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RDMW 3rd Quarter Monitoung Repan . 20456 Thornton Road Thornton October 13, 1992 Page 2 on Figure 2 Field methodology and analytical results are discussed in detail in RESNA's Interim Problem Assessment Report, dated January 13, 1992 GROUNDWATER MONITORING AND SAMPLING RESNA personnel performed quarterly groundwater monitoring and sampling on September 1, 1992 Field work consisted of measuring the depth-to-water (DTW) and subsequently purging, sampling, and analyzing the water from wells MW 1 through MW( for the presence of petroleum hydrocarbon constituents In addition a water sample was collected from the nearby municipal well The water table elevation has decreased an average of approximately 3 13 feet since March, 1992 The decrease in the water table is likely due to seasonal fluctuations The groundwater monitoring data is summarized in Table 1 The groundwater gradient beneath the site flows west-southwest under an overall gradient of approximately 0 00075 ft/ft, or 4 0 feet per male A groundwater gradient map for the most recent monitoring episode (September 1, 1992) 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 The analytical results are summarized in Table 2 The laboratory data sheets and chain-of-custody documentation are presented in Attachment 2 The Iateral extent of the dissolved contaminant plume associated with the former underground fuel storage tank has been essentially defiled, 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-sate contributing source In accordance with the PHS/EHD request, groundwater beneath the site will continue to be monitored on a quarterly basis The next QR090992 FNL