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Report of Secondary Investigation - 3 Prepared for Barbara Drake,26485 Thornton Rd ,Thornton,CA May 4, 1995 32 Wellhead Surveying/Groundwater Gradient On January 31, 1995, monitoring wells MWs-1, 2 and 3 were professionally surveyed to top of casing and top of well-box nms relative to mean sea level (Appendix A) The wellhead survey was conducted by Morrow Surveying of Sacramento Elevations were based on USGS Benchmark P 463 Wellhead elevations were combined with groundwater monitoring data collected in past quarters by Wallace-Kuhl to determine the groundwater flow direction beneath the site Table 1 shows the calculated groundwater elevations for the site based on groundwater monitoring collected by Wallace-Kuhl Based on the calculated groundwater elevations,the groundwater flow direction is to the north and northwest at a hydraulic gradient ranging from 0 001 to 0 01 ft/ft 33 Site Reconnaissance On February 9, 1995, Groundwater Technology personnel visited the site to perform general reconnaissance of site conditions and to mark the locations of the proposed soil/groundwater sampling locations During the site visit a map of site layout was prepared, photographs were taken of the current site condition,special conditions/obstructions to the proposed field work were also noted In addition, proposed boring locations were outlined in white paint to denote utility clearance areas for Underground Service Alert(USA) 34 Rationale for Soil/Groundwater Sampling Locations Based upon the results of the background review and the groundwater flow direction, Groundwater Technology specified ten locations for soil and groundwater sampling (131 through B10, Figure 2) The objective of soil and groundwater sampling was to further define the extent of hydrocarbon-impacted soil and groundwater identified during the installation of MWs-1 through 3 131 was positioned directly downgradient of MW-3 to determine the extent of dissolved hydrocarbons in groundwater to the north B2 and B10, located east of MW-2, were used to define the eastern boundary of hydrocarbon- impacted groundwater 33 and B5 were drilled to investigate groundwater quality south of the former UST pit B4 was drilled to more precisely identify the western boundary of hydrocarbon-impacted soil and groundwater B6 was located between MW-3 and the former UST pit to identify the source of dissolved hydrocarbons in MW-3 B7 was positioned in the northwest portion of the site to investigate the quality of groundwater west of MW-3 and downgradient of the former UST pit B8 and B9 were located downgradient of MW-2 for further definition of the northern and northeastern boundary of hydrocarbon-impacted soil and groundwater downgradient of the former pump islands 3,5 Site-Specific Health and Safety Plan Following a complete review of site conditions, Groundwater Technology prepared a site-specific Health and Safety Plan as required by the Occupational Safety and Health Administration (OSHA) i W53EAR RPT ❑❑li GROUNDWATER ❑❑❑ TECHNOLOGY