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casing could contribute to misinterpretation of subsidence movements based on <br />monuments affixed to well casings. Similarly, it is possible that some movement <br />of the well-pad monuments could be related to thermal effects on the underlying <br />formation rather than to a subsidence event. Consideration of this potential effect <br />will be necessary when interpreting these data. <br />2.2 Monument Locations for Future Mining Areas <br />As mining progresses into new mine panels, surface monitoring locations will be <br />established in the new mining areas. Surface monuments will be arranged in a <br />manner similar to that used in the initial mining panel. Additional section and <br />quarter-section corner markers within the potential area of influence of mining <br />will be added to the subsidence monitoring plan as mining progresses. <br />2.3 Surface Monitoring Frequency and Duration <br />Surface monument measurements will be conducted annually at section or <br />quarter-section corners. Upon reclamation of a well field panel (or area), <br />monitoring will continue until an adequate understanding of the date and the <br />potential for subsidence is attained. At that point, American Soda may apply to <br />the Colorado Division of Minerals and Geology (CDMG), Bureau of Land <br />Management (BLM), and Environmental Protection Agency (EPA) for <br />modification or termination of that monitoring. <br />Annual elevation measurements will also be taken for the well-casing and <br />well-pad monuments. Annual measurements for the well-casing and well-pad <br />monuments will continue until well closure and reclamation occurs. <br />3.0 SUBSURFACE MONITORING <br />The purpose of the subsurface monitoring is to ascertain movement of strata overlying the <br />solution cavities. A combination of time domain reflectometry (TDR) and borehole <br />geophysical techniques will be employed for determination of subsurface movement. <br />3.1 Time Domain Reflectometry <br />The previous subsidence plan dated December 23, 1999, called for the installation <br />of TDR cables in the first 23 of 26 wells in the initial mining panel. Although <br />TDR systems were installed in all of the first 27 wells, the TDR systems failed in <br />every well in which they have been installed. The heat generated in the well bore <br />during mining and water infiltration from surrounding aquifers are the primary <br />reasons for the failures. At this point, TDR has been demonstrated to be <br />technically infeasible during active mining. <br />FINAL: Modification No. 9, UIC Area Permit No. CO30858-00000 <br />Page 43 of 46 <br />