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Former Chevron Service Station No . 98632 Page 2 of 3 575 West Grant Line Road February 19; 2015 Tracy, California 95376-2551 I The EHD does not consider advancement of only two soil borings adequate to characterize the current distribution and concentrations of the contaminants of concern in shallow soil on the site and cannot approve the Vt/orl< Plan. The EHD is of the opinion that this scope of work should be adequate to resolve several issues that will forward the case to closure. The questions of concern to the EHD are as follows: 1 . What are the current distribution and representative concentrations of the contaminants of concern at this time and can the concentrations pass the LTCP criterion for direct contact and exposure to outdoor air? Such questions cannot be answered by two borings. 2. What are the effects of natural attenuation on the concentrations of contaminants of concern in soil and what is the rate of attenuation? CRA invoked natural attenuation as likely to have reduced benzene concentrations to acceptable levels _ in Low-Threat Closure Evaluation, dated 19 June 2013; this question(s) requires comparison with previously obtained analytical data. 3. Is there a systematic distribution of contaminants of concern in shallow sail ? The EHD has contoured the benzene concentrations for shallow soil which appear to show a linear high concentration ', airway' trending northeast-southwest between borings PP-3 and MW-4; concentrations decline perpendicular to the `fairway'. The most intensely impacted soil gas probe locations occur close to or on the `fairway'. i If the high concentration `fairway' actually exists, it may represent an old utility trench or some other such linear feature. Whatever the results from the soil_samples_fr_om_pr_oposed_borings�_1_and-B-2, theyxannot-in and of themselves address questions 1 or 2, but can help address question 3 , and their proposed locations are approved. To obtain a current soil data set robust enough to address question 1 while also addressing questions 2 and 3, the EHD recommends advancing six additional soil borings as described below: e One boring adjacent to the southwest side of VP-1 and one boring adjacent to the south side of VP-4 where high benzene concentrations were detected in soil gas samples ; groundwater data show groundwater to be an unlikely source of the Z benzene, leaving soil as the potential source. Data from soil from these borings can address questions 1 and 3. ® One boring adjacent to the east side of IVR/11-3 where benzene concentrations were detected at 24 milligrams per kilogram (mg/kg) approximately six-and-a-half feet bsg; i data from this boring may address question 2 as well as questions 1 and 3. One boring adjacent to the south side of the onsite trailer in line with boring PP-3 and MW-2; and one boring in line with borings MW-2 and MW-4, approximately twenty- five feet northeast of MW-4; the data from these borings may address questions 1 and 3 along the inferred axis of the high concentration 'fair niav' ; and