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Soil and Groundwater Investigation Report ARCO Facility No 6080 Lathrop, California . Page 3 Diisopropyl Ether (DIPE), Ethyl Tertiary Butyl Ether (ETBE), and Tertiary Amyl Methyl Ether (TAME) Well Installation Delta directed the installation of three (3) groundwater monitoring wells during the investigation The wells were constructed using four-inch diameter, Schedule 40 PVC Wells were screened from approximately 5 feet above, and 15 feet below first encountered groundwater at the time of installation A filter pack of#3 sand was placed in the annular space around the well from the bottom of the well screen to approximately one foot above the top of the well screen Bentonite was placed on top of the filter pack to provide a sanitary seal for the well Neat cement was used to backfill the annular space around the well to surface grade Traffic rated vault boxes were placed over the top of the well casings flush with the surrounding asphalt surface Monitoring wells were developed on June 21, 2000 using a surge block and groundwater pumping Three to four feet of sand and soil material were removed from the bottom of the wells during development Groundwater samples were collected approximately 48 hours after development of the wells Field data sheets from well development and groundwater sampling are presented in Appendix B Analytical results of monitoring well groundwater samples are presented in Table 1 Certified analytical reports and chain-of-custody documentation are presented in Appendix C The groundwater monitoring wells will be incorporated into a quarterly groundwater monitoring program Morrow Surveying Inc of West Sacramento, California established top-of-well-casing elevations and locations of all borings in July 2000 Well elevations were established to the nearest 0 01-foot, referenced to San Joaquin County Benchmark UV-34, elevation 12 50 feet mean sea level (MSL) The locations of the monitoring wells are shown on Figure 2 Findings Site Geology and Hydrogeology Soil conditions encountered during drilling consisted predominately of fine to coarse- grained sand Fine to medium grained sand was present from the ground surface to approximately 12 feet bgs at which point the sand becomes predominately coarse grained Coarse sand continues to the total depth explored with the exception of a relatively thin fine-grained material present in boring/monitoring well MW-2 and MW-3 At a depth of approximately 15 5 feet bgs and extending to approximately 16 5 feet bgs, clayey silt was encountered in MW-2 The clayey silt layer appears to grade laterally to a sandy silt layer in boring/monitoring well MW-3 This finer grained material was not • present in boring/monitoring well MW-1