SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Response to SJCEHD's 8-19-08 Letter--Page)--" Joe's Travel Plaza,15600 S.Harlan Road zone wells at this time (September 2008), there is nothing to suggest that further investigation of the C zone(or any deeper zone)is warranted, or that the vertical extent of the plume has not yet been defined. The SWQ►CB-CUF letter recommends the site be considered for regulatory closure. The EHD does not believe that sufficient data and information have been provided to justify a determination of "no further action is required" (closure) for this site. It must be demonstrated that the contaminant plume is defined,stable and contaminant concentrations are declining. 4,10 Attached is a table entitled "Exhibit A, Summary of Historical Contaminant Trends at Joe's Travel Plaza"which compares the highest historic concentrations of MTBE (since sampling began at the site) with current residual concentrations. The following bullet points summarize the observations and trends shown on the attached Exhibit. Also attached, as Exhibits B, C, and D, are MTBE isoconcentration maps illustrating the decrease in both aerial extent and concentration magnitude from the worst-case (hypothetical) historic plume dimensions to the current residual plume in groundwater zones A, B and C. Further, graphs illustrating concentrations of MTBE over time in select wells are included as Exhibit E. It is apparent from the attached Exhibits that MTBE concentrations have substantially declined and that consequently, the plume is naturally attenuating and shrinking. %/ • There are 22 wells at the site (15 in A zone; 4 in B zone; I screened across A and B zones, and 2 in C zone). • Four, of the wells (MW-5, 6, 14B and MWO-I) have been nondetect for all parameters for substantially all of their sampling history and should be dropped from the sampling program altogether. • With respect to MTBE, 6 of the 22 wells have been nondetect for all sampling events, and 15 wells have shown decreasing MTBE concentrations (T1 of which show a> 90% decrease). Only one well, MW-13A/B shows a statistically insignificant increase in MTBE concentrations. In addition, this increase is completely a function of the limited data set we have for groundwater in this area (in other words, if MW-13A1B had been installed 7 years ago, elevated concentrations of MTBE would likely have been observed during that period and have now attenuated in conjunction with all other wells that we can demonstrate have attenuated over time. • in Conclusion: * 94% of wells with detectable concentrations of MTBE have shown a decrease in concentrations of 59%-100%. As a further illustration of the magnitude of attenuation that has occurred, the highest concentration of MTBE ever detected at the site was 2,800 ug/L and the remaining highest concentration is only 130 ug/L (a 95%decrease); * With the exception of well MW-13A/B (in which the data set limits our ability to demonstrate attenuation over time), the above trends show a clear indication that the MTBE plume is diminishing in contaminant levels over time and consequently shrinking in size. * Exhibits A, B, C, D, and E clearly demonstrates that the site has been effectively remediated to this point(and will continue to be)by natural attenuation alone. The groundwater flow direction changed from predominantly to the northeast to the northwest.6 2