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SECG www,secor.com L INTERNATIONAL 3017 Kilgore Road , Suite 100 S E C O R INCORPORATED Rancho Cordova, CA 95670 916-861 -0400 TEL 916-861 -0430 FAX . 1' - I PM 24 3t0 April 27 , 2007 - Y Ms. Lori Duncan San Joaquin County Environmental Health Department 600 E . Main Street Stockton , CA 95202 RE: Work Plan for Additional Assessment, ARCO Service Station No. 548 1612 Hammer Lane Stockton , California SECOR Project No. : 77BP . 00548 . 01 . 0136 Dear Ms . Duncan : This document, prepared by SECOR International Incorporated (SECOR) on behalf of Atlantic Richfield Company, a BP Affiliated Company, was prepared in response to the San Joaquin County Environmental Health Department (SJCEHD) letter dated February 9 , 2007 (Attachment A) . As requested , this document presents an evaluation of the 1 , 2-dichloroethane ( 1 , 2-DCA) concentrations and a work plan for the installation of groundwater monitoring wells at ARCO Service Station No. 548 (the "Site') . SITE DESCRIPTION The site is located at the southwest corner of West Hammer Lane and Brentwood Drive in the City of Stockton , California ( Figure 1 ) . The site was formerly operated as an ARCO Service Station and AM/PM mini-mart. The site currently operates as a Valero gasoline service station and mini-mart. Current site facilities include four underground storage tanks (USTs) ; two pump islands , associated product lines and dispensers , and a convenience store . A site plan is presented on Figure 2. PREVIOUS ASSESSMENTS Site Investigations began in October 1990 with five soil borings (B- 1 through B-5) located around the USTs and product lines. Soil samples were collected and analyzed for petroleum hydrocarbons including gasoline range organics (GRO) and benzene . Hydrocarbons were detected in six of the 18 samples collected . The highest concentrations detected were 1 , 400 milligram per kilogram (mg/kg) for GRO and 1 . 2 mg/kg for benzene . These detections were located in samples around the USTs at 30 feet and 40 feet, respectively. The investigation concluded that hydrocarbon impacted soil was limited to the proximity of the UST complex. In October 1991 , four on-site groundwater monitoring wells (MW- 1 through MW-4) , and three soil vapor extraction (SVE) wells (VW-1 , VW-2 , and VW-3) were installed . Soil samples collected identified maximum GRO and benzene concentrations of 340 mg/kg and 0 . 031 mg/kg , respectively. These results also indicated that petroleum hydrocarbon impacted soil was limited to the area of the UST complex.