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Work Plan for Additional Subsurface Assessment Former Unocal Station#2859. March 6,20O1 s. ., ,.. s • : NP-1 (2.5 ppb)..'Methyl tert butyl ether(MtBE) was detected to wells MW 9 and NP-I at-concentratiotis'' . .of 6.4 and 5.6.ppb,•respectively. 'SUBSURFACE ASSESSMENT41 <. , �. .. In order to delineate the horizontal and vertical extent of petroleum hydrocarbons'm soil and groundwater ' in the immediate :Y icinity of the-subject site,.and.test the feasibility,<of vapor,exiraction'a's a remedial ` option, GR proposes,to install three off--site and.six'on-site.groundwater monitoring wells, and conduct a vapor extraction pilot test. .Field Methods and Procedures are included in Appendix A:r To implement ~ .- .this scope of work,F GR proposes'the following seven'tasks: Task 1. 4 ' Pre-field Activities GR will'generate a`site-specific;safety pian and obtain the necessary;monitoring well installation permits F from the SJCPHS, and encroachment'permit from the City:af Stockton. Underground :Service.Alert , a (USA)will be notified at least 48 hours prior,to initiating field work.' A private utility line locator.will be'- (USA) contracted to clear the monitoring well locations..; Task 2. Well Installation k f > i -.. ...-• , .„- -s .[, ' C '.. * v _ .r +_�, GR will install nine groundwater mohitortng wellsat the locations shown on Figure.2.. DrilIingand'well construction will be performed by a California=licensed•driller. Eight of the,groundwater monitoring wells will be installed in four sets•consisting.oftwo wells each. Each set of wells will consist-of.one well _ A -installed to a depth of approximately 60 feet,bgs and-the-othef io approximately 90 feet bgs. Because the`% two wells in each set will be installed 5 feet.apait,'soi14samp les,for description and possible chemical” 's analysis will only be obtained from the 90 foot well-boring at fve-foot.intervals,--as a minimum: One' additional 90 foot deep'groundwater monitoring well vriial be installed downgradient of the site adjacent.to - existing monitoring well- MW-7. -Soil.samples for description-and. possible chemical analysis will be� , obtained from this well baring at five-foot intervals, as a minimum. Soil samples will be collected with a s : split-spoon sampler.fitted with'-clean` brass sample rings.';The actual'number of.samples;submitted for, f chemical'analysts as described,in'Task 5 will depend on site conditions'and field screening data The monitoring welts`ivill be installed'in well;borings drilled with 8-inch diameter hollow=sfem augers: The wells will be constructed with 2-inch diameter Schedule 40.polyvinyl chloride{PVC)well.casing and ` 0.02-inch machine .slotted'-well screen: The- 60��foot deep wells will be.constructed with`a screened interval between approximately,50 to 60 feet bgs, and the 90'foot-deep wells will -be screened from ' approximately 85 to 90 feet,.bgs ;.The actual screen .interval will depend on the oundwater de th and lithologic conditions encountered'd'uring drillmg. 'Proposed well construction details are presented'in;' ; Figures 3 and 4.. Y Soil from each sampled interval-Will-be screened in'the field for the' presence of volatile.organic compounds using,a photoionization detector (PID): These data.will be collected .for reconnaissance purposes only, and:will not be used as'verification o_f.the presence or-absence. petroleum hydrocaibons. . . Screening data will be recorded on the boring logs.: .; • --`"' 'nom' ' _ _ w. � _ r x 240079 04 f 3 ;