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i ` 3. PRESENT INVESTIGATION To permit groundwater monitoring between former Chevron SS 9-7780 and domestic wells located cast and west of the site,EA installo:two groundwater monitoring wells at Chevron SS 9-7780 during the week of 6 August 1950. The wells were developed and sampled during the week of 13 August 1990. 3.1 BOREHOLE DRILLING The two groundwater monitoring wells (BA5 and EA6) were installed by Kvilhaug Drilling CO. (C57-482390)at the locations shown in Figure 4, under permit number 90-1963 from San Joa- quin Public Health Services, Environmental Division. A copy of the permit is included in Appendix B. The boreholes were drilled and sampled,and the wells were completed,developed, and sampled according to EA"s standard protocols that are consistent with guidelines of the San Joaquin Local Health District and Tri-Regional Recommendations (Appendix Q. The boring logs and well construction details are included in Appendixes D and E. 3.2 SOIL SAMPLING Undisturbed soil samples were collected at 5-foot intervals with a split-spoon sampler during borehole drilling, in accordance with the protocol. Because the exact elevation of the ground- water table was not known, a sample was taken both above and within the water table in bore- hole EA5. The sample in EA6 was collected almost at the water table. The sample depths in each borehole are indicated on the boring logs(Appendix D). Soil samples were delivered, under chain of custody, to Superior Laboratories, certified by the California Department of Health services(DRS)for hazardous materials analyses. The samples were analyzed for total petroleum hydrocarbons(TPH)as gasoline,with benzene, toluene, xyl- enes,and ethylbenzene (BTXE)distinction by DHS modified EPA Method 8015/8020. 3.3 GROUNDWATER MONITORING WELL INSTALLATION The boreholes were completed as groundwater monitoring wells after drilling. The wells were constructed using flinch diameter Schedule 40 PVC flush-threaded casing. The screened inter- val in each well consists of 0.010-inch slotted casing, placed from 6 to 7 feet above the water table,encountered at about 69 feet below ground surface, to 13-14 feet below it. A threaded end plug was placed on the bottom of each well, and a locking well cap was used to secure the top of the well from unauthorized entry. The wells were completed on the surface in slightly raised, 14-inch diameter traffic rated, water- tight steel well boxes that require a special wrench to open. 'Me well boxes were set in concrete. The well completion diagrams are included on the boring logs in Appendix D and are included . independently as Appendix E. c67/97780/tx 4