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U.S. EPA's subjective analysis of the data suggests that there may be a consistent and <br />relatively predictable relationship between waste disposal sites, high soil gas <br />concentrations, hydraulic and geologic features, and the size and shape of groundwater <br />plumes. <br />SHAD should utilize these relationships to the fullest extent possible in future <br />investigations. <br />4. Agricultural wells should be sampled during at least one quarterly sampling round. <br />Screened intervals and other construction details for these wells should be obtained, <br />if available. Impact of these wells on the migration of contaminants should be better <br />characterized. For example, water levels in adjacent wells could be continuously <br />monitored over an appropriate period of time. <br />Other Comments <br />Several of the findings need clarifications, and a number of questions remain <br />unanswered. For example, what is the relationship between potential source areas where <br />waste -generating and disposal activities occurred, and soil gas results, soil boring results, <br />and groundwater results? <br />1. Relatively Iittle is known about the volume of contaminants in soils. The most <br />mobile components, the low molecular weight VOCs such as TCE, PCE, and carbon <br />tetrachloride have appeared in groundwater at many locations where waste was <br />generated or disposed of. Ultimately, less mobile species may enter the groundwater. <br />Waiting for volatile contaminants to be flushed from the vadose zone into groundwater <br />by natural processes and then treating the groundwater may not be the most <br />effective long-term solution to the problem of VOCs in the vadose zone, and current <br />groundwater treatment facilities will not remove low -volatility or soluble compounds. <br />Sufficient data should be collected during the RI to assess the volume and extent of <br />all contaminants in the vadose zone that may eventually impact groundwater. <br />3 <br />