SITE INFORMATION AND CORRESPONDENCE_FILE 1

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ENVIRONMENTAL HEAI.TM DIVISION . ej,: ,W%CA Ave.Third Floor304P.O.Box 95201-0388 phone:. (209) 3420 O'Approved pp roved with changes: See Remarks Disapproved: See Remarks Remarks ,b Dade p� By WORK PLAN FOR SITE ASSESSMENT Valley Motors Approval of these plans extends only to that 800 East Main Street Ap ut Stockton,CA which is shown and describednhereon sion does not authorize or app Y 1.0 INTRODUCTION or deviation of requrrements of Stats laws or IN r91dinances. Upgradient Environmental Consultants(Upgradient)has been retained to continue the t o groundwater contamination at Valley Motors in Stockton,California(Figure 1).The scope of work, described in Upgradient proposal no.96006,is to drill four to six borings,converting two of them to monitoring wells.The primary objectives of this investigation are to fully delineate the lateral extent of the plume of.contaminated groundwater that was detected during the September-October 1995 investigation, to map the groundwater gradient and flow direction, and to establish groundwater monitoring wells that can be used for periodic monitoring of groundwater quality.A limited number of soil samples will be collected for laboratory analysis,but previous investigations have shown that there is very little soil contamination beneath the site. 2.0 BACKGROUND 2.1 Previous Work I It is our understanding that two undergrodnd storage tanks were removed from the site in 1990.The tanks were reportedly used to store gasoline,but had apparently not been used since the present owner acquired the property in 1987.Soil samples were collected from beneath the tanks during removal,and petroleum hydrocarbons were detected, although the laboratory results subsequently came into question. As a result, a subsurface drilling investigation was conducted by William Hunter & Associates in 1994. The 1994 investigation included the drilling of 5 borings and the collection of 40 soil samples for analysis(Figure 2).Only 4 of these contained hydrocarbons at concentrations above detectable limits, ' and concentrations in these four samples were very low. The hydrocarbons were reported by FGL Laboratory to be within the compositional range of Stoddard Solvent,although specific carbon ranges were not reported.The maximum depth at which hydrocarbons were detected was approximately 25 feet. r In September 1995,Upgradient Environmental installed monitoring well VM-I in the area between the former dispenser island and one of the former USTs.Although hydrocarbon odor was evident in some soil samples to a depth of 35 feet,laboratory analysis demonstrated that hydrocarbons(both gasoline and diesel)were present only at a depth of 25 feet.However,both gasoline and diesel and all of the volatile aromatic compounds(BTEX)were detected in the groundwater sample(depth 50 feet).These results were confirmed when groundwater was re-sampled in October,and additional analysis for the presence of chlorinated hydrocarbons demonstrated that 1,2-dichloroethane,trans-l,2-dichloroethene, cis-l,2-dichloroethene, trichloroethene, and tetrachloroethene were present at concentrations well above the Maximum Contaminant Levels allowed in California. Since these compounds are typically i I ! i I