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�ACffM� Fourth Quarter Momtonng Report 26056 Thornton Road,Thornton January 12, 1993 Page 2 GROUNDWATER MONITORING AND SAMPLING RESNA personnel performed quarterly groundwater monitoring and sampling on December 3, 1992 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 In addition, a water sample was collected from the nearby municipal well (Thornton Water Well #2) The water table elevation has increased an average of approximately 1 03 feet since September, 1992 The rise in the water table is likely due to natural seasonal fluctuations Groundwater monitoring data is summarized in Table 1 Groundwater beneath the site flows southwest under an overall gradient of 0 0007 ft/ft, or approximately 3 7 feet per mile A groundwater gradient map for the most recent monitoring episode (December 3, 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 (PHSIEHD) 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 MW1, MWS, MW6, or Thornton water well #2 This is 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 PHSIEHD request, groundwater beneath the site will continue to be monitored on a quarterly basis The next sampling event should be completed by March 31, 1993, and the subsequent monitoring report submitted by April 20, 1993 QR121592 DRF