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<br /> CONSULTANTS INC
<br /> ENVIRONMENTAL 0 GEOTEC HNICAL ❑ MATERIALS
<br /> ' Project No S8475-06-24 '
<br /> June 21,2002
<br /> ' Mr Saiyed S Ali
<br /> California Department of Transportation
<br /> ' Central Sierra Environmental Services Branch
<br /> Post Office Box 2048
<br /> Stockton, California 95201
<br /> ' Subject SHOP 10 MAINTENANCE STATION
<br /> STOCKTON, SAN JOAQUIN COUNTY,CALIFORNIA
<br /> CONTRACT NO 43AO078
<br /> TASK ORDER NO 10-987901-MU
<br /> GROUNDWATER MONITORING REPORT-FIRST QUARTER 2002
<br /> ' Dear Mr Ali
<br /> In accordance with Caltrans Task Order No 10-987901-MU, dated February 22, 2002, Geocon
<br /> Consultants, Inc has provided groundwater monitoring services at the Shop 10 Maintenance Station
<br /> located at 1603 South B Street in Stockton, California (the Site) The Site consists of a maintenance
<br /> facility utilized for the storage of vehicles and equipment used for the maintenance of California
<br /> highways The scope of services provided by Geocon included depth to groundwater measurements,
<br /> the sampling of twelve onsite monitoring wells, and the submittal of the water samples to a
<br /> Califorma-certified laboratory for analytical testing The site location is depicted on the Vicinity
<br />' Map, Figure I
<br /> BACKGROUND
<br /> ' In 1978, one 5,000-gallon underground storage tank (UST) with unknown contents was reportedly
<br /> removed from the Site due to a detected leak In 1989, three 5,000-gallon gasoline USTs, one 5,000-
<br /> gallon diesel UST and one 1,000-gallon waste oil UST were removed from the Site Maximum total
<br /> petroleum hydrocarbons as gasoline and diesel (TPHg and TPHd) concentrations of 3,700 and 1,700
<br /> milligrams per kilogram (mg/kg), respectively, were detected in soil samples collected from beneath the
<br /> ' gasoline, diesel and waste oil USTs Between 1987 and 1996, twenty-one groundwater monitoring
<br /> wells were installed at the Site to determine the vertical and lateral extent of contamination Twelve
<br /> wells are currently located onsite and are included in the quarterly monitoring program Maximum
<br /> TPHg, TPHd, methyl tent-butyl ether (MTBE) and benzene concentrations of 231,000, 6,100, 96 and
<br /> 28,000 micrograms per liter (µg/1), respectively, have been reported for water samples collected from
<br /> the wells Tert-butanol, tetrachloroethene, tert-butyl benzene 1,2-dichloroethane (1,2-DCA), iso-
<br />' propylbenzene, sec-butyl benzene, p-isopropyltoluene, naphthalene, n-propyl benzene, 1,2,4-
<br /> trimethylbenzene and 1,3,5-trimethylbenzene have historically been detected in the groundwater
<br /> samples collected from wells VM-1 and VM-2 1,2-DCA has also been detected in the groundwater
<br /> samples collected from well MW-13 The Site has been on a quarterly groundwater monitoring
<br />' schedule since 1990 Wells MW-1 through MW-8, MW-10, MW-26 and VW-1 have been abandoned
<br /> The groundwater flow direction beneath the Site has generally been directed to the east and
<br /> northeast The former UST locations and approximate locations of the groundwater monitoring wells
<br /> are presented on Figure 2 A summary of historical analytical groundwater data is tabulated in
<br /> Appendix A
<br /> 11375 Sunrise Park Drive, Suite 100 11 Rancho Cordova, California 95742 a Telephone (9161 852 9118 ® Fax (916) 852 9132
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