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• The drinking water well located at St. Joseph's Hospital, approximately 500 feet -- away from the former USTs represents the most likely exposure pathway for t� groundwater ingestion. 1,.� • On-site commercial and construction workers are potentially exposed to CDCs in site soil via dermal contact and ingestion pathways. Exposure pathways associated with volatilization to outdoor air potentially exist for on-site construction or commercial workers, and for commercial workers at St. Joseph's Hospital. An indoor air exposure pathway is also evaluated for on- site construction and commercial workers. • A combined surface soil/Johnson & Ettinger model is used to estimate volatilization of COCs to outdoor air. A Johnson & Ettinger model is also applied to evaluate volatilization of COCs to indoor air. 0 The Soil Attenuation Model (SAM) is used to quantify leaching of COCs from site soil to groundwater. SAM is an adjustment to the ASTM leachate model, which accounts for attenuation of COCs concentrations due to sorption to vadose zone soil. �. • The 3-D Gausian Air Dispersion Model is used to estimate the ratio of off-site to �.. on-site air concentrations due to dispersion. • The Groundwater Dilution Attenuation Factor is calculated using the Domenico Equation with no biodegradation. kw� • A Transient Domenico Analysis is used to model fate and transport mechanisms. • The depth to water used in the RSCA assessment is 44.76 feet which is the average depth to water measured in site monitoring wells during 2005 quarterly monitoring events. • The depth of the affected soil zone ranges from 5 feet below ground surface (bgs) to 25 feet bgs. These values are based on August 2000 soil analytical data from boring CPT-1A. A circle with a 20-foot radius approximates the area of affected soil. This estimate is consistent with the petroleum hydrocarbon mass calculations presented in response to Item 13, above. UCSC parameter values for silty clay soil are representative of site soil parameters including dry bulk density, porosity, and hydraulic conductivity. .► • Average annual rainfall at the site is approximately 14 inches which corresponds to a 47 year average for Stockton, California. . The average groundwater gradient is approximately 0.003 according to quarterly groundwater monitoring elevation data. • The MTBE plume dimensions are based on the isoconcentration map presented as Figure 8 and the range of well screening elevations for impacted wells.