SITE INFORMATION AND CORRESPONDENCE

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Mr. Frank Barone is - 3 - 22 February 2006 Table 1. Threshold concentrations' for protection of human health. Compliance Point Indicator Threshold Reference Monitoring Wells Constituent Concentration US EPA Draft Guidance for Evaluating DBCP 170 ug/L the Vapor Intrusion to Indoor Air Pathway, 2002 MW-2, MW-5, MW-7, 13-TCP 15 uDTSC Vapor Intrusion Guidance MW-23, MW-25, MW-26 ,2, g/L , Interim Final, December 2004 MW-27, MW-28, MW-29 RWQCB, San Francisco Region, 1,2-DCP 280 ug/L Screening for Environmental Concerns, Interim Final, Volume 2, February 2005. Thresholds must be exceeded in two consecutive semi-annual monitoring events. Geomatrix proposes that if the threshold concentration is exceeded in any of these downgradient monitoring wells for two consecutive monitoring events, and an increase is not also observed in upgradient monitoring wells, then MSRM will implement the preferred remedial alternative of enhanced insitu bioremediation. Geomatrix' proposal to identify thresholds in monitoring wells adjacent to and within the residential subdivision will be protective of residents given the present distribution of pollutants. Groundwater Migration In Regional Water Board staff's 18 August letter, we requested that MSRM propose a revised threshold and contingency plan for the possibility of plume migration. In the Revised Contingency Plan, Geomatrix proposed to take action if constituents of concern appeared in concentrations exceeding cleanup goals in off-site monitoring wells MW-26 or MW-27 for two consecutive monitoring events. If an exceedarce is confirmed,then MSRM proposes to install a monitoring well about 350 feet downgradient of these wells. If the cleanup goal is exceeded in two consecutive monitoring events in this new well, then MSRM will take remedial action. MSRM needs to include MW-28 as an off-site monitoring well subject to the plume migration contingency plan. Similarly to the contingency plan for MW-26 and MW-27, if constituents of concern exceed the cleanup goal in MW-28, then MSRM needs to submit a work plan for a monitoring well downgradient of MW-28 and implement remedial action if cleanup goals are exceeded in the new well. Regional Water Board staff also requested that MSRM provide a means to assess if groundwater concentrations are declining. In the Revised Contingency Plan, Geomatrix stated that it will not be necessary to assess declining trends since the nature of hydraulic control exerted by the photoremediation precludes movement of constituents off-site. Regional Water Board staff believe that observing trends from graphical representations of the analytical data, as required annually in Monitoring and Reporting Program No. R5-2002-0831, will suffice to assess whether declining trends are observed.