Contaminants of Potential Concern in the 300-FF-5 Operable Unit: Expanded Annual Groundwater Report for Fiscal Year 2004

Abstract

these estimates is caused by the need to make certain assumptions, such as the thickness of the contaminated layer. However, for the period evaluated, the trends in parameters suggest a relatively constant level of contamination, but with some variability. At the 618-11 sub-region, monitoring results since 1999 show decreasing tritium concentrations at wells closest to the source and variable concentrations at wells along the downgradient migration pathway. This plume has not reached the Energy Northwest water supply wells, nor the Columbia River. At the 316-4/618-10 sub-region, COPC are currently at levels below the drinking water standards, except for very recent samples from two wells near the 316-4 cribs excavation site that show concentrations near the 30-ug/L standard for uranium. A revised strategy for categorizing waste constituents in groundwater as a COC or COPC, along with the implications for remedial actions and regulatory decisions, is proposed. As a result, the lists developed during the remedial investigation have been shortened, primarily because of improving conditions and lack of evidence suggesting unacceptable risk. Conceptual Site Model for 300 Area Uranium. The 300 Area uranium plume can be characterized as persistent, i.e., the area and concentrations have remained similar to early 1990 conditions. There has been variability in spatial and temporal distribution patterns, primarily as a consequence of (a) cessation of liquid waste disposal to the ground, (b) large-scale source excavation activities, (c) unusually high and prolonged water table conditions during 1996 and 1997, and (d) seasonality because of river-stage fluctuations. During the most recent years, the plume appears to be relatively stable, with evidence showing gradual downgradient migration to the Columbia River. The highest concentrations observed currently are along the shoreline, and probably reflect the last significant input from beneath former major waste sites, such as the 316-5 process trenches. Uranium is lost from the plume via discharge to the river and groundwater withdrawal at a water supply well. Some amount of re-supply to the plume is believed to occur as a consequence of long-term release of uranium that has been sequestered on vadose zone and aquifer solids. The mobility of uranium and controls on dissolved concentrations are influenced by the geochemistry of the original waste effluent, the receiving sediment, and pore fluids, all of which vary in the 300 Area environment. The compositional and spatial variability leads to complexity in computer models designed for predicting plume behavior. The heterogeneity in conditions also drives the need for more field data on the locations, inventory, and geochemical characteristics of uranium in potential source zones. Conceptual Site Model for 618-11 Tritium. The tritium plume associated with the 618-11 sub-region has apparently been created by episodic release of tritium gas from irradiated materials in the burial ground. The gas interacts with moisture in the vad...

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

“…The record of decision imposed restrictions on the use of 300 Area groundwater until health-based criteria were met for uranium, tricholoroethene, and cis-1, 2-dichloroethene; of these, uranium is the most prominent contaminant (Peterson et al 2005). Most 300 Area wells are monitored at least twice per year, and a subset of wells between source areas and the river are monitored more frequently (approximately quarterly).…”

“…Most 300 Area wells are monitored at least twice per year, and a subset of wells between source areas and the river are monitored more frequently (approximately quarterly). One GPAP objective was to confirm that contaminant concentrations in the riverbank spring water do not exceed ambient water-quality criteria or established-remediation goals (DOE 2002;Peterson et al 2005). To this end, the GPAP installed aquifer tubes to collect samples at eight locations along the 300 Area shoreline (Peterson et al 2005).…”

“…One GPAP objective was to confirm that contaminant concentrations in the riverbank spring water do not exceed ambient water-quality criteria or established-remediation goals (DOE 2002;Peterson et al 2005). To this end, the GPAP installed aquifer tubes to collect samples at eight locations along the 300 Area shoreline (Peterson et al 2005). Three tubes were installed at each location: a shallow tube to sample groundwater near the water table (typically 1 to 3 m depth), a deep tube to sample as deep as logistically possible; and a "middle" tube between the shallow and deep locations.…”

“…Peterson et al (2005) reports average hydraulic conductivity values of 14,000 meters per day for the highly transmissive Hanford Site gravels, and 125 meters per day for the underlying Ringold Unit E gravels and associated sands, based on aquifer testing in 300 Area wells.…”

“…The hydraulically tight nature of the fine sand layer impedes horizontal groundwater flow, and the sand serves as a confining layer to vertical movement. The channels incised into the Ringold Formation might act as preferential pathways for groundwater flow, as these thoroughfares are filled with the less-compacted gravels and sands of the Hanford formation, which have hydraulic conductivities several orders of magnitude higher than their respective Ringold Formation counterparts (Peterson et al 2005). …”