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Closure Report 26 Golden Grain Manteca Bean Facip ,299 Moffat Boulevard,Manteca,California July 16,1996 I ' A baseline nsk assessment and a nsk-based approach for closure have similanties and differences A baseline risk assessment estimates the numerical risk posed by current chemical concentrations detected at the site based on a no-action alternative The risk estimates are based on current and future exposure scenarios at a site As a policy, the USEPA has defined that a cumulative site risk range of 1 in 10,000 to i in 1,000,000 and a cumulative hazard index of 10 will not warrant remedial ' action A cumulative site risk of 1 in 10,000 is considered the upper end risk estimate that will warrant remedial action The results of a baseline risk assessment will, therefore, demonstrate whether remedial action may be required or not If remediation is required, then chemical-specific ' cleanup levels may be proposed based on different exposure scenarios The implementation of any remedial action is contingent on a regulatory consensus regarding the exposure scenario which, in ' tum, is determined by the land use deemed applicable for a site The RBC approach, on the other hand, is more proactive because the rationale is based on ' establishing, in the forefront, a regulatory consensus regarding the exposure scenano and the level of risk deemed acceptable for the site These two decision criteria may be established without any prior knowledge of the site data but are essential in the development of cleanup levels for the detected chemicals at the site Therefore, in contrast to a baseline risk assessment that estimates risk based on scenarios that may or may not be considered acceptable for the site, the RBC I ' approach decreases the number of iterations in the risk assessment by obtaining, in the forefront, concurrence among the risk managers regarding the exposure scenano and acceptable level of risk Through this methodology,the efforts for a risk-based closure can be more focused and the number of iterations in the process is reduced, thus saving time and resources ' The following sections will discuss and identify the COPCs that were evaluated, the rationale supporting the proposed exposure scenano, the proposed acceptable level of risk, and the proposed ' cleanup levels ' 62 Identification of Chemicals of Potential Concern ' The COPCs at the Manteca Bean Facility primarily consist of petroleum hydrocarbons from the fuel products (gasoline) stored at the site Gasoline may be categorized as fuel which consists of a blend of distilled process streams Gasoline is a light distillate which consists primarily of hydrocarbons in ' the C4-C12 range (ATSDR 1993) The average percentages of aliphatic and aromatic fractions in gasoline are 60 and 40 percent, respectively BTEX comprises the largest fractions of the t42020213 CR FLUOR DANIEL GTI