiltrate the riverbank during periods of high-river stage and mix with the approaching groundwater. Contaminants carried by groundwater may become diluted by the infiltrating river water, thus reducing concentrations at locations of exposure, such as riverbank springs and upwelling through the riverbed. There have been limited studies of contaminant concentrations, physical properties, or the extent of the hyporheic zone near the Hanford Site's 300 Area, yet this zone is a major interface for discharge of groundwater contamination into the Columbia River.The Remediation Task of the Remediation and Closure Science Project conducts research to meet several objectives concerning the discharge of groundwater contamination into the river at the 300 Area of the Hanford Site in Washington State. This report documents research conducted to meet these objectives by developing baseline data for future evaluation of remedial technologies, evaluating the effects of changing river stage on near-shore groundwater chemistry, improving estimates of contaminant flux to the river, providing estimates on the extent of contaminant discharge areas along the shoreline, and providing data to support computer models used to evaluate remedial alternatives. This report summarizes the activities conducted to date, and provides an overview of data collected through July 2006. Recent geologic investigations (funded through other U.S. Department of Energy [DOE] projects)have provided a more complete geologic interpretation of the 300 Area and a characterization of the vertical extent of uranium contamination. Extrapolation of this geologic interpretation into the hyporheic zone is possible, but little data are available to provide corroboration. Penetration testing was conducted along the shoreline to develop evidence to support the extrapolation of the mapping of the geologic facies. While this penetration testing provided evidence supporting the extrapolation of the most recent geologic interpretation, it also provided some higher-resolution detail on the shape of the layer that constrains contaminant movement. Information on this confining layer will provide a more-detailed estimate of the area of riverbed that has the potential to be impacted by uranium discharge to the river from groundwater transport.Water sampling in the hyporheic zone has provided results that illustrate the degree of mixing that occurs in the hyporheic zone. Uranium concentrations measured at individual sampling locations can vary by several orders of magnitude depending on the Columbia River and near-shore aquifer elevations. This report shows that the concentrations of all the measured constituents in water samples collected from the hyporheic zone vary according to the ratio of groundwater and C...

Modulating factors of hydrologic exchanges in a large‐scale river reach: Insights from three‐dimensional computational fluid dynamics simulations

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