SITE INFORMATION AND CORRESPONDENCE

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i Response to Letter October 18, 1990 Former Texaco Station, Manteca, California AGS 38053-2 i water. Therefore, with the authorization of the CRWQCBCVR, the excavation was backfilled with the stockpiled soil. On November 13, 1989, we wrote a letter to the SJPHSD/EHD requesting authorization to backfill the excavation and indicating we would initiate a quarterly monitoring program to assess concentrations of hydrocarbons in the ground water over a period of one year. We indicated in this letter that if concentrations decrease over this period,we would re-evaluate and possibly recommend terminating the monitoring program. We also indicated that if hydrocarbons concentrations in ground water increase or remained above Maximum Contaminant Levels recommended by the California Department of Health Services, we would evaluate the need for active remediation. On December 20, 1989, an Applied GeoSystems geologist conducted the first quarterly. monitoring event. The direction of groundwater flow Across the site was evaluated to be to the northwest, and the average gradient was evaluated to be 0.0012. Results of the laboratory analyses of the ground water from the monitoring wells indicated that, in e�neral, the concentrations of hydrocarbons had decreased since the preliminary investigation (Applied GeoSystems, January 24, 1989). On March 29, 1990, ,an Applied GeoSvstems geologist conducted the second quarterly sampling event, The direction of ground-water flow. beneath the site was evaluated to be slightly west of north, and the average gradient was evaluated to be 0.0012. Results of laboratory analyses of the ground water from the wells indicated that since the December 20, 1989, sampling event, concentrations of TPHg, total petroleum hydrocarbons as diesel fuel (TPHd), and BTEX had decreased or remained below detection levels in monitoring wells MW-1 and MW-3 and increased in monitoring well MW-I On July 2, 1990,:.,an Applied GeoSystems geologist conducted the third quarterly sampling event. The direction of ground-water flow beneath the site was evaluated to be slightly west of north,'`and the average gradient was evaluated to be 0.0019. Results of laboratory analyses of the ground water from the wells indicated that since March 1990, concentrations of TPHg, TPHd, and BTEX had decreased or remained below detection limits in each monitoring well. The concentrations of TPHg and BTEX in samples collected from each monitoring well were below detection limits for the analyses used_ Results of laboratory analyses of water samples showed 0.28 ppm and 0.48 ppm TPHd in water samples collected from wells MW-2 and MW-1, respectively. TPHd was not detected in water collected from monitoring well MW-3. Previous work performed by Applied GeoSystems and Canonie is documented in the reports cited in the attached references list. The cumulative results of chemical analyses of water samples are summarized in Table 1. 3 - APPlied GeoSystems