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F w y 'i J U L 1 9 '"lt ENVIRONMENTAL HEALTH Mr. R.R.Zielinski PERMIT/2111010551 02320 0 0475 Page 2 The water samples were analyzed using modifier! EPA methods 5030/8020/8015, which measure concentrations of total petroleum hydrocarbons-as-nasoline(rPH-G),and benzene,toluene,ethylbenzene and xylenes(BTEX). Copies of the laboratory analysis reports and chain-of-custody manifest are presented in Attachment C. GROUNDWATER ANALYYICAL RESULTS Concentrations of TPH-G in the groundwater samples ranged from 240 parts per billion(ppb)to 83,000 ppb; the range for the previous quarter was from 640 to 84,000 ppb. Two mr;Aoring wells recorded substantial changes in dissolved TPH-G concentrations, MW-4 decreased frcrn 84,U04 to 38,OQC"ppb and MW-5 Increased from 30,000 to 83,000 ppb TPH-G. The benzene concentrations ranged from 0.9 to 21,400 ppb. TPH-G and benzene concentrations in groundwater for February 8, 1991 are shown in Figures 2 and 3 (Attachment A). Table 2(Attachment B),presents results of the laboratory analyses for the last five sampling events. SOiL VAPOR EXTRACTION TEST BACKGROUND Analytical results of soil samples collected from 15 soil borings, and an angle boring, Installed during the course of the subsurface Investigation,were used to define the extent hydrocarbon Impacted soils beneath the Texaco service station site. Those soil samples were analyzed by a State-certifiad laboratory. Fifteen of the samples collected contalneJ detectable concentrations of TPH-G ranging from 2 to 2,500 parts per million (ppm);9 samples contain ld detectab:e concentrations of benzene ranging from 1 to 11 ppm. Based on the laboratory analyses of the soil samples, hydrocarbon-Impacted soil appears to be confined beneath the northeast corner of the site and has been defined in all directions except to the north, underneath Charter Way. Assuming that the Impacted soil does not extend further than 20 feet north of the site, (under Charter Way),the vo€vme of impacted soil is estimated to be approximately 3,000 cubic yards. The average concentration of petroleum hydrocarbons in the soil has been estimated io be between 500 and 1,000 parts per million(ppm)yielding a total weight of hydrocarbons between 4,300 and 8,700 pounds. Since Charter Way Is StatQ Route 4 and within the Cal Trans right cf way,further subsurface exploration Is not feasible because Cal Trans does not allow this type of encroachment. Soils encountered during drilling operations consisted predominately of clay to a depth of 30 to J5 feet below ground surface(BGS). The clay is underlain by water-bearing s€It and a fine micaceous sand. SOIL VAPOR EXTRACTION PILOT TEST METHOD A soil-venting pilot test was conducted on May 21, 1991. Permisslon was obtained from the San Joaquin County Air Pollution Control District prior to conducting the test. The test was conducted to determine the radius of Influence and the concentration of hydrocarbons in the soil vapor extracted from MW-1,MW-4 and MW-S. Vapors were drawn from each extraction well by a 1.5 horsepower regenerative blower. Flow rate during the test was monitored with a Kurz model 441 air velocity meter,and wau varied between 10 and 63 cubic feet per minute (cfm). Effluent air from the blower was discharged directly to the atmosphere and concentraticns were monitored during operation with a portable photo-Ionization detector(PID). A sample from extraction wells MW-5 and MW-4 was collected in Tedlar bags and shipped with a chain-of-custody manifest to GTEL for analysis according to EPA methods 5030/8015/8020 for BTEX,and TPH-G. No sample was collected from MWA because of the presence of separate-phase petroleum in the well. l'� GROUNIMIMt•:It TFumoi.ocy.isc.