SITE INFORMATION AND CORRESPONDENCE

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440 / 441 / 508 W. Charter Waytes 2 18 August 2010 Stockton, San Joaquin County ! a 0 water varied from 17 feet below ground surface (' bgs) to 21' bgs. Groundwater flow direction historically has varied based on hydrologic zone and site. The Report states "Groundwater flow direction and gradient will be presented in the next monitoring report based on the re-evaluation of zoning of the groundwater monitoring wells." Maximum groundwater concentrations (Chevron) were total petroleum hydrocarbons as gasoline (TPHg), 170,000 micrograms per Liter (ug/L); TPH as diesel (TPHd), 18,000 ug/L; benzene, 24,000 ug/L; toluene, 29,000 ug/L; ethylbenzene, 4,100 ug/L; xylenes, 22,000 ug/L; methyl tert butyl ether (MTBE), 40,000 ug/L; tert butyl ether (TBA), 5,700 ug/L; 1,2-dichlorethane (1,2- DCA), 120 ug/L; ethyl tert butyl ether (ETBE), 140 ug/L; and tert amyl methyl ether (TAME), 130 ug/L. The ARCO soil vapor extraction system was operational in pulse mode (six weeks off/ two weeks on) for 1,101 hours during the second quarter of 2010, removing approximately 26.5 pounds of petroleum hydrocarbons. The ARCO air sparging system was shut down during the second quarter of 2010, due to a blown transformer and the pulse mode of operations. The Report concludes the increase in groundwater concentrations across all four sites may indicate plume instability (migration) or be a result of biodegradation (e.g., MTBE breaks down into TBA). A site conceptual model is currently being developed to identify data gaps. Groundwater will be monitored during the next quarter to verify the increasing trends. The Report recommends continuing the current monitoring and sampling schedule for the site (semi-annual), re-surveying the wells, and submittal of the low flow purging work plan (Workplan). I concur to the recommendation for additional sampling (beyond semi-annual) proposed for the 3`d quarter of 2010, to include a combination of the wells with unstable concentration trends, any additional wells that might be helpful to.track the unstable trends, and the wells proposed for the low flow purge comparison. The well resurvey was approved during our meeting, and is appropriate considering the complexity of the sites and questions on accuracy of former surveys. The Workplan is approved, however, there is a US Environmental Protection Agency (USEPA) paper that was not mentioned in the Workplan, and describes the low flow protocol in detail; that paper will be provided to you and Stantec electronically as an attachment in a separate email. The USEPA recommends a lower flow rates (<0.5 L per minute) which might be more easily reached with a USEPA approved for Volatile Organic Compound submersible purge and sampling pump. I suggest you consider a submersible pump, as decontaminating that pump may be more cost effective than replacing bladders for the bladder pump, and the Workplan stated that using a submersible pump for the conventional purge is an option. Use of the same style of pump for each sampling method would also provide a closer comparison of each sampling method. Other concerns include the type of dedicated sample tubing suspended in the well; as certain types may absorb petroleum hydrocarbons and bias the results higher than actual concentrations. You may want to research this issue further. The water level tape needs to be suspended in the well just below water level while low flow purging; when checked periodically, the lack of an audible alarm would indicate that drawdown has occurred. Please provide a schedule of the approved additional sampling event (fieldwork and reports) to me in an email at the address below by 30 August 2010. If you have any questions, you can contact me at (916) 464-4615 or I can be reached by e-mail at b ton waterboards.ca. ov. II ' mes L. Barton, P.G. ngineering Geologist cc: See CP0057 Distribution List