ARCHIVED REPORTS_REMEDIAL ACTION DOCUMENT

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REMEDIATION COMPLETION REPORT-LATHROP DEHYDRATOR STATION 3.0 FIELD ACTIVITIES 3.5 Soil Excavation The goal of the excavation activity was to remove soil with petroleum hydrocarbons from around the former sump and reboiler. Results from previous investigations at the Site showed that the highest concentrations of TPH in soil were present near the former reboiler, sump,and discharge line. TPH-g was detected in soil at concentrations up to 1,800 mg/kg. The highest TPH-g concentration was detected in a sample(SB-1)collected at 6.5 feet bgs adjacent to the former reboiler. TPH in the kerosene range (TPH-k)was detected in soil at concentrations ranging up to 4,300 mg/kg.The highest concentration of TPH-k was detected in a sample collected at 9 feet bgs near the former reboiler. TPH-d was detected in soil at concentrations between 2.3 mg/kg and 35 mg/kg. Benzene was detected at four on- site locations,all at concentrations lower than 1 mg/kg.The highest benzene concentration (0.23 mg/kg)was detected in a sample collected at 9 feet bgs near the former reboiler(MW- 1). Benzene was detected in only one off-site location at a concentration of 0.019 mg/kg (approximately 20 feet west of the Site at MW-4). Low concentrations of ethylbenzene, toluene and xylenes were also detected in some borings. Sampling results have delineated the vertical extent of petroleum hydrocarbons,with the highest concentrations of petroleum hydrocarbons in soil detected within the zone of groundwater fluctuation between 6 and 12 feet bgs. The excavation area at the Site was originally proposed to be approximately 40 feet wide by 50 feet long and 12 feet deep.Approximately 800 to 1,000 cubic yards of soil were originally estimated for excavation. During excavation,soil was field-screened for the presence of petroleum hydrocarbons. Field screening included observing the excavation for suspected source areas,visual inspection of the soil for staining,and noting the presence of odors believed to be associated with petroleum hydrocarbons.In addition,a photoionization detector(PID)was used to periodically measure the presence of volatile organic compounds in soil and ambient air. On-site field representative used the field screening observations to manage the excavation activities and to guide the final excavation limits.Verification soil from the bottom and side walls of the excavation area were collected and analyzed by a mobile laboratory to confirm excavation boundaries prior to backfilling. Excavation proceeded as planned;however,field observations of staining and odor,as well as analytical soil results,necessitated additional excavation laterally and vertically.The excavation was extended as feasible within the constraints imposed by remaining structures and/or utilities.The excavation started in the northeast corner and moved westward.The northern limit of the excavation was extended as far as feasible without compromising the stability of the active odorizer tank.The northwest corner was extended to within 3 feet of well MW-7. All excavations near or around utilities were performed with direction from PG&E personnel and followed PG&E guidelines and protocols. Groundwater was observed to enter the excavation at a depth of approximately 10 feet bgs. Wet soil or soil excavated below the water line was drained in the bucket or mixed with dry soil before transfer into dump trucks.Consistent with the RAD,free groundwater was not pumped from the excavation. Approximately 2,370 cubic yards of soil were excavated and transported by DenBeste Transportation,Inc.to Forward Landfill in Manteca for disposal. This volume includes the soil removed from the former odorant storage tank and production fluids tank area.Copies of the waste manifests are included in Appendix C. SFO\LATHROPCOMPLETIONREPORT 12-1-04.DOC 7