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First Quarter Monitoring Report 26056 Thornton Road,Thornton Project No W0309 01 Page 2 on Figure 2 Field methodology and analytical results are discussed in detail in RESNA's Intenm Problem Assessment Report, dated January 13, 1992 GROUNDWATER MONITORING AND SAMPLING RESNA personnel performed quarterly groundwater monitoring and sampling on March 24, 1993 Field work consisted of measuring the depth-to-water (DTW) and subsequently purging, sampling, and analyzing the water from wells MW 1 through MW6 for the presence of petroleum hydrocarbon constituents The water table elevation has increased an average of approximately 5 38 feet since December 1992 The nse in the water table is likely due to groundwater recharge associated with the recent rain season Groundwater monitoring data is summarized in Table 1 Groundwater beneath the site flows southwest under an overall gradient of 0 0008 ft/ft, or approximately 4.2 feet per mile A groundwater gradient map for the most recent monitoring episode (March 24, 1993) is shown on Figure 2 • Groundwater samples were collected from the site wells after stabilization of physical parameters (pH, electrical conductivity, and temperature) The monitoring wells were purged and sampled according to RESNA's Quality Assurance/Quality Control Plan, on file with the San Joaquin County Public Health Services, Environmental Health Division (PHS/EHD) Well purge logs are included in Attachment 1 LABORATORY ANALYSIS Collected groundwater samples were placed on ice and transported under chain-of-custody protocol to state certified RESNA Environmental Laboratories Samples were analyzed for BTEX and TPHG by EPA methods 8020 and modified 8015, respectively Laboratory analyses detected elevated concentrations of petroleum hydrocarbons in the samples collected from wells MW2, MW3, and MW4 No hydrocarbon analytes were detected at or above the method detection limits in wells MW 1, MWS, or MW6 Results are generally consistent with the previous sampling round The analytical results are summarized in Table 2 The laboratory data sheets and chain-of-custody documentation are presented in Attachment 2 The lateral extent of the dissolved contaminant plume associated with the former underground fuel storage tank has been essentially defined, except in the vicinity of well MW4, north of the former tank It is unclear whether contamination at MW4 can be attributed to the former tank . or from a potential off-site contributing source In accordance with the PHS/EHD request, groundwater beneath the site will continue to be monitored on a quarterly basis The next IQR-1993 FNL