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EXECUTIVE SUMMARY In accordance with Caltrans Contract No. 53W202 and Task Order No. 10-911175-12, Geocon Environmental Consultants, Inc. (Geocon) is submitting the enclosed Problem Assessment Report and Corrective Action Plan(PAR/CAP)for the Caltrans Stockton Yard maintenance facility located at 1604 South B Street in Stockton,California.This PAR/CAP presents the cumulative results of environmental site assessment work and vapor extraction pilot testing performed at the subject site. The work described herein was performed to delineate the vertical and lateral extent of petroleum hydrocarbons impacts to soil and groundwater, to evaluate interim soil remediation options, and to present preliminary soil vapor extraction design criteria. In December 1989, one gasoline and one diesel fuel underground storage tanks (USTs) were removed from the site. The initial subsurface investigation was performed at the site in 1992 by Alton Geosciences. This investigation included the performance of two soil borings, the installation and monitoring of three groundwater monitoring wells and the chemical analysis of soil and groundwater samples. The results of these studies indicated the presence of total petroleum hydrocarbons as gasoline (TPHg)impacts to soil and groundwater resulting from the former fuel UST systems. Between May 1995 and August 1996 Geocon perfonned one soil boring, installed nine groundwater monitoring wells and two vapor extraction test wells, and performed quarterly groundwater monitoring and a vapor extraction pilot test (VET). The VET was performed to collect site specific data and evaluate the.feasibility of using vapor extraction as a soil remediation alternative. Based on Geocon's review of the cumulative soil and groundwater data obtained at the site, the majority of the residual TPHg currently remaining in soil is located beneath the gasoline and diesel UST excavation to a depth of approximately 65 feet below the ground surface. The vertical and lateral limits of residual TPHg in on-site soil have been established as described in the PAR/CAP. The volume of TPHg-impacted soil is estimated at 1,500 cubic yards. The soil analytical data obtained from this investigation provided the ability to reasonably estimate the distribution of petroleum hydrocarbons in soil beneath the site and initiate corrective action planning. Evaluation of the site conditions and data obtained from the vapor extraction pilot test indicated that soil vapor extraction is feasible and the most cost effective remediation technology for the impacted soil. The results of the VET performed at the site indicate an initial start-up vapor extraction system (VES) TPHg mass extraction rate of approximately 300 pounds per day. Based upon an extraction flowrate of 50 standard cubic feet per minute (scfm) per extraction well, an effective radius of influence (ROI) of approximately 35 feet could be established to capture petroleum hydrocarbons existing in on-site soil. It is estimated that a 100 scfm therm-ox based VES operated for a 12 to 18 month period of time would be required to volatilize and capture the extractable TPHg mass in soil. The estimated cost to construct, operate and maintain such a VES for a period of 24 months (including the purchase of a new therm-ox) ranges from approximately $90,000 to$125,000. Upon approval of this PAR/CAP, the process of designing, installing and operating a VES can be initiated. Project No.58100-06-11C -i- December 17,1996