Hydraulic Containment of TCE Contaminated Groundwater at the DOE Portsmouth Gaseous Diffusion Plant - 8069

摘要

This paper will describe the progress of a groundwater remedial action at the Portsmouth Gaseous Diffusion Plant (PORTS), a Department of Energy (DOE) facility that enriched uranium from the early 1950s until 2000. The X-749 southern boundary hydraulic containment system, combining a four-well extraction system with a previously constructed subsurface barrier wall, has been employed at PORTS. The hydraulic containment project has been implemented as part of containment and remediation of the X-749/X-120 area trichloroethylene (TCE) contaminant.The X-749/X-120 groundwater contaminant plume is located in the south central section (Quadrant I) of the PORTS facility. The plume is associated with the former X-120 Goodyear Training Facility and a landfill known as the X-749 Contaminated Materials Disposal Facility. The principal contaminants of concern are chlorinated solvents (primarily TCE) and technetium-99 (Tc-99). A subsurface barrier wall (X-749 South Barrier Wall) was completed in 1994 at the PORTS southern reservation boundary as an interim remedial measure to slow the advancement of the leading edge of the contaminated groundwater plume or to prevent the plume from migrating off DOE property. Remedial measures identified by Ohio Environmental Protection Agency (Ohio EPA) included installation of a barrier wall around the eastern and southern portions of the X-749 landfill to provide source control and installation of a phytoremediation system to help contain groundwater flow and remove volatile organic compounds. Previous remedial measures that were implemented as elements of 'closures' on the X-749 landfill included a multimedia cap, barrier walls, and a groundwater collection system. Despite these measures, the X-749/X-120 groundwater plume has migrated beyond the southern DOE property boundary. Current TCE concentrations in off-site groundwater monitoring wells are below the preliminary remediation goal and drinking water maximum contaminant level for TCE of 5 μg/kg, but continue to increase.Hydraulic containment was selected as the method for controlling the plume at the southern DOE property boundary. Recent borings and pumping tests indicate that approximately a 400-foot section of the existing subsurface barrier wall near the DOE property boundary may been improperly keyed into the Sunbury Shale bedrock which underlies the unconsolidated uppermost Gallia sand and gravel aquifer (Gallia). This gap is reported to be as much as 4 vertical feet. In addition, the X-749 groundwater plume is migrating around the western end of the X-749 South Barrier Wall.Four groundwater extraction wells were installed at the DOE property boundary to provide hydraulic control of the plume currently flowing under and around the existing subsurface barrier wall. Placement of the new extraction wells was based on groundwater modeling and data collected from pumping tests in the area. The extracted groundwater is being sent to the on-site X-622 Groundwater Treatment Facility via subsurface piping.The hydraulic containment system began operation in June 2007. The preliminary water elevations from monitoring wells in the vicinity of two of the four extraction wells demonstrate a significant decrease in groundwater potentiometric head in the southern boundary area. The current extraction rates should be adequate to contain the leading edge of the contaminant plume. Monitoring wells in the area will continue to be sampled on a quarterly basis.