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` Ms. Victoria McCartney S E C 0 R — . June 26, 2003 Page 4 of the drawdown and recovery data is 15,000 gpd/ft. The hydraulic conductivity was calculated by dividing the transmissivity at each well by its respective aquifer thickness, which ranged from 26 feet in well MW-5D to 5 feet in well MW-2. The calculated hydraulic conductivities ranged from 0.01 cm/sec in wells MW-5D and MW-8 to 0.53 cm/sec in well MW-2. The geometric mean for hydraulic conductivity was calculated at 0.07 cm/sec. This is consistent with the materials described in the saturated zone on the boring logs (Freeze and Cherry, 1994). Storativity of the aquifer can be calculated from analysis of the observation well data only. Storativity ranged from 0.0016 to 0.05, with a geometric mean calculated at 0.02. The storativity values are typical of a semi-confined aquifer system. A summary of hydraulic properties determined from aquifer testing is presented in Table 2. Analysis of the drawdown data from wells MW-8, MW-1, and MW-2 indicated the presence of an impermeable boundary or a stratigraphic change to less permeable aquifer material in an area north of wells MW-1 and MW-2. This was identified by an increase in the rate of drawdown in these wells after approximately 500 minutes from the start of pumping. The boundary condition is anticipated to have minimal impact at the site except for increased drawdown in the northern portion of the site. CAPTURE ZONE CALCULATIONS Assuming that semi-confined conditions exist across the site, a capture zone for pumping well MW-8 was calculated using the Javandel and Tsang (1986) method. According to Gorelick et. al. (1993), the Javandel and Tsang (1986) method may be applied at sites with semi-confined conditions based on the following: "...semi-confined conditions seem to indicate that the delayed yield effects from gravity drainage are insignificant. Confined-aquifer hydraulics can be applied with reasonable confidence in such cases." Using a discharge rate of 8.5 gpm, an average hydraulic gradient of 0.0039, and an average T of 15,000 gpd/ft, a capture zone of approximately 35 feet and 52 feet were calculated in the downgradient and cross-gradient directions, respectively(Figure 3). PROPOSED INTERIM REMEDIATION MEASURE SJCEHD indicated in their letter, dated January 17, 2003, that interim remediation at this site should be considered. Therefore, as an interim measure to mitigate the presence of dissolved MtBE in groundwater near the USTs, SECOR proposes to extract groundwater on a monthly basis using a vacuum truck. Approximately 8,000 gallons can be extracted by and stored in a single vacuum truck, therefore approximately 8,000 gallons will be removed from MW-8 each month. This measure will be performed at least once each month, once approved by SJCEHD, through the end of 2003. During the period of interim remediation, SECOR will evaluate the need for additional soil and/or groundwater assessment to fully define the lateral and vertical extent of MtBE impacts at the site. In addition, SECOR will study the feasibility of other alternatives for restoring and protecting the beneficial uses of groundwater at the site. At the end of 2003, results of the monthly extractions will be evaluated. If concentrations of MtBE in groundwater at the site have been reduced sufficiently, the extraction episodes may be curtailed. If elevated MtBE concentrations persist, the monthly episodes will either continue or alternative remediation methods will be recommended based on the SECOR's feasibility study. I:\Chevron\91452\REPORTSWquifer Test Report and Plan for Interim Remedial Measure.doc i {