PNNL

Abstract

An in situ injection of polyphosphate amendment was conducted to address a uranium source zone for a plume near the Columbia River at the U.S. Department of Energy Hanford Site. This treatment was applied as an enhanced attenuation approach, targeting uranium in a periodically wetted zone where groundwater inundates as the water table rises in response to the seasonal high river stage. Uranium within this zone is mobilized into the groundwater at the high river stage, resulting in a persistent groundwater plume. The polyphosphate amendment creates precipitates that decrease uranium mobility so that the amount of uranium flushed into groundwater when the zone is periodically wetted is significantly decreased. With a reduced flux of uranium into the plume, the plume concentrations are expected to decrease over time. During injection, real-time geophysical monitoring was applied to provide hourly images of injected-fluid distribution. The real-time autonomous data collection, processing, and reporting enabled the electrical resistivity tomography images to provide operational feedback during injection. These data were used in conjunction with operational information (e.g., flow rate, pressures) and subsurface data from 24 monitoring wells placed within and downgradient of the treatment area (e.g., water level and specific conductance probes, uranium concentration, phosphate concentration) to assess overall performance of the treatment.