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SECOR www secor corn INTERNATIONAL 3017 Kilgore Road Suite 100 S E C O R INCORPORATED Rancho Cordova CA 95670 916 861-0400 TEL r 916 861-0430 �nx February 12, 2003 Mr Ed Ralston ConocoPhdtips 76 Broadway Sacramento, California 95812 Re Results of Additional Site Assessment Tosco Service Station No 1205 16470 Cambridge Street, Lathrop, California SECOR Project No 77TO 43870 00 0222 Dear Mr Ralston This letter, prepared by SECOR International Incorporated (SECOR) on behalf of ConocoPhillips, presents the findings of assessment activities at the site referenced above (Figure 1) The objective of this work was to delineate the lateral extent of petroleum hydrocarbons in groundwater downgradient of the site The scope of work consisted of the advancement of nine geoprobe borings (GP-1 through GP-9), the installation of six groundwater monitoring wells (MW-10 through MW-15), and performance of a sensitive receptor survey This work was performed as described in SECOR's Work Plan For Additional Off-Site Assessment, dated February 27, 2002, Addendum to the Work Plan dated July 16, 2002, and Addendum 92 to the Work Plan dated October 31, 2002, and was approved by San Joaquin County Environmental Health Division (SJCEHD) in their letters dated February 7, 2002, July 19, 2002, and August 27, 2002 (Attachment A) SITE DESCRIPTION AND BACKGROUND The site is located on northeast corner of Cambridge Street and Louise Avenue in Lathrop, California, and is an operating Circle K Mini Mart and 76 service station In November 1997, Pacific Environmental Group (PEG) performed oversight and sampling during the removal of one 4,000-gallon and two 10,000-gallon underground storage tanks (UST's), product dispensers, and associated product piping from the site during facility upgrade activities Approximately 1,241 tons of hydrocarbon-impacted soil was excavated from beneath the UST's and from the new tank pit excavation Additionally, approximately 17,000 gallons of hydrocarbon- impacted groundwater was pumped from the new tank pit excavation Soil samples collected from the excavated former tank pit contained total petroleum hydrocarbons as gasoline (TPHg) at concentrations ranging from 130 ppm to 5,800 ppm Methyl tert-butyl ether (MtBE) was reported in the excavation soil samples at concentrations ranging from 0 19 ppm to 3 6 ppm (PEG, 1998) In July 1998, one Geoprobe 0 boring was advanced to a depth of 40 feet below ground surface (bgs) downgradient (north) of the former UST excavation Laboratory analysis of soil samples collected from GP-1 at 10 feet bgs reported maximum concentrations of TPHg and MtBE at 1,900 ppm and 0 31 ppm, respectively Laboratory analysis of grab groundwater samples collected from GP-1 contained concentrations of TPHg and MtBE of 52,000 ppb and 61 ppb, respectively (PEG, 1999) From August through September 1998, IT Corporation (IT) drilled and installed four groundwater f monitoring wells (MW-1 through MW-4) to approximate depths of 26 to 28 feet bgs During the