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[PEOCON ,
<br /> CONSULTANTS INC
<br /> GEOTECHNICAL ❑ ENVIRONMENTAL ❑ MATER I A L S ®/
<br /> Project No S8475-06-24H
<br /> October 23, 2003
<br /> Mr Sawed S All
<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-THIRD QUARTER 2003
<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 California-
<br /> certified laboratory for analytical testing The site location is depicted on the Vicinity Map, Figure 1
<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 wells
<br /> were installed at the Site to determine the vertical and lateral extent of contamination Twelve wells are
<br /> currently located onsite and are included in the quarterly monitoring program Maximum TPHg, TPHd,
<br /> methyl tert-butyl ether (MTBE) and benzene concentrations of 231,000, 6,100, 96 and 28,000
<br /> micrograms per later (µg/1), respectively, have been reported for water samples collected from the wells
<br /> Tert-butanol (TBA), tetrachloroethene, tent-butyl benzene, 1,2-dichloroethane (1,2-DCA), iso-
<br /> propylbenzene, sec-butyl benzene, p-isopropyltoluene, naphthalene, n-propyl benzene, 1,2,4-
<br /> tnmethylbenzene and 1,3,5-trimethylbenzene have historically been detected in the groundwater samples
<br /> collected from wells VM-1 and VM-2 1,2-DCA has also been detected in the groundwater samples
<br /> collected from well MW-13 The Site has been on a quarterly groundwater monitoring schedule since
<br /> 1990 Wells MW-1 through MW-8, MW-10, MW-26 and VW-I have been abandoned The groundwater
<br /> flow direction beneath the Site has generally been directed to the east and northeast The former UST
<br /> locations and approximate locations of the groundwater monitoring wells are presented on Figure 2 A
<br /> summary of historical analytical groundwater data is tabulated in Appendix A
<br /> 3160 Gold Valley Drive, Suite 800 0 Rancho Cordova, California 95742 0 Telephone(916( 852 9118 13 Fax (916) 852 9132
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