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880 Victor Road,Lodi Former Shell Service Station Page 2 EHD agrees the aquifer test has demonstrated the effectiveness of groundwater extraction for dissolved MTBE removal,but the proposed one-week per-quarter groundwater extraction schedule is insufficient as interim remediation. Currently the vertical extent of the contamination plume has not been determined. However,if one assumes the immediate volume of the contamination plume to just be a 10-foot thick saturated soil zone corresponding to the screened interval (53 to 63 feet bsg) within a 60-foot radius (approximate distance between MW-4 and MW-2),with 30%reflective porosity and 7.48 gallons per cubic foot, the calculation yields a volume of 253,790 gallons of impacted groundwater. The report proposed groundwater extraction at approximately 0.7 gpm for one week (assuming 7 days rather than 5 days) once per quarter. At this rate only 28,224 gallons of groundwater would be extracted in a year,which is approximately 11 % of the immediate groundwater plume. As an interim remediation this appears to be inadequate and is unlikely to exert any significant hydrogeological control of contaminant migration. MTBE was detected in multiple soil samples (Table 2) from both the vadose zone and the saturated zone. Groundwater table fell from approximately 48 feet bsg in 1999 to approximately 52 feet bsg in 2002. Based on the well logs and the CPT logs, significant sand intervals exist from about 10 feet bsg to 30 feet bsg, and from 35 feet to 60 feet bsg. Due to the high MTBE concentrations in the vadose zone,and the lack of a significant barrier against vertical migration of residual soil contamination to groundwater,EHD recommends that interim remediation in the vadose zone also be considered. Soil vapor extraction (SVE) may be a good remediation candidate at this site. SVE is particularly effective against MTBE due to MTBE's high vapor pressure (250 millimeter of Hg in pure phase,versus 86 mm of Hg for benzene) and its tendency to volatilize. Table 2: Concentrations of MTBE in Micrograms Per Kilogram (mg/kg) in Soil Samples by Method 8260B Sample MW-1 I MW-2 MW-3 Sample SB-1 SB-2 SB-3 SB-4 CPT-1 Depth Nov-99 /DTW 48'bs Depth Jan-99 /DTW— 50'bs Oct-01 30.5' 1.79 1.42 0.867 25' 92.9 2.01 0.226 46.6 ------ 40.5' 1.65 0.0378 ------ 35' 15 1.76 ------ ------ ------ 45.5' 0.0205 ------ 2.13 40.5' 1.09 7.94 ------ ------ ------ 57.5' 0.00806 0.0205 ------ 50' ------ ----- 2.36 0.507 ------ ' ------ 0.294 56' ------ 0.135 ------ ------ 60.5 ------ ------ ------ ------ ------ 60' ------ — - — 0.459 ------ ----- 62' ------ ----- 3.5 Submit to EHD by May 2, 2003 a work plan addendum to implement a continuous groundwater extraction interim remediation system as a measure to remove the dissolved contaminant mass and to establish migratory control over the contaminant plume. The work plan addendum should also include a proposal for removing the residual MTBE mass in the soil.