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Jim Tjosvold, P. E. Page 2 April 29, 1988 Proposed Well Changes The report states that 10 wells will have sampling frequency modifications . It was difficult to determine which wells and the precise modifications. Variations Over Time Evidence presented to substantiate variability with time was incomplete. The proposal calls for a reduced sampling frequency at ten wells. Graphs were presented for only two of the wells. The Department requires time graphs to be presented for all wells with proposed changes . Constituents to be plotted include pentachlorophenol , napthalene, acenaphthene, and metals. Characterization of Contamination Discontinuation of copper , chromium, lead, and zinc may be allowed if M&B can provide information to show that the concentrations originate from background levels in the soils. The use of Table 3 to present a summary of affected wells and the contaminants presently known appears to be incorrect . The Department believes there are additional wells and contaminants that are not listed. Examples include wells DSW 6C and 4B. Additional contaminants include chlorinated phenols. The Department finds the proposal modifying groundwater monitoring at the McCormick & Baxter facility requires additional supportive data. The objective of monitoring program is to provide an adequate characterization of the areal and vertical extent of soil and groundwater contamination. In addition, the program' s intent is also as a tool to protect public health. All wells in close proximity to the edge of the contamination plume are monitored to provide current information on the extent of contamination and must be monitored on the current quarterly system, unless shown that the concentrations are constant over a longer period of time. We support McCormick and Baxter ' s desire to make the on-going monitoring as cost-effective as possible. We will need the information discussed above to establish our minimum data needs. JV:mr