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Beacon Station No 641 Apn130, 1999 Stockton, California Groundwater Monitoring Re rt oxygenates by modified EPA Method 8260B The laboratory results are summarized in Table 1 Copies of the laboratory data sheets and the chain-of-custody report are included as Attachment C Conclusions Water-level data collected on March 25, 1999 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the five groundwater-monitoring wells was approximately 414 feet Water levels have increased approximately 3 feet since December 1998 The flow direction is shown to be toward the northeast at an average magnitude of 0 001 foot/foot Detectable concentrations of benzene were measured in water samples collected from monitoring wells MW-2, MW-5 and all three of the vapor wells sampled (VW-1, VW-3, VW-7) A Benzene Concentration Map was constructed for the March 25, 1999 sampling (Figure 3) Dissolved benzene concentrations Have decreased m monitoring wells MW-1 and MW-2 since the last quarterly sampling An increase m benzene concentrations was reported in MW-5, while MW-3 and MW-4 were below laboratory detection limits MtBE was detected in all the wells sampled except for monitoring well MW-3 Additional oxygenate compounds were detected in some of the monitoring well analyses included with this report (see Table 1) Recommendations Horizon recommends continued quarterly monitoring and additional assessment to complete the delineation of hydrocarbons horizontally and vertically beneath the site area Distribution A copy of this report should be forwarded to Mr Michael Infurna Jr San Joaquin County Public Health Services Environmental Health Division 304 East Weber Avenue, Third Floor P 4 Box 388 Stockton, California 95201-0388 Mr Gordon Boggs Regional Water Quality Control Board Central Valley Region 3443 Routier Road Sacramento, California 95827-3098 641QNM9 q1 2 HORIZON ENVIRONMENTAL INC.