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® ��o LLC k Llnvaronmenm Ql] mind CcnsuMnz Phone- Q7707D 4°?IM-6227 �Q)Ko QDOD) t,'g,a Dcn't'least t. kwer on Your lavdroaav, on&I pre.590M.9. 130'soffve v hemi . Uoy 13, 4 998 Seffff Wong Sara Joaquin County, Public Health Serv6ces Environmental Health Division, Site Migaatbon MnK 304 East Weber Avenue, 3rd Floor P.O. Box 388 Phone: (209) ,Q68-0335 Stockton, CA 952® -0388 Mia FAC: (209) 468-3433 Sub�en 2KCAMMOM Von OM2 2001L TOROMO Von: Ronald Mun¢anfion Jorn0gan for�%e Vormer MSU sole Doc¢a�ed ®4 she: Uvum & Coun4ry Chevron, �3336 C;lM 88, do¢{�ei'rora{l Gla Dear Mr. Wong: The attached subsurface investigation report summarizes the results of excavating a borehole to a depth of 100 feet below ground surface (bgs). This borehole was the seventh of six previous investigatory boreholes. The reason for excavating this borehole was to further define the vertical extent of gasoline contamination in the vicinity of the highest concentrations of benzene and TPHg identified in soils during the previous Investigation. The Central Malley Regional Board required that a photo-ionization detector (PID) be used to screen the soil samples for further laboratory analyses. The POD identified 4PHg concentrations which indicated the presence of old gasoline since most of the B TEX components have long since biodegraded and/or volatilized, as was demonstrated during the initial subsurface investigation for the subject site. All samples were analyzed for gasoline oxygenates and yielded non-detectable laboratory results as did the some analyses performed during the initial subsurface investigation. Even though significant levels of benzene were identified in soil at a depth of approximately 40 feet bgs, it is clear that benzene, the indicator chemical, is not a significant threat to the beneficial uses of groundwater, because very low to non-detectable levels were identified from 40 to 100 feet bgs. A silt layer which was identified between 42 and 60 feet bgs and is most likely the mainC r factor preventing the vertical migration of contaminants to groundwater. Also, a groundwater grab sample was collected from a depth of 96.4 feet bgs which yielded a concentration of 4.3 ppb benzene. Since seepage velocity calculations indicate that this low of t level of benzene would take approximately 205 years to reach the nearest supply well, I recommend that the site be closed. Sincerely, Franklin J. Goldman a t CEO/GeoSoly, LLC Registered Geologist No. 5557 Certified Hydrogeologist No. 466