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UNK-1010-M-1 Quarterly Monitoring, Second Quarter 1990 October 31, 1990 Former Unocal Station No. 289, Stockton, California AGS 38012-4 Applied GeoSystems began an environmental investigation in conjunction with underground- storage tank removal in March and April 1988. During this investigation, a geologist observed removal of the underground storage tanks and collected soil samples for laboratory analysis of hydrocarbons. Results of analyses indicated that hydrocarbons were present in the soil underlying the tank cavity(Applied GeoSystems,April 28, 1988,Report No.038012- 1). In June 1988, we observed the drilling of three soil borings (B-1 through B-3) and installation of three ground-water monitoring wells (MW-1 through MW-3) in the borings (Plata P-2). Soil and ground-water samples were collected for laboratory analysis of hydrocarbons. In August 1989,soil borings B-4 through B-6 were drilled onsite and offsite and two monitoring wells were installed in two of the borings(Plate P-2). Soil and ground- water samples were collected for laboratory analyses. Hydrocarbons were detected in soil and groundwater samples collected during both site visits. In response to the detection of hydrocarbons in soil and ground water,Applied GeoSystems began periodic monitoring of the ground water in June 1938. Results of previous monitoring events indicated that hydrocarbons were detected in water samples collected from the five wells;the highest concentrations of hydrocarbons were detected in wells MW-2 and MW-3. Ground-Water Sampling An Applied GeoSystems geologist was onsite May 30, 1990,to collect groundwater samples from wells MW-1 through MW-5 and examine the samples for subjective evidence of hydrocarbons using procedures summarized in an attachment to this report. The geologist noted no subjective evidence of hydrocarbons. Cumulative results of subjective analyses are presented in Table 1. The geologist purged the wells and collected samples from each well for laboratory analyses. A Chain of Custody Record was initiated in the field and accompanied the samples to the laboratory. A copy of that record is attacheri. The methods of ground-water sampling are discussed in an attachment to this report. The purge water was stored onsite in properly labeled 17-E steel drums pending the results of laboratory analyses. The water was subsequently removed by Armour Petroleum of Fairfield, California, and recycled. Laboratory Analysis sis ! Tire samples were analyzed for total petroleum hydrocarbons as g&.&line (TPHg) using modified Environmental Protection Agency (EPA) Method 8015 and benzene, toluene, —...----=---_ -.-..--ethylbenzene;-and-total--xylene isomers-(BTEX) using-EPA- Method-602.----Laboratory.------------- --------- 2 A,OPAad CeoSystsms