GROUNDWATER AND THE HYPORHEIC ZONE OF THE COLUMBIA RIVER, HANFORD SITE, WASHINGTON

Groundwater beneath the Hanford Site contains chemicals and radionuclides from past operations associated with plutonium production. Some contamination discharges into the Columbia River, which is a gaining stream as it crosses the Site. The Hanford Reach of the river is considered excellent in water quality, and is used for municipal water supplies and recreational activities. Degradation of water quality of the main stream is negligible, because of the low rate of groundwater discharge when compared to the flow of the river. However, the riverbed supports important habitat, e.g., gravelly substrate used by fall Chinook salmon for spawning. Thus, water quality in the hyporheic zone is an important consideration when evaluating remedial action alternatives for groundwater plumes.

Because of daily and seasonal river stage cycles (range one-half to several meters, respectively), a dynamic zone of interaction is present between the groundwater and river systems. Field research and computer simulation activities have shown this zone to be characterized by rapidly changing gradients and flow patterns; layering and/or mixing of river water with groundwater; and variable water chemistry. Mixing with river water dilutes contaminants that reach the hyporheic zone, and river water characteristics may enhance sorption to aquifer solids for some dissolved contaminants, thus reducing their mobility.

Examples of these processes are shown using a uranium plume that enters the river in an area where waste effluent from nuclear fuel fabrication activities have contaminated the vadose zone and aquifer. Near the river, the plume is monitored by analyzing samples from wells, plastic tubes implanted in the aquifer along the shore, and riverbank springs. Impacts to the river ecosystem are evaluated using samples of river water and channel sediment, and by monitoring biota. Clams are effective indicators of where uranium-contaminated groundwater upwells through the riverbed. Computer simulations of groundwater flow and uranium transport provide estimates for rates associated with discharge from the aquifer into the river. The end product of these investigations is a comprehensive characterization of a contaminant plume, which can be used in risk assessments and in selecting appropriate and effective remedial actions.