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k ' Former Beacon Station 494 October 30, 1995 Stockton,Cahfortua QuarterlyGroundwater Momtarm Groundwater samples were collected from the monitoring wells on October 11, 1995 Collected samples were submitted under chain-of-custody to Western Environmental Science & Technology, a California Department of Health Services-certified analytical laboratory(ELAN No 1346) located in Davis, California Groundwater samples were analyzed for Total Petroleum Hydrocarbons as gasoline (TPHg) according to EPA Method 8015 and for benzene, toluene, ethylbenzene, and xylenes (BTEX) according to EPA Method 602 Laboratory results are summarized on Table 1 Copies of the laboratory reports and chane-of-custody are included in Attachment D Laboratory results tables from sampling prior to April 1995 are shown in Attachment C Results Water-level data collected on October 24, 1995 were used to construct a Groundwater Contour Map (Figure 2) The groundwater elevation data indicates an overall groundwater flow direction toward the north with a gradient of approximately 0 02 Data collected before the start-up of the treatment system had shown a groundwater flow generally toward the northeast The gradient appears to have resumed its natural northward trend since groundwater extraction has ceased When sampled in the previous quarter, gasohne and/or BTEX compounds were detected in four of the five wells (Table 1) Concentrations of dissolved TPHg and BTEX have now decreased to below laboratory detection limits m all monitoring wells We recommend that two additional groundwater monitoring events be performed and, if at that time, the petroleum hydrocarbon concentrations are acceptable, the site be submitted for closure and the monitoring wells destroyed In our opinion, two additional quarters of groundwater data is adequate to evaluate groundwater conditions relative to hydrocarbons for closure Report No 494gmr95 q4 Job No 1494 31 2 HORIZON ENVIRONMENTAL INC