SITE INFORMATION AND CORRESPONDENCE

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• • DON RODGERS and JUNIUS MCKELVEY 515 W. Charter Way, Stockton, California Page 2 of 3 installed wells to an established benchmark on April 21, 2003, but this data was not used in the current report to calculate a ground water flow direction. All of the wells at this site screened in the same hydrological unit that are monitored should be used for determining the ground water flow direction. If a licensed surveyor did not survey the original wells to an established benchmark when they were installed, they should be surveyed in at this time. In reporting the results of the feasibility testing, Shaw concludes that the aquifer is not able to sustain flow rates that would make groundwater extraction cost-effective, but that the addition of air sparging greatly enhanced the effectiveness of the SVE system. Based on the detection of a helium tracer injected during the test, Shaw determined that the radius of influence of air sparging would be at least 46 feet. The groundwater extraction test was performed on the newly installed well MW-4RS, which was built to a total depth of 50 feet bsg. The report states that the average sustainable extraction rate was 0.4 gallons per minute (gpm). However, the field notes from the test show rates of 2, 1, and 0.9 gpm, which do not average to 0.4 gpm. The field notes from the well development show that both wells MW-4RS and MW-2R went dry during the development purge. They also note the total depth of well MW-4RS as 47 bsg. At the May 29, 2003 quarterly monitoring event, it was noted that MW-4RS was very slow to recharge, and that it was under-pressured. The depth to water in this well was approximately five feet lower than the other wells onsite at that time. SJC/EHD has concerns with the results of the air sparge test. The average depth to water at the site is currently about 21 feet bsg, which places the top of the water table in soil comprised predominately of clay and silt. The wells proposed for air sparging are screened in sand underlying the clay/silt interval. During the test, the first wells to show detections of the helium tracer were MW-4RS and MW-4RD, which are both screened in the sand, and next were VW-2D and MW-1 R that are also screened in the sand. As helium is a very light, inert gas with little affinity to sorb, it is not surprising to have it detected quickly in these observation wells that are screened in the same highly permeable interval into which it was injected. None of the observation wells showed increases in dissolved oxygen concentrations. SJC/EHD is concerned that actual capture of total sparged air may not be possible. If sparged air is not recovered, _ groundwater concentrations may drop due to offsite or down-gradient migration of stripped hydrocarbons moved by the injected air rather than by mass reduction. In addition, the influent air samples, which Shaw concludes indicate a significant increase in captured contaminants, actually had a TPH-g concentration that fell within the range of historical data fluctuations over time. As an influent sample was not collected for laboratory analysis immediately prior to initiation of the sparge test, EHD cannot determine that the concentration detected represents a response to the sparging or is merely a normal high fluctuation concentration. Though the report states that VOC's were not detected in VW-2 or VW-3 at the start of the sparge test, it does not specify which VW-2 and VW-3 screen interval they were referring to, the shallow or deep. The information presented in Figure 5 is confusing. Please have you consultant explain where the data points shown on this figure came from. There appears to be mixing of analytical lab results with PID readings. All points on the figure should be presented with identifying collection dates and analytical method.