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McDowell&Davis Towing ` Risk Assessment and Conceptual Site Model �- Boring B-4 was drilled to 20 feet in depth northeast of boring B-2. Laboratory soil L-' samples, collected at 15 and 20 feet, were reported as "non-detect for the compounds r analyzed" by the analytical laboratory. Three groundwater-monitoring wells were installed to a depth of 70 feet November 11, 2000 to define the direction and magnitude of groundwater flow and recognize �- possible groundwater impacts from the releases. Soil samples, collected from the lower portions of the borings did not contained detectable concentrations of analyzed contaminants Five quarters of groundwater monitoring has been conducted at McDowell and Davis. Laboratory analyses of the May 2002 monitoring event (report in preparation) detected a maximum of 1.1 ugll MTBE and .G-ug/I toluene in the groundwater samples. Laboratory analyses of groundwater samples collected during the monitoring events are presented in Table 1. Potable drinking water is supplied by the City of Escalon municipal water system. The closest municipal well is located approximately 1000 feet to the south south west of the site. The nearest water well identified in the well survey is the Escalon Unified School District well at 1500 east Yosemite Blvd., approximately 750 feet northeast of the site. 2.0 SCOPE OF WORK 2.1 Conceptual Site Model Hydrogeologic and contaminant cross-sections illustrating the shallow geology groundwater occurrence, and contaminant distribution were prepared. An estimate of the contaminant mass was calculated based on the laboratory analyses of tank removal grab samples and soil samples collected from the p oratory borings and ` monitoring wells. A survey of water wells within a radius o 100 feet of the site was conducted and information on significant wells used in the si del. ` 2.2 ASTM—Risk Based Correction Action Evaluation The soil and groundwater contamination present at the McDowell and Davis site was evaluated through the use of a computer application, the "RBCA Tool Kit for Chemical Releases", prepared by Groundwater Services, Inc. Houston, Texas. The application is composed of the an EXCEL spread sheet that completes all calculations required for Tier 1 and Tier 2 Risk Based Corrective Action evaluation as defined in ASTM standard PS-104. The spreadsheet calculates baseline risk levels for soil and groundwater remediation based upon the conceptual site model and measured contaminant concentrations. a„ Calculated parameters include site-specific exposure concentrations, average daily uptake, baseline risk and risk based cleanup levels. Fate and transport analytical models are used to evaluate site-specific soil, groundwater, and air exposure pathways. 2