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1r, The mass of MTBE remaining in groundwater is calculated as follows: MTBE mass = nr2 75.45W 1(0.35)(62.4t )(f 8,0 Ib MTBE) = 0.13 Il] MTBE • (ew,ater) (109 fb-wateF} 14. Assumptions parameters, calculations and model used in risk assessments, and fate and transport modeling. A Tier 2 Risk Based Corrective Action (RBCA) assessment was completed for the site using Groundwater Services, Inc. (GSI) RBCA Tool Kit for Chemical Releases, Version 1.3b. The objective of the RBCA assessment was to determine if residual concentrations of TPH-as-gasoline in site soil and MTBE and 1,2 DCA in site groundwater pose excessive carcinogenic risks or toxic effects to humans. The following assumptions are used in the RBCA assessment. • TPH-as-gasoline, MTBE and 1,2 DCA are the constituents of concern (COCs) at the site. These CDCs comprise the analyte list for the RBCA assessment. • The target cancer risk limits are 1.0 E-6 for individual Class AIB carcinogens, 1.0E-5 for cumulative Class AIB carcinogens, and 1.0 E-5 for Class C carcinogens. The target hazard quotient for individual CDCs is 1.0 E+0, and the cumulative hazard index is also 1.0 E+0. • Groundwater analytical data collected during the 2005 quarterly monitoring ` events comprise the groundwater data set used in the RBCA assessment. The 98% upper confidence limits (UCLs)for COC concentrations are considered representative of site conditions. • Soil samples collected from confirmation soil boring CPT-1A in August 2000 comprise the soil data set used in the assessment. Again, the 98% UCL for COC .. concentrations are considered representative of site conditions. • The RBCA program does not allow for input of TPH data without breaking down values into carbon ranges and distinguishing between aliphatics and aromatics. For this assessment, it was assumed that each detected TPH-as-gasoline concentration is comprised equally of aromatics in the C5-C7 and C7-C8 carbon ranges. Potential exposure pathways include exposure to site soil via dermal contact or ingestion, inhalation of outdoor or indoor vapors, and ingestion of groundwater. Potential receptors include on-site commercial and construction workers.