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i ( i Letter of Response 6194"F"Street.Banta,California Groundwater monitoring has been conducted at the site since August 1988. Concentrations of TPH, as gasoline and diesel, and BTEX have been periodically detected in five of the six monitoring wells on site. The highest reported concentration of TPH, as gasoline, in groundwater beneath the site was 2.0 parts-per-million (ppm) in well MW-1 in October 1991. The highest reported concentration of TPH, as diesel, in groundwater beneath the site was 6.2 ppm in well MW-1 in May 1991. The highest reported concentration of benzene in groundwater beneath the site was 0.15 ppm in well MW-1 in June 1989. Table 2 (Attachment 2) summarizes groundwater quality data collected at the site. This data has been submitted to your office in previous reports. As shown by the data in Table 2, the concentration of TPH, as gasoline and diesel, and benzene in January 1992 did not exceed 0.76, 1.7, and 0.0021 ppm, respectively. The data thus demonstrates concentrations of petroleum hydrocarbons in groundwater have either remained stable or decreased with time. Figure A is a graph of average concentrations of benzene through time in groundwater beneath the site. Average concentrations were calculated by summing the concentrations of benzene in samples collected from wells MW-1 through MW-5 on the dates the wells were sampled. Only samples that contained positive results for benzene were used to calculate the average. If a well was not sampled on a particular date, or if the analysis was negative for benzene, the data was not used to calculate the average. Additionally, samples from well MW-6 have never contained petroleum hydrocarbons and data from the well was not used to calculate the average. Even with the conservative use of the data, Figure A indicates concentrations of benzene in groundwater have been declining with time and are nearing the maximum contaminant level. Figure A also indicates the average concentration of benzene in groundwater has fluctuated less than 3 parts-per- billion in the past 18 months. Additionally, the data summarized in Table 2 and the distribution of petroleum hydrocarbons in groundwater, as documented in the quarterly groundwater monitoring reports, indicate the extent of petroleum hydrocarbons in groundwater has remained stable over the years. Based on the falling benzene concentrations and the stability of the extent of petroleum hydrocarbons in groundwater, a corrective action plan for the site appears unjustified at this time. Aegis requests a re-evaluation of the requirements for a corrective action plan, and that groundwater monitoring be continued until such time as the groundwater data indicate a significant change in the concentrations or extent of petroleum hydrocarbons in groundwater beneath the site. 90-099F,LTR -2-