SITE INFORMATION AND CORRESPONDENCE

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CVATC A S S O C I A T E S I N C . presented in ATC's report titled Summary Report of Additional Subsurface Investigation, dated November 1,2002. On April 29 and 30, 2003, ATC supervised the installation monitoring well, MW9. MW-9 was installed in the northern portion of the site to a depth of 113 feet bgs to correspond with the deeper water-bearing zone encountered in boring CPTL Laboratory analytical results of a groundwater sample collected from MW-9 on May 5, 2003, indicated that hexavalent chromium and total chromium were detected at concentrations of 0.0049 and 0.0081 mg/l, respectively. Refer to ATC's report Summary Report ofAdditional Subsurface Investigation Deep Well Investigation, dated May 8, 2003 for additional information. Conceptual Remediation Design Historical analytical data indicate that there is not a feasible natural mechanism for attenuation of dissolved hexavalent chromium (Cr VI), except for dilution. The oxidizing environment in the subsurface will likely tend to maintain chromium ions in the +6 valence. Absent a reducing environment in the subsurface, monitored natural attenuation is not recommended, and an engineered solution is proposed. Based on the depth of impacted soil and the proximity to structures at the site, soil excavation and removal is not feasible. Stabilization of the chromium is recommended and should serve to inhibit its further migration. As a combined remediation effort of soil and groundwater, we propose to extract shallow groundwater, remediate the groundwater, introduce a ferrous hydroxide solution, and reinject the treated groundwater near Vaults 7 and 8 to reduce Cr VI in the subsurface to Cr III. The remediation system consists of shallow groundwater extraction and treatment, and installation of an infiltration system located near Vaults 7 and 8 (Figure 2). The purpose of this system will be to produce a dilute ferrous hydroxide solution from extracted groundwater, and infiltrate the solution as closely as possible through the migration pathways that originally contaminated the groundwater. The ferrous hydroxide solution is intended to reduce the Cr VI in the subsurface to Cr III, and stabilize Cr III as a precipitate of chromium hydroxide [Cr(OH)3]. The groundwater treatment system is expected to consist of one extraction well installed in the shallow aquifer near Vaults 7 and 8, an extraction pump, a ten gallon per minute capacity electrochemical cell with sacrificial iron anodes, a dual cartridge-type filter with pressure sensor and valves to allow switching chambers when one side becomes plugged, a PLC with sufficient inputs and outputs to control amperage of the rectifier based on flow rate, level control valves on each sump based on sump level, alarm activated pressure readings for the filter, and adjustable valve positions for the filter. The groundwater treatment system will also include interconnecting piping, fittings and valves, level sensors and level control valves for each sump, a flow transmitter, a pressure sensor for the filter, a three-way control valve to switch the filters, a power supply panel, electrical distribution wiring from the plant,and perforated or removable vault bottoms. Currently, monitoring well MW-5 is used as a sentinel well with respect to WP-1. Should elevated concentrations of chromium be detected in groundwater samples collected from MW-5, a second component is proposed as a contingency measure consisting of a hydraulic barrier that would be intended to prevent further migration of contaminants in the downgradient direction toward Stockton W:\62583Vetten\Concept Design Outine.doc 3