SITE INFORMATION AND CORRESPONDENCE FILE 2

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Mr. Dan Silva and Ms. Tina Mayhew 15 September 2009 West Clay Properties Page 3 of 4 639 West Clay Street, Stockton, California 95206 well before ozone injection was started in the area. The TPHg concentrations in MW-9 and EW- 1 (redrilled in late 2005) do not display a strong declining trend, except the MW-9 samples were frequently non-detect for TPHg. The data trend suggests that a different mechanism(s) had had a profound effect on these contaminant concentrations in the remediation area long before ozone injection started, perhaps the result of the over-excavation work and shifting groundwater flow directions; the data does not convincingly demonstrate the effectiveness of ozone injection on the site contaminants. The EHD recommends that ozone injection cease and that the groundwater in the area be monitored for significant rebound. Unless it can be shown that ozone injection is likely to be effective for reducing contaminant concentrations on your site, the EHD is reluctant to approve expansion of the system to deeper intervals or other areas on the site. Also, if not already completed, the effect of ozone injection on the site must be evaluated to provide data to the Central Valley Regional Water Quality Control Board (CVRWQCB) to evaluate the need to issue Waste Discharge Requirements (WDRs) before the system can be expanded or applied to other areas on the site. This may be accomplished through a bench-scale test or by collecting appropriate data from the active ozone injection area prior to rebound testing. AGE identified three potentially applicable remediation technologies for your site; ozone injection, in situ air sparging (IAS), or application of a solid chemical oxidizing agent. Prepare and submit to the EHD a feasibility evaluation of the three methods and a recommendation as to the preferred technology. As the contaminant mass estimate and distribution submitted in December 1999 is available, a rough estimate of the resource requirements of each technology should be part of the evaluation for cost effectiveness. Please submit this evaluation to the EHD by 15 December 2009. From the second bulleted point, AGE appears to conclude that installation of wells will not reduce the contamination on the site and would not be of environmental benefit, and therefore is not required or cost effective. In addition, AGE states that grab groundwater samples should be proposed for collection prior to installation of monitoring wells to satisfy the Resolution. The EHD disagrees with the general conclusion that monitoring wells are not required or cost effective; while perhaps not direct agents of contaminant mass reduction, monitoring wells are necessary for adequate plume characterization and management. The EHD directive to install additional monitoring wells was a concurrence with the AGE recommendation in the fourth quarter 2008 monitoring report to install an additional well east of MW-1; the EHD believed additional assessment of the sand interval locally present at 65 to 70 feet below surface grade (bsg) in the former UST cluster area was also needed as indicated in the EHD letter of 19 May 2009, due to the unstable plume indications coming from monitoring well MW-14B. If AGE believes that obtaining grab groundwater samples as an initial step is more cost-effective, a work plan proposing such should be submitted to the EHD for technical review. The EHD notes that grab groundwater samples collected at CPT borings 1 through 3 at depths ranging from 50 feet to 180 feet bsg were intensely impacted, but the nine wells installed (MW-16 through MW- 24), presumably sited and justified by the CPT data, yield groundwater samples that are not impacted. The CPT groundwater samples apparently were false positives induced by cross contamination from shallower groundwater. The zone of primary concern to the EHD in the former UST No.3 area is the 40- to 60-foot bsg interval screened in monitoring wells MW-1 and MW-2, the most impacted monitoring wells on the site. With the exception of MW-14B, shallower and deep monitoring wells typically are not Comment Letter 0909