SITE INFORMATION AND CORRESPONDENCE

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• Mr. Philip S. Isorena November 19, 1998 Page 2 remediation to non -zero risk-based or health -based standards such as the drinking water MCLS may be possible. 3. We plan to conduct at least two separate phases of extraction and treatment to a point of 90% reduction of initial VOC concentrations for both soil vapor and groundwater. In addition, if an achievable health -based standard is not agreed upon, then the approach to closure will be to perform a feasibility study to show that cleanup has been accomplished to the extent technically and economically feasible. Sufficient data will be collected periodically during remediation to support the feasibility study. This study would will consider the seven factors listed in the RWQCB letter and other criteria drawn from regulatory guidance such as the Guidance for Evaluating the Technical Impracticability of Ground -Water Remediation (USEPA 1993). Responses to Additional Comments 1. A memorandum report of findings of short term pumping tests at the site was completed and submitted on October 19. Parsons considers the information gained from the tests sufficient for designing a groundwater pumping plan. In order to provide further assurance that groundwater flow through the contaminated source zone can be captured by the proposed extraction wells, we intend to model the extent of the anticipated capture zone based on several pumping rates. The proposed analysis would utilize the program MODFLOW® with a flow line depiction tsubmipa meme, MOPATHO. As indicatd orandum report b November 20 n the attached project schedule we planto 2. In the two most recent quarterly monitoring events, samples collected from well MW - 9 located just north of the source zone, yielded detections of 64 µg/L and 58 µg/L total VOCs. This location is not bounded to the north by sample points with non -detect analyses. Therefore, in response to your request for additional definition of the extent of contamination, Parsons collected four additional groundwater samples from boreholes arranged parallel to the north property line fence in the vicinity of MW -9. Sampling was conducted on November 2 and 3, by direOct push ne sammlehod was sing a collect colw stem lected theughaler dlolllw rig and a HydropunchTM same P groundwater zone in each boring. 3. A natural attenuation evaluation was performed in accordance with the Technical Protocol for Evaluating Natural Attenuation of Chlorinated Solvents in Groun&vater (Wiedemeier et al, 1996) as part of the 2nd 1998 Quarterly Groundwater Monitoring Report (Parsons, September 16, 1998). As discussed in the report, some evidence for geochemical conditions favorable to natural attenuation was found, although it was limited. Parsons plans to sample the newly installed extraction well, GW -2, and selected other wells for additional evidence for natural attenuation as part of the 4th 1998 quarterly sampling in mid-November. The quarterly monitoring report is scheduled to be submitted January 6, 1999. However, and laboratory ae otraroundwater from GW -2 byed and Decemberd to 18 reach a decision on whether top p contingent upon completion of all laboratory analytical work. A decision not to pump from this well would be made only if the analyses indicate "adequate evidence for \\11;NRA\\T