SITE INFORMATION AND CORRESPONDENCE

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Joe Lentini - 2 - 8 April 2010 Shell Mountain House CRA installed 15 monitoring wells (MW-2 through MW-16) and one soil vapor well (SVP-1-6/16) in Neighborhoods C and D, and 19 monitoring wells (MW-1 through MW-19) in Neighborhood B. The monitoring wells were generally screened between 20 and 40 feet bgs. The Report states that the soil vapor well will be sampled during the first quarter. However, in verbal communication on 2 March 2010, CRA estimated that the sampling would be delayed to April 2010 to allow field conditions to dry out sufficiently. Shell concluded that the contaminants in soil have been vertically and laterally delineated since TPH in the deepest samples in all borings was below detection limits, including MW-6. Shell had previously excavated soil from this area to 18 feet bgs. Groundwater sampling was conducted in September 2009. Dept to static groundwater ranged from about 27 to 35 feet bgs. Samples were submitted for laboratory analysis of TPHd, TPHc, benzene, toluene, ethylbenzene, and total xylenes (collectively BTEX), naphthalene, sulfate and sulfur. No TPHc, TPHd, BTEX or naphthalene was detected in any of the groundwater samples. Maximum sulfur and sulfate concentrations included 78.8 milligrams per liter (mg/L) and 220 mg/L, respectively. The highest concentrations were detected in Neighborhoods C and D. Groundwater flow generally varied between the northeast and northwest at gradients that ranged from 0.002 to 0.006. As requested by Central Valley Water Board staff, Shell conducted a background analysis of the soil and groundwater sulfate and sulfur data to evaluate if the presence of this contaminant was attributable to the crude oil release. The comparisons show that sulfate and sulfur concentrations are similar between background and source area samples. The 26 March 2010 Groundwater Monitoring Report Fourth Quarter 2009 (Fourth QMR) summarized the results of groundwater monitoring conducted on 29 and 30 December 2009. Groundwater generally flowed to the northwest in Neighborhood B and to the north-northeast in Neighborhoods C and D at gradients ranging from 0.002 to 0.005 feet per foot. All groundwater samples from Neighborhoods B, C, and D were non-detect (ND) for petroleum hydrocarbons. Groundwater monitoring for sulfur and sulfate was not conducted. Due to access restriction caused by seasonal wet weather, SVP-1-6/16 could not be sampled. Our comments are presented below. 1. We concur that the groundwater monitoring results confirm that sulfur and sulfate are ubiquitous and naturally occurring in groundwater in this area. 2. The ND groundwater monitoring results appear to indicate that the most of the petroleum hydrocarbon pollution mass is either sorbed to the porous media solids or generally insoluble. We will require Shell to conduct monitoring and submit reports for at least four consecutive quarters. If monitoring results are consistently ND or below water quality objectives, Central Valley Water Board staff will consider developing a regulatory decision for this Site. 3. Page 2 of the Fourth QMR ends in an incomplete sentence that appears to have been addressing soil vapor sampling at SVP-1-6/16. Since this proposed sampling was delayed due to weather and field conditions, we anticipate that the sampling will be conducted some time during the second quarter when drier weather conditions prevail.