SITE INFORMATION AND CORRESPONDENCE_FILE 2

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Mr. Russell Chapin 1766 W. Monte Diablo Avenue Page s of 7 Criterion 4: There are no existing water supply wells, surface waters or other receptors threatened by the remaining contaminants in soil or groundwater. The main threat posed by the plume, should it expand under natural conditions, would be to additional volumes of groundwater currently not impacted by petroleum hydrocarbons. Criterion 5: Pollutants remaining in groundwater do not create or threaten to create risk to human health and safety, or to future beneficial use(s) of groundwater. Patterns of existing and future demands for usable water resources in the area must be considered in determining what period of time is reasonable to reach non-detectable (or background) concentrations. EHD does not believe the models presented allow evaluation of the first portion of this criterion and a health risk assessment, based on an accepted site model, was not performed. Criterion 6: The plume is stable and sufficiently limited in lateral and vertical extent and contaminant concentrations detected in ground water show a decreasing trend with time. One hydrologic cycle (four quarters) of monitoring after active remediation measures have ceased is usually considered to be the minimum necessary to determine site groundwater and plume conditions. ATC states the plume is delineated vertically and laterally, and decreasing in size and TPHg concentrations at a faster rate than predicted by the MT3D model — but the site is under active remediation while the model is for natural attenuation. Conclusions/Recommendations — ATC formally requests an immediate no further action required (NFAR) concurrence and immediate cessation of groundwater monitoring. If this site does merit immediate closure, the models presented in this report do not appear to demonstrate or justify such a conclusion. As described above, in EHD's opinion the site model provided by ATC for their closure request does not seem adequate as it fails to predict the dissolved plume's fate in a convincing manner under natural attenuation once the active remedial system is discontinued. This is demonstrated by comparing the predicted plume response under natural attenuation to the actual response under active remediation. The model does not analyze plume response to active remediation and provides no up-to-date estimate of sorbed contaminant mass or contaminant flux rate for sorbed contaminants in the saturated zone. Overestimating the mass is not necessarily a conservative approach if the model that utilizes the mass estimate is itself inadequate. To improve the models, EHD recommends the following: • Carefully prepare an estimate of the sorbed contaminant mass in both the vadose and saturated zones; the estimate should incorporate recent data that reflects the effects of the remediation efforts on the site. The MT3D model may have to be adjusted or `tweaked' to provide loading of the plume due to reflux of contaminants from the submerged impacted soil. • Prepare an estimate of the remediated hydrocarbon mass through natural attenuation (if possible), soil vapor extraction, in-situ air sparging, and ground water extraction. This should be helpful with evaluating the efficiency of the current remedial effort.