SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr.Russell Chapin 1766 W.Monte Diablo Avenue Page 6 of 7 plans to utilize the area groundwater, and if the model presented by ATC, after being validated and accepted, predicted a return to background conditions in half of that time (10 years), then that portion of Criterion 5 may be satisfied. The EHD does not find the models sufficient to confidently predict the time period required for impacted groundwater to return to background conditions. Indeed, Run 2 of the MT31D model predicts that the TPHg concentration in MW-1 will drop from 3,969 micrograms per liter (µg/I) in 2001, to 3,931 µg/I in 2004, and hold steady at 3,930 µg/I from 2007 to 2026. This is a 25-year period of essentially no return to background conditions, which seems to be an unreasonably long period of time, particularly when one considers the accuracy with which future needs can be predicted with a rapidly growing population, changing water treatment technologies and the constant threat of drought or other restrictions of water supply. Point 11: ATC quotes Criterion 6: The plume is stable and sufficiently limited in lateral and vertical extent and contaminant concentrations detected in groundwater show a decreasing trend with time. One hydrologic cycle (four quarters) after active remediation measures have ceased is usually considered to be the minimum necessary to determine site groundwater and plume conditions. ATC concedes their model is based on natural attenuation and notes that the Run 2 was started in 1995 utilizing the 1995 data. The model for MW-1, located in the area directly affected by SVE, nearly matches the dissolved concentrations actually measured prior to initiation of groundwater extraction, but the model for down-gradient MW-3, intuitively the well where conditions have been more like natural attenuation, had actual TPHg concentrations of 2,100 µg/I in 2004 as opposed to the 714 µg/I predicted from the model. These data suggest to the EHD that the model for natural attenuation is overly optimistic. In summary, the EHD remains concerned that the model produced by ATC is not sufficient to confidently predict future behavior of the plume of dissolved contaminants such that EHD can concur with the closure recommendation. In the opinion of the EHD, the model presented has several deficiencies: • The same technique of matching model output with an actually plume disposition was utilized to assign values to multiple critical model variables: hydraulic conductivity, contaminant reflux charging of the plume from submerged impacted soil, and incorporating the effect of SVE on the plume. These are all important factors that should have values validated by independent methods. • The model sensitivity to a reasonable range of all estimated variables has not been tested and reported. Sensitivity analysis is essential to address the uncertainty inherent in the model. At a minimum, the key variables should be tested utilizing the most plausible case, the upper limit bounds and lower limit bounds. The results should be evaluated in terms of providing a conservative plume response scenario. • The EHD is not sure how the hydraulic gradient, the primary transportation force in the model, was averaged, as most of the groundwater flow directions determined from field data have been oblique or counter to the contaminant transport direction. • Site-specific data should be incorporated into the model. The single-layer model of 25- foot thickness does not appear to adequately represent the site complexity posed by