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Mr. Greg Vaughn Project OC42-001.23 March 10, 1995 Page 2 measurements. Four piezometers (PS/P-21, PS/P-23, PS/P-26, and PS/P-29) are sampled as upgradient monitoring points under the provisions of Quarterly Monitoring Program (QMP) 93-807. The shallow completion depth of 10 feet and narrow casing diameters are impractical for groundwater remediation applications. The STTC proposes to decommission 20 of the piezometers, including the 6 that are dama_ed. In addition to the damaged piezometers decommissioning rationale includes location with respect to groundwater monitoring wells and distribution of A-zone impacts, and limited utility as summarized on Table 1. A licensed drilling contractor will decommission the piezometers by drilling out the surface seal, filter pack, and casing to a total depth of 10 feet below ground surface (bgs) using 6- to 8-inch diameter hollow-stem auger drilling equipment. The borings will be sealed to ground surface with a cement grout containing 5 percent bentonite. Decommissioning debris will be temporarily stockpiled in STTC terminal lease areas west of Stork Road. Drilling equipment will be steam cleaned after each piezometer is decommissioned to reduce potential cross-contamination and rinsate will be containerized for disposal. The STTC proposes ten piezometers remain in service. Four piezometers (PS/P-21, PS/P-23, PS/P-26, and PS/P-29) will continue to be sampled as defined in the QMP. Piezometers PS/P-11, PS/P-12, and PS/P-13 are located within the zone of free phase hydrocarbons in the southern part of the site and will be useful in monitoring source removal during A-zone remediation. Piezometers PS/P-9 and PS/P-10 are located near the proposed interim remedial action plan (IRAP) trench. Piezometer PS/P-14 is located behind the former UNOCAL facility and will continue to provide groundwater le%el data during quarterly monitoring events. Task 3 Decommission Monitoring Wells PS/MW-3 and PS/MW-13 Woodward Clyde installed groundwater monitoring wells PS/MW-3 and PS/MW-13 in 1987. The wells are screened from 5 feet to 30 feet bgs. This interval includes the A-zone and the upper part of the B-zone and provides a potential avenue for downward migration of dissolved petroleum hydrocarbons. Monitoring well PS/MW-3 is located within the dissolved petroleum hydrocarbon plumes in the A- and B-zones and downward hydraulic gradients have been observed in nearby well pair PS/WC-ls,m. Monitoring well PS/MW-13 lies outside of the A-zone dissolved hydrocarbon plume and near the periphery of the B-zone dissolved hydrocarbon plume. A licensed drilling contractor will drill out the surface seal, filter pack, and casing to a total depth of 32 feet bgs using 10-inch-diameter hollow-stem auger drilling equipment. The borings will be sealed to ground surface with a cement grout containing 5 percent bentonite. Grout will be tremied from the base of the borings to ground ,urface. SA(\N r f7lTIX'42tKJ4200t2.3AL-94Vfc:2