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<br />proximity to the injection fluid stream will make those zones subject to a greater drop in fluid <br />temperatures. Therefore, the solution would lose its ability to mine or dissolve nahcolite within a <br />short distance from the cavity. The principle behind the solution mining process is that the mining <br />fluid needs to be hot to dissolve nahcolite. The rock in the cavity does not transmit heat to the <br />rock outside of the cavity in sufficient quantities to heat isolated mining fluids that may have <br />moved outside of the main cavity. If fluid were to exit the cavity via a narrow pathway ("finger"), <br />precipitation of nahcolite with cooling could seal the pathway. Evidence ofself-healing fractures <br />was documented at the Horse Draw Ivfine and at White River Nahcolite's commercial operation. <br />Additionally, the fact that nahcolite precipitates rapidly with cooling has been verified by the <br />occurrence of several blockages in well tubing at the Yankee Gulch test facility operation. <br />Thermomechanical analyses of the commercial mine plan done by American Soda using varying <br />cavity size, cavity spacing, and mining temperatures showed that in a 200-foot-diameter cavity <br />with an average cavity temperature of 300°F, rock 50 feet beyond the outer edge of the cavity <br />would have a temperature of only about 100°F. This temperature is much lower than the <br />required solution mining temperatures. Thus, the solubility of nahcolite and the properties of the <br />cavity rock should limit the growth of "lingers" or pathways around the gas cap of solution <br />mining cavities. <br />As a result of the review of all comments and the available information, EPA has <br />determined that no additional changes are needed to the Permit in response to these <br />comments. <br />Comment 12: Draft SOB indicated that test Qhase began in July 1997 as a Class V experimental <br />test - no information regarding the results of the experimental test mine was presented. We are <br />concerned that the Permit will allow well operatine conditions outside of the range oftest <br />conditions. <br />Response: EPA reviewed available monitoring data from the experimental cavities and compared <br />the results with the operating parameters proposed for the commercial operations. The test phase <br />operations have included operations using pressures that are representative of the proposed <br />commercial operating pressures (up to 695 psig). Throughout the test phase operations, the <br />operator was able to maintain operating pressures and monitoring wells have not indicated any <br />cavity failures. There were no indications. of leakage from test cavities. There were no <br />unexplained differences between injection and production fluid flow rates. It should be <br />emphasized that the Permit limits injection pressures to a maximum of 700 psig which is <br />consistent with the test pressures. <br />As a result of the review of this comment and the associated information, EPA has <br />determined that no changes to the Permit are needed in response to this specific concern. <br />Comment 13: The last pazagraph on paee 7 of the SOB is self-contradictory. The Mahogany <br />Zone is first described as an effective barrier and later as a zone that may Permit some vertical <br />exchange of water. <br />Response: EPA agrees that this discussion is confusing. The local and regional data in the basin <br />on the geologic and hydrologic conditions indicates that the Mahogany zone is impermeable. <br />30 <br />