Plan for the characterization and remediation of chromium plume west of the 100-D reactor

Sampling of the Columbia River substrate pore water in November 1995 revealed chromium concentrations in excess of 630 microg/L in the river bed offshore of the 100-D/DR area. Shoreline drive point sampling at multiple depths also revealed chromium concentrations increasing with depth to a maximum of 869 microg/L at approximately 3.05 m below the water table. Because the 1907-DR Process Sewer Outfall is located at the point where the highest concentrations of chromium were found, it was thought that the sewer outfall could be the source of chromium for these drive point and pore water samples. Drive point sampling and substrate pore water sampling upstream of the 1907-DR Process Sewer Outfall also revealed elevated concentrations of chromium. The drive point sampling revealed chromium concentrations were as high as 414 microg/L at a location 457 m upstream of the 1907-DR Process Sewer Outfall, while the substrate pore water samples near the same location had a chromium concentration of 68 microg/L. Since the 1907-DR Process Sewer Outfall is within 60 m of the Columbia River, it is unlikely that chromium from this source could have moved that far upgradient. Therefore, the question remained: Why are there elevated concentrations of chromium along this shoreline? To help answer this question, a single well was drilled approximately 152 m inland from the drive point sample with the highest chromium (869 microg/L). The groundwater sampling results from the new well showed a chromium concentration of approximately 1030 microg/L. This well identified groundwater as the source of chromium in the pore water and drive point samples. This document presents the chemical sampling results from well 199-D4-1 and the plans for gathering data to identify the areal extent of the chromium plume for a possible remedial action