TECHNICAL EVALUATION OF THE INTERACTION OF GROUNDWATER WITH THE COLUMBIA RIVER AT THE DEPARTMENT OF ENERGY HANFORD SITE 100-D AREA

Groundwater beneath much of Hanford's 100 Areas is contaminated with hexavalent chromium (Cr{sup +6}) as a consequence of treating reactor cooling water to prevent corrosion. Several treatment systems are in place to remove Cr{sup +6} from the groundwater; however, these systems currently do not reduce Cr{sup +6} to concentrations below aquatic standards. Of concern is the transport of Cr{sup +6} to areas within the channel of the river, as sensitive species inhabit the river and its associated transition zone. The aquatic standard for Cr{sup +6} is currently 11 ug/l under the Record of Decision (ROD) for Interim Action and Department of Energy (DOE) currently plans to pursue remediation of the groundwater to achieve the 11 ug/l standard. Because the compliance wells used to monitor the current remediation systems are located some distance from the river, they may not provide an accurate indication of Cr{sup +6} concentrations in the water that reaches the riverbed. In addition, because salmon spawning areas are considered a high priority for protection from Hanford contaminants, it would be advantageous to understand (1) to what extent Cr{sup +6} discharged to the near-shore or river ecosystems is diluted or attenuated and (2) mechanisms that could mitigate the exposure of the river ecosystems to the discharging Cr{sup +6}. The current concentration target for Cr{sup +6} at near-river groundwater monitoring locations is 20 {micro}g/L; it is assumed that this groundwater mixes with river water that contains virtually no chromium to meet Washington Department of Ecology's (Ecology) water quality standard of 10 {micro}g/L in the river environment. This dynamic mixing process is believed to be driven by daily and seasonal changes in river stage and groundwater remediation system operations, and has been validated using analytical data from numerous groundwater samples obtained adjacent to and within the banks of the river. Although the mean mixing factor of river water and site groundwater in this zone has been estimated to be equal parts of groundwater and river water, a wide range of mixing ratios likely occurs at various times of the day and year. The degree of mixing and dilution appears to be greatly influenced by the river stage and other groundwater/surface water interaction. The extent of mixing, thus, has implications for the design and operation of the groundwater remediation systems. Improved understanding of this 'dilution' mechanism is needed to design an optimum 'systems approach' to accelerate remediation of the near-shore contaminant plumes. More information on the pathway from near-river mapped plumes to riverbed receptor locations is also needed to develop a defensible proposed plan for a future ROD for final remedial action of contaminated groundwater. In April 2008, an expert panel of scientists was convened to review existing information and provide observations and suggestions to improve the current understanding of groundwater surface water interactions in the 100 Areas (primarily focusing on 100-D Area), and to identify what additional analyses or approaches may provide critical information needed to design and implement remediation systems that will minimize impacts to river aquatic systems. Specific objectives provided to the panel included: (1) comment on approaches and methods to improve the current understanding of groundwater-surface water interactions, specifically how contaminated groundwater enters the riverbed and how this relates to remediation of chromate in the groundwater in the 100 Areas; (2) evaluate past and current data collection methods, data analysis techniques, assumptions, and groundwater transport and mixing mechanisms; (3) evaluate the current monitoring network (monitoring wells, aquifer tubes, and shoreline/river monitoring); (4) evaluate the role played by modeling; and (5) suggest additional research to fill data gaps and perform modeling