SITE INFORMATION AND CORRESPONDENCE_2001-CURRENT

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Former Unocal#5098 6 August 2001 5606 Pacific Avenue, Stockton - 2— 4. Connect monitoring wells MW-19S and MW-25S to the Wards soil vapor extraction system, and connect monitoring wells MW-19D,MW-23D,MW-25D, and the new air sparge well to the Wards air sparge system. 5. Connect monitoring wells MW-21D,MW-22D,MW-24S, and the new biosparge well to a 20 standard cubic feet per minute, 30 pounds per square inch biosparging unit on the Delta College property. Staff agree to items No. 1 through No. 4 above,but do not agree to using existing groundwater monitoring wells MW-22D and MW-24S as sparge wells. These wells are needed to monitor contaminant trends at the selected depths, and using these wells as sparge wells will eliminate them as effective monitoring points. Monitoring wells MW-19D,MW-23D,MW-21D, and MW-25D, which are also proposed to be used as sparge wells,are in data overlap areas where nearby monitoring wells can be used to monitor contaminant trends. We agree that biosparging on the Delta College property will have a positive benefit;however, we are also concerned that biosparging may cause contamination to disperse from the sparge well and spread laterally across the site. Therefore, continued monitoring of all existing monitoring wells is required to assure that contamination is being remediated. If the proposed biosparging system does not show a decrease in groundwater concentrations at monitoring wells MW-21 S, MW-22S/D, and MW-24S/D within two quarters,you may be required to implement additional remedial actions to address the groundwater contamination in this area. The Delta College Robinhood Irrigation Well has not operated since August 2000, and the groundwater flow direction has returned to an easterly/northeasterly flow. Monitoring wells MW-6 and MW-16,which were the easterly boundary for contamination, are currently showing an increase in TPHg and BTEX constituents. Therefore, additional monitoring wells are needed to identify the lateral extent of groundwater contamination east of the Unocal and Wards sites. The supplemental assessment work and Remediation Plan is approved with the following additions: • Collect a minimum of one depth-discrete groundwater sample from each of the two CPT locations between 90 and 150 feet bgs to define the vertical extent of groundwater contamination. • Install two or more additional air sparge wells as needed to remediate groundwater on the Delta College property. • Monitoring wells MW-19D, MW-23D,MW-25D,which will be used, as air sparge wells must be sampled annually at a minimum.