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WORK PLAN • ADDITIONAL SITE INVESTIGATION ACTIVITIES SAN JOAQUIN COUNTY MOSQUITO AND VECTOR CONTROL DISTRICT BECKMAN ROAD FACILITY LODI, CALIFORNIA 1.0 INTRODUCTION This work plan has been prepared in response to a letter dated November 18, 1993 from the San Joaquin County Public Health Services Environmental Health Division (EHD) to the San Joaquin County Mosquito and Vector Control District (SJCMVCD) which requested submission of a work plan for further site investigation activities at the SJCMVCD facility located at 200 North Beckman Road, Lodi, California (Figure 1) 1.1 Back round In 1986, the SJCMVCD removed three underground storage tanks (USTs) from the site and excavated soil beneath the gasoline UST to a depth of 31 feet below grade A soil and groundwater investigation conducted by Weiss Associates on behalf of the SJCMVCD in 1987 ' consisted of soil borings and the installation of three groundwater monitor wells (MW-1, MW-2 and MW-3) to a depth of approximately 50 feet While the investigation identified the presence of hydrocarbon compounds in soil beneath the location of the former gasoline UST, no detectable hydrocarbon compound presence was found in the three groundwater monitor wells A low permeability soil layer at approximately 32 Leet below the ground surface was believed to have acted as a barrier to downward migration of hydrocarbon compounds to groundwater ' The Weiss Associates study also concluded that, based on groundwater elevations in the three monitor wells, groundwater beneath the site flowed in a north-northeasterly direction ' In December 1991, ROY F WESTON, INC (WESTON©) began a quarterly groundwater monitoring program on behalf of the SJCMVCD WESTON's first sampling effort found that ' only one of the three monitor wells contained enough water to sample Consequently, the San Joaquin County Environmental Health Division (EHD) requested that an additional monitor well be installed on the site northeast of the former tank area to evaluate the possible downgradient ' migration of hydrocarbons in groundwater The new well (MW-4) was installed in July 1992 according to the EHD-approved WESTON work plan A detailed description of the well installation and sampling activities was presented in the WESTON report entitled Well Installation and Quarterly Groundwater Monitoring Report, dated October 14, 1992 Analysis of the samples from the initial sampling of the new well (MW-4) on August 25, 1992 ' showed total petroleum hydrocarbons-gasoline fraction (TPH-G) and benzene, toluene, ethylbenzene and xylenes (BTEX) to be non-detectable Furthermore, all groundwater sampling events from that time to the present (November 1992 and January 1993) have shown non- detectable results for TPH-G and BTEX in the new monitor well and any other wells containing sufficient water to sample rrKNRF rs\.vcmvcn wr 1 02M9,94