Final Remedy

Final Remedy Proposal

The September 2010 document, Feasibility Study (Study), evaluates technologies capable of remediating hexavalent chromium in groundwater to achieve clean up to adopted background levels (1.2 pg/l average Cr6 and 3.1 IJg/1 maximum Cr6). Five alternatives were evaluated in detail within the Study. The alternatives include “no action” which allows natural attenuation processes, such as dilution and dispersion, to reduce chromium concentrations in groundwater. Another alternative proposes plume containment using groundwater extraction and agricultural land treatment within the plume. Yet another alternative discusses plume-wide in-situ (below ground) remediation using multiple treatment lines. In addition, one alternative proposes a combination of the prior two alternatives. A final alternative involves physical removal of chromium from extracted groundwater in an aboveground treatment facility. This latter alternative is the only one proposing complete removal of chromium from the environment. In contrast, the first four alternatives leave chromium mass in the groundwater or in soil in levels that are not toxic.

PG&E recommends Alternative 4 for final site clean up. This alternative proposes to continue operating the in-situ technologies already in place in the Source Area, Central Area, and South Central Re-injection Area and add additional treatment locations out to the 50 ug/l total chromium plume boundary with the goal of achieving a maximum chromium concentration of 50 ug/l hexavalent chromium within this boundary. Pilot tests conducted over the past years demonstrate the ability of in-situ remediation to achieve clean up of hexavalent and total chromium to concentrations of less than 1 ug/l. In addition to groundwater extraction and land treatment currently being conducted at the Desert View Dairy, Alternative 4 proposes a new agricultural land treatment unit to the north for the purposes of plume containment. Upon reaching the maximum concentration of 50 ug/l total chromium throughout the plume area, Alternative 4 proposes to cease in-situ treatment and continue with agricultural land treatment units to contain plume migration. Natural attenuation processes are anticipated to reduce the remaining chromium concentrations to the average background level of 1.2 ug/l Cr6, with no concentration being greater than the maximum background level of 3.1 ug/l Cr6.

Project construction and operation will be completed under County permits. The estimated cleanup time for Alternative 4 to achieve the 50 ug/l total chromium drinking water standard throughout the entire plume area is 6 years. The estimated cleanup time to reach average background of 1.2 ug/l is 220 years.

This remedial action was selected by PG&E over other technologies because it appears it would be effective for remediation, its costs are less than costs for other alternatives, and would prevent significant public exposure to chromium.