Laserfiche WebLink
Mr. Maurice Benson <br /> November 19, 2008 <br /> Page 2 <br /> DTSC comments are provided below: <br /> Comments: <br /> DTSC agrees with DESJC that at some point in the future a rebound test for <br /> DDJC-Tracy groundwater would provide useful and necessary information to <br /> evaluate the groundwater contaminant concentration changes without active <br /> pumping. Future rebound efforts could help to determine whether optimization <br /> efforts of the groundwater system are warranted, if a passive groundwater <br /> remedy would remain protective of human health and the environment, and still <br /> meet the remedial action objectives of the selected remedy. DTSC considers the <br /> proposal premature for the reasons listed below: <br /> Delineation of NE Banta Road TCE Plume: <br /> Currently, the TCE plume northeast of Banta Road is not defined (vertically and <br /> horizontally). The source of the TCE plume east of Banta Road originates in the <br /> EW024B/EW025B area, detected at Long-Term Monitoring Well (LM) LM151 B, <br /> and has migrated down-gradient in a north/northeast direction over one-quarter <br /> mile. Cone penetrometer testing (CPT) groundwater analytical results taken in <br /> 1994 (CPT023, CPT022), and 2001 (CPT0392, CPT0394), identify TCE above <br /> the Maximum Contaminant Level (MCL) beneath the farmer's field north of the <br /> Rose residence, indicating that the plume from EW024B/EW025B and <br /> LM157A/LM158B are not bifurcated as shown on reports, but rather one <br /> continuous plume. <br /> TCE concentrations over the MCL at both LM157A, the Aquifer Cleanup Level <br /> (ACL) Compliance Well, and LM158B, the Guard Well, indicate that the actual <br /> TCE toe-of-plume is further down-gradient and not defined. If monitored natural <br /> attenuation is the intended remedy for the TCE plume East of Banta Road, <br /> DESJC must demonstrate that the plume is stable and not migrating by defining <br /> the lateral and vertical extent of contamination with validated groundwater <br /> analytical data. <br /> The fieldwork efforts should vertically and horizontally delineate the NE Banta <br /> Road TCE plume. The associated fieldwork data should also be used to <br /> demonstrate, if possible, that natural attenuation processes are occurring for <br /> TCE plume, as no data are available to prove these mechanisms exist in this <br /> area. <br />