SITE INFORMATION AND CORRESPONDENCE

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Theresa Lanctot � 2651 S. Airport Way, StocTcron, California Page 4 of 5 Preparation and implementation of a work plan to complete the vertical assessment of impacted groundwater; assess soil in the immediate area of contaminant release(s); confirm effective remediation of vadose zone soil; and collect sufficient data to develop an adequate hydrogeological model for the site — EHD suggests a CPT investigation to develop the hydrogeological data and assess the vertical extent of impacted ground water; Inclusion of 1,2 -DCA and EDB in the analytical regime for samples collected during future soil and groundwater sampling events; and Collection of groundwater samples from the irrigation well for laboratory analysis for the chemicals of concern to demonstrate that the well has not been impacted or is influencing the plume of impacted groundwater. The plume of impacted groundwater must be fully delineated in all directions. 1,2 -DCA should be reported at 0.5 micrograms per liter, if possible. Ideally, the work plan for the next phase of work should be presented in the context of an SCM. Simply stated, the purposes for such a model are to demonstrate where the contaminants came from, where they are at the present time, how they move through the subsurface, how they will respond to changes in the groundwater flow characteristics or to potential remediation efforts, what the contaminants' ultimate environmental fate will be, and to help evaluate the risk posed by the contaminants to groundwater supplies and sensitive receptors. To be sure that the SCM will adequately address the assessment, remediation and, ultimately, final closure requirements for this site, please include the following items, as appropriate, in the SCM: • Local and regional plan view maps with sources, boring and monitoring well locations, lines of cross section, extent of contaminants in each media, direction and rate of groundwater flow, and receptor locations, including water supply wells within 2,000 feet of the site. • Cross sections showing subsurface geological features, depth to groundwater, man-made conduits, monitoring well construction and an interpretative drawing of the vertical extent of soil and groundwater contamination. • Cross -plots of key chemical concentrations verses time for representative wells that demonstrate significant contaminant concentration trends. • Summary tables of contaminant concentrations in the different media. • Well and boring logs. • A narrative description of the SCM that describes controls on contaminant distribution; contaminant migration mechanisms, pathways, and rates; plume disposition over time; and sorbed and dissolved contaminant masses. While not necessarily showing all data in diagrams and drawings, an adequate SCM can be utilized to account for most of the known data and to illustrate additional assessment needs. All analytical data should be included in tables. Methods and calculations of contaminant masses should be included in the SCM, and may be submitted as an appendix. The SCM should be updated each time new data are acquired that causes a change of interpretation or expands the model until model modifications are not needed to explain subsequently acquired data. As the SCM becomes complete, it can be utilized to reasonably predict additional investigation results and plume response to various remedial technologies or to changes in groundwater flow regime.