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31 August 2005 AGE-NC Project No 96-0232 Page 29 of 30 • Based on back-calculations,the RISC Workbench program determined that no MTBE clean- up was necessary for the typical site models because the site MTBE concentrations did not exceed the site-specific target cleanup levels(SSTL) However,the SSTL were exceeded for maximum input values for hydraulic conductivities and soil types,these maximum values are considered anomalous and are not expected to be encountered at the site • The acceptable range for carcinogenic risk defined by US EPA is from 10'to 10-6(e g , one case per 10,000 or 1,000,000 inhabitants) Residential risk is often assessed at the 10'level, however, risks less than 10' are difficult to quantify relative to background health hazards Where the cumulative carcinogenic site risk to an individual based on reasonable maximum exposure for both current and future land use is less than 10' and the non-carcinogenic hazard quotient is less than 10,corrective action generally is not warranted(US EPA, 199 1) • AGE concludes that MTBE-impacted soil and ground water remaining beneath the site appear to pose minimal risk to human health and beneficial use of ground water down- gradient from the subject site 9.0. RECOMMENDATIONS Based upon data collected from the site, AGE recommends the following • No further subsurface investigation at this time Flag City Chevron has adequately assessed MTBE-impacted soil and ground water associated with the Flag City Chevron UAR • Recision of Cleanup and Abatment Order (C&A) 5-00-700 Because the Banner Avenue MTBE plume encroached onto the Flag City Chevron property and has essentially interfered with the on-going cleanup efforts,the status of Cleanup and Abatement Order(C&A) 5-00- 700 should be modified AGE is of the opinion that all the prior conditions of C&A order 5- 00-700 order have been diligently met, and that Flag City Chevron should not be held to the C&A order for the cleanup of the Banner Avenue MTBE plume Furthermore, it is our understanding that the primary reason for the issuance of the C&A Order was because of the MTBE-impact to the domestic drinking water well formerly located at the Rocky's Restaurant Since the Rocky's domestic well has been abandoned and is no longer a threat for contamination impact, the C&A Order is unnecessary • Consideration of the facility for closure as a low risk ground water site From previous data, AGE has documented that MTBE concentrations in ground water at the site have been at non-detectable or near non-detectable levels Fate-and-transport model techniques have been used to demonstrate that concentrations of remaining MTBE in ground water would not impact the CSA well, though there is the potential for limited impact to water above 50 feet + bsg in well MW-10 Furthermore, the Banner Avenue MTBE plume that migrated onto the i Flag City Chevron property is no longer being drawn by pump-treat ground water extraction Advanced GeoEnvironmental,Inc