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a Beacon Station No 641 October 12, 1998 Stockton California Groundwater Monitortn Re ort ' Table 1 Copies of the laboratory data sheets and the chain-of-custody report are included as Attachment C Results Water-level data collected on September 8, 1998 was used to construct the Groundwater Contour Map (Figure 2) The average depth-to-water for the monitoring wells was approximately 49 5 feet Water levels have decreased approximately 3'/z feet since wells MW-1 through MW-5 were measured on June 10, 1998 The flow direction on September 8, 1998 is shown to be towards a trough trending toward the east at a magnitude of 0 0015 foot/foot, as opposed to the June 10, 1998 northwesterly gradient direction w1 a magnitude Of 0 0011 foot/foot However, this flow direction is similar to the October 2, 1997 easterly gradient direction with a magnitude of 0 0088 foot/foot, and may indicate some sort of seasonal groundwater fluctuations Detectable concentrations of benzene were measured in water samples collected from each of the five monitoring wells at concentrations between 2 9 parts per billion (ppb) and 6,500 ppb A Benzene Concentration Map was constructed for the September 8, 1998 data(Figure 3) Dissolved benzene concentrations have decreased in well MW-4 since the since the June 1998 sampling event, while dissolved benzene concentrations have increased in wells MW- 1, MW-2, MW-3, and MW-5 IDistnbution 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 O Box 388 Stockton, California 95201-0388 Mr Gordon Boggs Regional Water Quality Control Board Central Valley Region 3443 Routier Road Sacramento, California 95827-3098 641 gmr98 q3 Job No 1641 41 2 HORIZON ENVIRONMENTAL INC.