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CLEARWWTER G R O U P, I N C EnirronnienW Services 1.0 INTRODUCTION ' This report presents the results ofuarterl groundwater monitoring performed b q Yg gP Y Clearwater Group, Inc (Clearwater) on Februa 27 1996 at 1002 North Yosemite ' Street in Stockton, California (Figure 1) The tasks performed during this phase of work included groundwater gauging, well purging and groundwater sample collection 1 2.0 BACKGROUND ' No aboveground structures are present at the subject location, it is a vacant lot surrounded by a chain-link fence. Records indicate that the subject site was used as a ' gasoline service station from 1961 to 1978, and that the underground fuel storage tanks were removed in 1980 An environmental assessment conducted in 1990 by Evax Technologies, Inc consisted of a limited soil gas survey. The purpose of this ' investigation was to identify any potential subsurface contamination in the vicinity of the three former underground storage tanks The soil gas survey indicated the possible presence of petroleum hydrocarbons in the soil Following this initial discovery, GeoAudit was retained to conduct a more comprehensive subsurface investigation The investigation consisted of drilling six, soil borings to depths from between 29 5 and 36.5 feet below ground surface (bgs), and collecting soil samples at five foot intervals Laboratory analysis of soil samples collected by GeoAudit indicated the presence of relatively low levels of ethylbenzene in the soil around the western edge of the property Subsequently, San Joaquin County Public Health Services Environmental Health Division (PHS/EHD) directed the property owner to install monitoring wells and to begin quarterly sampling. ' In accordance with PHS/EHD directives, GeoAudit completed installation and sampling of three groundwater monitoring wells (MW-1 through MW-3) in October 1993 The highest concentrations of volatiles were found in groundwater samples collected from MW-2, located near the former tank area. A detailed presentation of the initial site assessment can be found in GeoAudit's November 1993 Monitoring ' WeIl Installation and Quarterly Report ' To further evaluate the extent of hydrocarbon distribution in groundwater, Clearwater installed morutormg wells MW-4 and MW-5 on August 31, 1994 MW-5 was installed approximately 50 feet downgradient of MW-2 At the request of the PHS/EHD, MW-4 was installed as an additional data point to evaluate possible fluctuations in the groundwater flow direction. Water level and surveying data compiled over the past year indicates a fairly consistent northwestern groundwater ' flow direction D-101 1Q96 1 Apnl 18, 1996