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RESPONSE TO WiHT WATER ENGINEERS' CQ~IMENTS Oi• PENDIX F OF THE DRAFT NAHCCIITE ENVIR(hyi~'~NTAL IMPACT STATEMENT COMMENT #1 The commenter believes that "there is general agreement among the various groundwater scientists that; there is hydraulic connection between these "two" aquifers. The two aquifers in question are the "upper" and "lower" aquifers overlying and underlying the Mahogany Zone. Their conclusions are based on data which include results from a groundwater model. RESPON SE #1 A re view of available infornation, and discussions with various experts on the Piceance Basin, indicates there are conflicting opinions about the hydraulic gradients at the ;Jolf Ridge Corporation (WRC) site. There is also concern about the quality control associated with early drilling and testing in the area. Some of the input data used for the ground-water simulations appear to have been output data obtained from the results of simulations run by O.J. Taylor (Open-File Report 8Z~37). Under certain circumstances, this practice might be acceptable. However it may also cause the results to suggest conditions different than reality. EPA would like to evaluate the Wright computer model effort and to draw conclusions based on sound technical analysis. COMMENT #2 The commenter indicated that the WRC plugging & abandonment plan of operation was considered to be adequate. The discussion involving abandonment procedures does not provide any suppo rt data to indicate that mud gel will function as stated at this particular site. COMMENT #3 The commenter pointed out that, in his opinion, "the mere volume of water available, and a total dissolved solids concentration of less than 10,000 mg /1 does not make a drinking water source." In addition, the commenter questions the "necessity for the EPP~ to arbitrarily consider the Unita-Green River Aquifer a source of drinking water." RESPONSE #3 According to 40 CFR Fart 144 Subpart A Section 144.3, if an aquifer contains a sufficient quantity of ground water and this supply contains less than 10,000 mg/1 total di<_;solved solids (TDS), then the aquifer is an underground source of drinking water (USDIJ). It should be made clear that the EPA has NOT "arbitrarily" considered the Unita-Green River Aquifer an USDW, the federal regulations define it as such. The applicant for the' Class III UIC permit may propose aquifer exempti on s, i f he desires..