SITE INFORMATION AND CORRESPONDENCE

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Remo Canepa Page 2 of 4 6230 Pacific Avenue March 5, 2012 Stockton , CA • The presence of highly volatile organic compounds in groundwater, but a lack of the same in soil gas samples purportedly showing that the release occurred circa 2002, with biodegradation of the organics occurring in the shallow soil zone , but with less degradation in the saturated zone ; and • Recent UST system pressure testing demonstrating that the purported second UAR has been stopped , but not before the second UAR occurred . The EHD has considered the lines of evidence and has determined that some are consistent with the two-release model [(pre- 1993 and post-1993) , but none conclusively demonstrate that two separate releases occurred . The EHD evaluation of the eight lines of evidence is as follows: • The EHD , vendor and on-site maintenance records demonstrate that some system components had integrity breaches such as cracks and ' leaks ' , however the presence of a crack or ' leak' does not necessarily mean that a release had occurred . Only one of the cited incidents noted the presence of liquid , but the liquid was not identified or described. While not demonstrating that a second release had occurred , the given UST system history does show that there was the potential for such a release to occur. • The EHD has interpreted the concentration trends in MW-1 previously , noting that the decline of overall concentrations appeared to correlate with increasing groundwater elevation rising significantly above the screened interval of the well , and the EHD concurs with AGE that the significant increase between January 2005 and January 2008 was a result of the groundwater extraction (GWE) operation . The EHD does not note an increasing concentration trend immediately prior to initiation of GWE in MW- 1 ; overall MTBE concentrations had been declining since July 1999 and the increase of TPHg correlates closely to rising groundwater elevation . The EHD considers the MW-1 concentration trend to be neutral as far as indicating a post-1993 UAR . • The EHD is not bound by `precedent' of data interpretations made for other sites for a variety of reasons , the most pertinent being that such interpretations are site-specific and entail more than just the one line of data . In the case cited , groundwater samples had been analyzed for MTBE prior to the inferred release , and those samples yielded non-detect or very low MTBE concentrations that compared sharply with the inferred second UAR . In your case , there is no MTBE groundwater data collected immediately after the UST system removal and replacement in 1993 that demonstrates insignificant or no detectable MTBE concentrations . The earliest MTBE data are from groundwater samples collected in 1996 , when MTBE was 480 micrograms per liter (Ng/L) , and groundwater was approximately 7 feet over screen in MW- 1 . Shallower groundwater in nearby SV-4 was significantly impacted when first sampled in 1998 , but the MW- 1 MTBE concentrations did not markedly increase except when groundwater extraction pulled in more impacted groundwater, probably from shallower depths . • The EHD does not consider the increase of contaminant concentrations in MW-1 during the period of GWE , putatively masking a concentration increase due to a second UAR to be actual evidence of a second UAR ; sometimes you don 't see the signal because there is no signal . • The EHD has responded previously to the inference of a second UAR based on the presence of butane , pentane and isooctane; the technical issues and questions raised by the EHD related to these data have not been addressed , and therefore the EHD does not consider the data to provide strong support to the second release model . The EHD notes that the monitoring well most impacted by butane and pentane ( Figures 5 and 6) UAR Request Response Letter 0312