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Beacon Station No. 474 April 30, 1999 ' 1 Stockton California Groundwater Monitoring Re ort Conclusions Water-level data collected by Doulos on March 25, 1999 was used to construct the Groundwater Contour Map (Figure 2). The average depth-to-water for the wells was approximately 55 feet below surface grade (bsg). Water levels have risen approximately 2 feet since the last monitoring in December 1998. The flow direction at the time of this sampling is generally toward the west, with an average magnitude of 0.0002. Data collected from this sampling event was used to construct a Benzene Isoconcentration Map (Figure 3). Deeper wells with submerged screen were not utilized for creating the isoconcentration map. Detectable concentrations of benzene were reported in the water samples collected from four(MW-2, MW-6, VW-1, and VW-2) of the eleven wells sampled. Dissolved benzene concentrations have decreased in wells MW-6 and VW-4 since the December 1998 sampling event while benzene concentrations have increased in well VW-1. Concentrations of MtBE were only detected in the sample collected from monitoring well MW-2 at 0.62 parts per billion (ppb). Other oxygenate compounds were not detected for all the wells sampled with the exception being MW-2 and MW-6 which both reported concentrations of Tert-butanol. Recommendations Horizon recommends continued quarterly monitoring and to proceed with the proposed assessment work referenced in Horizon's June 5, 1998 Work Plan For A Remedial Investigation and December 28, 1998 Letter Addendum. Report Distribution We recommend a copy of this report be forwarded to: Mr. Michael J. Infurna Environmental Health Division San Joaquin County Public Health Services P.O. Box 2009 Stockton, California 95201 Mr. Gordon Boggs CRWQCB - Central Valley Region 3443 Routier Road, Suite A Sacramento, California 95827-3098 QMR99.Q1 2 HORIZON ENVIRONMENTAL INC.