SITE INFORMATION AND CORRESPONDENCE_CASE 1

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M. Scott Mansholt - 2 - 28 July 2008 Chevron OVP Vernalis Offsite 31 feet bgs. SAIC concluded the downgradient extent of affected soil was adequately defined. • Analysis of groundwater constituents reported TPH as diesel (TPHd) at concentrations of 160 micrograms per liter (pg/L) and 240 pg/L, in samples from SB-26 and SB-33 respectively. The other remaining samples were ND at the detection limit for TPHd. BTEX was ND at respective detection limits for all samples. SAIC concluded that this was at or near the downgradient extent of affected groundwater. • PAHs were ND at respective detection limits for five of eight groundwater samples (including a duplicate from SB-31) with the exception of two PAHs that slightly exceeded the Environmental Screening Levels (ESLs)' of 0.029 pg/L for benzo(a)anthracene and benzo(a)fluoranthene. Analytical results for SB-33 reported benzo(a)anthracene detected at a concentration of 0.039 pg/L. Analytical results for borings SB-28, 31, and 33 reported concentrations of benzo(a)fluoranthene detected at 0.039, 0.031, and 0.038 pg/L respectively. The Water Quality Objective 2 (WQO) for the PAH compounds is 0.04 pg/L as cited in the Basin Plan. SAIC stated that since the results were only slightly greater than the ESLs and less than the WQO, additional PAH characterization was not needed. (The minimum method detection limits used by the laboratory for these compounds was 0.030 pg/L). • SAIC indicated that since 240 pg/L TPHd was detected in groundwater from SB-33, is downgradient of the former TAOC Ohm Pump Station property, additional groundwater characterization associated with this boring would be addressed as part of the future investigation(s) proposed for this site.- • SAIC recommended that groundwater sampling be reduced to semi-annual frequency to coincide with the former Vernalis Pump Station monitoring and sampling schedule. • SAIC recommended that a Human Health Risk Assessment be prepared as the next step to move the project to a No Further Action Requested determination. Based on our review, Regional Water Board staff has the following comments: • We concur that affected soil and groundwater has been delineated on the Vernalis Offsite property and needs no further characterization, and that additional groundwater sampling for PAH analysis is not needed. • The extent of affected groundwater at SB-33 should be evaluated as part of the former TAOC Ohm Pump Station characterization to be conducted this year. • A Human Health Risk Assessment may be completed. 1 Screening for Environmental Concerns at Sites with Contaminated Soil and Groundwater, Interim Final November 2007, San Francisco Bay Regional Water Quality Control Board. 2 Central Valley Regional Water Quality Control Board - Water Quality Control Plan (Basin Plan) for the Sacramento River Basin and the San Joaquin River Basin, 4th edition(revised August 2006).