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v ASS O C I ATE S I NI C . Groundwater samples were collected after physical parameters (pH, temperature, and conductivity) had stabilized. Dedicated disposable bailers with bottom-emptying devices were used to collect samples from the wells. Field purge logs are presented in Attachment 1. LABORATORY ANALYSES Groundwater samples were analyzed by Argon Laboratories, a State-certified laboratory, for total petroleum hydrocarbons as diesel (TPHd) utilizing EPA method 8015. Analytical results are summarized in Table 3. Laboratory data sheets and chain-of-custody documentation are included in Attachment 2. Laboratory results indicate that TPHd was detected in the groundwater sample collected from well MW4 on January 22, 2003 at a concentration of 4,400 micrograms per liter (ug/1). Analysis of water samples collected from wells MW1, MW2, MW3, MW5, MW6 and MW7 on January 22, 2003 did not detect concentrations of TPHd above laboratory-reported method detection limits. CONCLUSIONS TPHd has not been detected above the method detection limits for at least three quarters in groundwater samples collected from all wells except well MW4. Review of the laboratory analysis indicates that the aerial extent of the TPHd plume in groundwater is defined and has significantly receded when compared to results of previous groundwater sampling events. As an example of the general decrease of diesel concentrations in groundwater, a graphical representation of groundwater elevation and diesel concentrations versus time for well MW3 is shown in Figure 3. Well MW3 is located hydraulically down gradient of the former diesel underground storage tank. Diesel concentrations have rapidly decreased since the concentration peak of 1,610 ug/l on July 14, 2000. Diesel has not been detected above detection limits in groundwater samples collected from MW3 since May 31, 2002. The groundwater elevations in well MW3 did not appreciably fluctuate (less than 2 feet) during the corresponding decrease in diesel concentrations. The general decrease of diesel concentrations demonstrates that diesel source removal (free product removal) has been effective. The continued decrease in diesel concentrations in groundwater also indicates that natural attenuation is occurring at the site. SITE CLOSURE REQUEST ATC Associates Inc. previously requested regulatory approval to grant No Further Action (NFA) status for this site in a document entitled Closure Report dated March 15, 2001. The closure request was conducted using the criteria listed in the Central Regional Water Quality Control Board's (CVRWQCB)NFA requirements as outlined by the CVRWQCB (Appendix B of the Tri- Regional Board Staff Recommendations for Preliminary Investigation and Evaluation of w:162574.021quartlyllgmr2003.doo 2