SITE INFORMATION AND CORRESPONDENCE

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0 • Mr. Frank Barone - 2 - 18 August 2005 The trees are irrigated by drip irrigation using municipal water and supplemented with flood irrigation with stormwater, which MSRM must contain on-site. MSRM is capturing stormwater, treating it with carbon filtration, analyzing the treated water, and storing it on-site until it is blended with municipal water and used as irrigant on the trees. When the trees have grown sufficiently to contact groundwater, irrigation will be reduced to the extent needed to distribute treated stormwater during the summer months, and the quantity of municipal water added to the stormwater will be reduced. Contingency Plan The Contingency Plan proposes threshold concentrations of constituents in groundwater that would signal that natural attenuation and hydraulic control were not containing the groundwater contaminants. The Contingency Plan identifies Enhanced Insitu Bioremediation as an alternative remedial strategy that will be implemented if the threshold concentrations are exceeded, or if phytoremediation and natural attenuation are not containing the groundwater plume. The Contingency Plan is contingent upon the trees developing root structures sufficient to exert hydraulic control on groundwater as evidenced by a groundwater depression in the planted area. The trees were planted in 2002, and MSRM expects that the trees could exhibit the first indication of influence on groundwater elevations by the summer of 2006, and be evident by summer 2008. In 2003, at the direction of Regional Board staff, MSRM contained all stormwater onsite. This adds an additional hydraulic burden that creates a groundwater mound beneath the site. MSRM captures, stores and treats up to 42,000 gallons of stormwater in above-ground tanks for use as irrigant in the summer season, thereby removing some of the wet-season contribution to groundwater. Even so,the contribution of infiltrating rainwater to groundwater appears to be influencing the distribution of groundwater pollutants beneath the site, and as of this writing concentrations have not stabilized. The thresholds that MSRM proposes and Regional Board staff comments are as follows: • If the groundwater flow direction shifts more than 90 degrees east in two consecutive monitoring events, MSRM will submit a work plan for additional monitoring wells. Regional Board staff concur. • If on-site shallow monitoring well concentrations exceed historical highs for two successive monitoring events and increases cannot be attributed to corresponding concentrations observed in upgradient monitoring well MW-16,then a remedial alternative will be implemented. After hydraulic control is demonstrated, then MSRM needs to assess if concentrations of constituents are declining. MSRM needs to identify a means of ascertaining declining trends. If trends are not declining, then it would be appropriate to propose an alternative remedial action. Before hydraulic control over groundwater elevations is established, MSRM needs to identify movement of constituents off-site and implement a remediation before constituents migrate beyond the reach of practical remediation strategies, and before they become a health threat to adjacent residents. • MSRM proposes to identify off-site migration of constituents in shallow groundwater by evaluating concentrations in downgradient wells MW-23, MW-25, or MW-29. If constituents exceed cleanup levels in these wells,then the remedial alternative will be implemented. These monitoring wells are about 1,000 feet downgradient of the site and a residential neighborhood is located between these wells and the site. If a constituent of concern is confirmed in these wells, it