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ERE LIMINAILY NYDROCEOLOGIC ASSESSMENT WORK PLAN b491"F"Street,Banta,California Delta Project No.40-88-481 Pr,ga 12 4.0 PROPOSED,OIL BORINGS_AND MON1T RIN WELL Because of the possible presence of petroleum constituents on soils and ground water at the site,two additional soil borings will be drilled to characterize the subsurface geology and to evaluate the presence of petroleum constituents in the soils at the site. Both soil borings will be completed within the upper-most water-bearing zone as ground water monitoring wells. These wells,with the existing monitoring well at the site, will serve to establish the natural flow gradient and hydraulic l ° characteristics of the water-bearing materials,and they will provide information for determination -., of whether dissolved hydrocarbons and/or free product are present in ground water at the site. 4.1 Borings/Monitoring Wells Previous work at the site and Delta's experience at similar locations indicate that the area of the site 1 most likely to have soils and ground water containing petroleum constituents would be adjacent or 1: in close proximity to the underground storage tanks(USTs). For.this reason,two soil borings, each t completed as monitoring wells, will be located adjacent to or in close proximity to the lank excavation (Figure 2). it is anticipated that well MW-3 will serve as a down-gradient well, with monitoring wells MW-1 and MW-2 either up- or across-gradient in relation to MW-3. i > 4.2 Soil Sample Collection and Screening I., A California modified split-barrel sampler will be used to extract soil samples from the sail borings. Three six-inch-long brass tubes will be'inserted into the California sampler to retain the soil sample. Upon retrieval,one brass tube will be capped, packaged,and preserved according to accepted EPA procedures for possible chemical analysis. One sample from each sampler will be extracted and. ! placed in a sealed jar until completion of drilling activities. ] s Samples placed in jars will be brought to room temperature and the head space of each jar will be screened for total organic vapors utilizing a portable photoionization detector. Appendix C,Section 1.3, describes the detailed soil-screening procedures. Based on this screening information, soil samples will be selected for chemical analyses to determine the vertical extent of the presence of 1,3 petroleum constituents in the soil. 4 !v cnitorinv We 11 Installation The proposed monitoring well specifications are illustrated on Figure 4. The wells will be constructed of two-inch diameter,Schedule 40(flush-threaded)blank casing and two-inch diameter, r' Schedule 40 well screen with 0.010-inch slats. The screened interval of each monitoring well will ------extend to approximately-IS_feet below and.five-feet-ebo.e the water table. Actual screened intervals --- — -- and well depths will be determined in the field by a Delta geologist or engineer.