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Joe Lentini • 3 _ • Former Pipelines, Tracy 5 November 2010 detection of TPH in soil at the MW-1 location, Shell opted to complete sentinel well MW-1 as a single shallow well in lieu of the originally proposed triple-screened cluster. MW-5C, the deepest well, is screened across a sand layer at about 165 feet bgs. This deep zone is separated from the shallow zone by silt units of up to 40 feet thick. Groundwater occurs at 60 feet bgs in the deep zone. This deep zone is likely a separate hydrostratographic unit based on the presence of an aquitard separating the shallow zone from the deep zone and differences in water levels between the water table and.the potentiometric surface measured in MW-5C. Because there is only one deep zone well, the flow direction and gradient could not be calculated. All monitoring wells were sampled in August, and MW-5A was re-sampled in September. The groundwater samples collected from this well contained the maximum TPHc and TPHd concentrations at 24,000 micrograms per liter (Ng/L) and 13,000 pg/L, respectively. Benzene concentrations ranged from 89 pg/L to 380 pg/L in MW-2 and MW-5A, respectively. Toluene, ethylbenzene, and xylene were not detected in any wells. Laboratory analytical results for the groundwater sample collected from MW-5C were ND. The total depth of MW-5C was based on the results of grab groundwater samples collected in February and March 2004 from two borings, B-66 and B-67, which were sampled at 171.5 and 238 feet bgs, respectively. Laboratory analytical results showed that TPHc was detected in B-66 at 74,000 pg/L and in B-67 at 380 Ng/L. Shell conducted a forensic analysis of the B-67 sample and concluded that TPHc below 171 feet bgs is naturally occurring. The main goal of sampling MW-5C was, through further forensic analysis, to confirm that TPHc below 171.5 is naturally occurring crude. However, the laboratory analytical results of the groundwater sample collected from 180 feet bgs in MW-5C was ND. Therefore, the proposed forensic analysis was not performed. Based on the results of the groundwater investigation, Shell concludes that the shallow plume is defined in each of the areas except Area 2. The Report contains no recommendations for further investigation or cleanup. Our comments are presented below. 1. The lack of petroleum contamination observed in the soil sampled in the upper 15 feet of MW-4 and MW-5 may be attributable to the soil excavation performed in 2001. The soil contamination observed below this depth indicates that the excavation was not deep enough to complete the cleanup. Therefore, the contaminated soil observed in the borings of MW-1, MW-2, MW-4 and MW-5A must be delineated and characterized in terms of its potential to further degrade groundwater. As stated in our 2 February 2010 letter, soil cleanup levels that are protective of groundwater quality may be calculated using the Central Valley Water Board's June 1989 Designated Level Methodology for Waste Classification and Cleanup Level Determinations (DLM). 2. The laboratory analytical result of ND for the groundwater sample collected from MW-5C preliminarily indicates that the vertical extent of the TPH release from the