SITE INFORMATION AND CORRESPONDENCE

Home Browse Search

Mr. Maurice Benson October 24, 2008 Page 2 concentrations are below the cleanup goal under the current operational conditions. The remaining PCE mass in leachate is acting as a persistent source to groundwater thus prolonging the operation of pump-and-treat remedy until remedial goals are achieved. DTSC recommends that additional fieldwork be proposed to target the residual mass in the tighter soil formations for Area 1-1 to eliminate the ongoing persistent source to groundwater. 2. Figure 4-8, Area 1-2; Similar to comment#1, the VLEACH modeling analysis indicates that the PCE leachate in Area 1-2 will be a continuing source of groundwater contamination. VLEACH modeling calculates that Area 1-2 cleanup goals would be achieved in approximately 90 years under current operational conditions. DTSC recommends that additional fieldwork be proposed to target the residual mass in the tighter soil formations for Area 1-2 to eliminate the ongoing persistent source to groundwater. 3. Figure 2-3; Analytical results from Area 3 soil gas sampling at CP0820 show concentrations of Tricholorethene (TCE) in soil gas above the cleanup objective of 350 parts per billion volume (ppbv) with concentrations of 1900 ppbv at 7 feet below ground surface (bgs) and 1200 ppbv at 12 feet bgs. In addition, CP0827 and CP0824 recorded PCE and TCE soil gas concentrations of 920 and 560 ppbv, respectively. These soil gas concentrations correspond to localized groundwater contamination of both PCE and TCE of 5.7 pg/L and 25 pg/L, respectively. According to the Sitewide Record of Decision for DDJC-Tracy, Section 9.6.5 specifically states that vadose zone cleanup will be achieved when all the following conditions are met: 1) concentrations of PCE and TCE in soil gas are equal to or less than the cleanup standard; 2) it is demonstrated that the remaining TCE and PCE can no longer cause leachate concentrations to exceed the aquifer cleanup standards; and 3)TCE and PCE have been removed to the extent technically and economically feasible. The above-mentioned ROD requirements must be achieved prior to regulatory approval for no-further action determination at the remaining SVE sites at DDJC-Tracy. Recommendations: 1. DTSC recommends that DESJC conduct either a removal action to target the high concentrations of PCE in soil gas in Area 1 or optimize the SVE system with additional extraction wells to target the residual mass in the tighter formations.