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1.0 INTRODUCTION <br />American Soda had submitted the Subsidence Monitoring Plan dated April 1999, to the <br />Bureau of Land Management (BLM), Colorado Division of Minerals and Geology <br />(CDMG), and Environmental Protection Agency (EPA). The 4th revision of this plan, <br />dated December 23, 1999, was incorporated into the Underground Injection Control <br />(iJIC) Permit No. C03858-00000, issued on March 10, 2000, as Appendix I. At that time, <br />American Soda was required to conduct several activities related to subsidence. Based on <br />operating and monitoring experience since the UIC permit was issued, some of these <br />measures were not reasonably providing the intended information and benefits. This <br />subsidence monitoring plan has been modified to incorporate changes resulting <br />from experience with implementing monitoring measures identified in the previous <br />plan. <br />This subsidence monitoring plan includes both surface and subsurface monitoring of <br />vertical ground movement resulting from solution mining at the Yankee Gulch Lease. <br />Subsidence monitoring will be performed throughout the commercial operating life of the <br />Yankee Gulch Project. Results of the subsidence monitoring program (both surface and <br />subsurface) maybe to verify and calibrate numerical models used in the design of the <br />well field layout. <br />Surface monitoring will be conducted by surveying monument locations. Subsurface <br />monitoring will consist of a test program using downhole instrumentation over a cavity <br />and borehole geophysical logging of the overlying strata. <br />2.0 SURFACE SUBSIDENCE MONITORING <br />The purpose of the surface monitoring is to determine if surface movement occurs as a <br />response to solution mining. <br />2.1 Initial Mining Area Surface Monuments <br />A total of 18 section or quarter-section corners and 9 benchmark ground surface <br />locations have been established in and around the initial mining panel. Production <br />well casings established in the initial mining panel will have a survey monument <br />affixed to casing. Aground surface monument will also be established on the <br />well-pad near the instrumentation skid. The well-casing and well-pad monuments <br />will be located on wells to be developed on 600-ft spacing planned for the first <br />pass of solution mining. These .monuments will be established upon construction <br />of the respective wells. <br />It should be noted that data collected from the well-casing monuments and <br />possibly the well-pad monuments have some limitations. Both upward and <br />downward movement resulting from thermal expansion and contraction of the <br />FINAL: Modification No. 9, UIC Area Permit No. CO30858-00000 <br />Page 42 of 46 <br />