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• • Mr. Ron Rowe Page 4 of 6 • The criteria for proposing additional groundwater monitoring wells will be the identification of significant lithological units that are not adequately monitored by the currently existing wells. This may included discrete intervals of higher permeability. Other than proposed monitoring well MW14, described below, no additional monitoring wells are currently proposed simply in order to screen the current water table surface. Existing monitoring wells MWI, MW2, MW3 and MW13 are currently screened at or just below the water table. Vapor wells VW2, VW3, VW5 and VW6 are also screened across the water table surface. Together these provide more than adequate onsite monitoring density. However, it may be advisable in the future to install additional water table wells downgradient of the site. This issue will be addressed following evaluation of the data obtained from the proposed phase of work. One monitoring well (MW14) will be installed at the approximate location depicted on Figure 2. The purpose of this well is to investigate groundwater conditions in the indicated preferential flowpath (sand channel) present along the southern portion of the subject site (Figure 3). Well MW14 will be drilled using hollow stem auger equipment and will be constructed of 2 inch diameter PVC and screened from approximately 30 to 55 feet bgs. • We disagree that monitoring well MW13 provides a conduit for vertical migration of contaminants and should be replaced. The base of the screened interval in this well (75 feet) is similar to that in most of the other wells on the project, including MW8. The only difference is that the screen in MW13 extends up to 20 feet. Groundwater in the lower strata has been contaminated for years, while the upper screened strata in MW 13 contained no detectable soil contamination when the well was installed. In recent years contaminated groundwater has risen into the upper strata. MW13 was installed under permit and the screened interval was acceptable to the PHS/EHD. We do not recommend replacement of this well. TASK 4 UPDATED CONTAMINANT MASS AND DISTRIBUTION ESTIMATES, FATE AND TRANSPORT MODELING AND RISK EVALUATION Utilizing data obtained from the proposed investigation, Ground Zero will estimate the current mass and distribution of contaminants in the subsurface. If appropriate, we will also model and evaluate the fate and transport of contaminants and the action of natural attenuation. The evaluation will include estimates of cleanup time by natural attenuation alone and conjunction with continued active remediation. A Risk Evaluation will be prepared which will identify exposure pathways and potential sensitive receptors and quantify health risks due to contamination at present concentrations. The evaluation will . be used to assist in establishing site-specific soil and groundwater cleanup goals which are protective of human health and the environment. mek\c]mure\addmdum