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December 23, 1999 • ~ page <br />1.0 INTRODUCTION <br />The subsidence monitoring plan includes both surface and subsurface monitoring of vertical <br />ground movement resulting from solution mining at the Yankee Gulch Lease. Subsidence <br />J monitoring will be performed throughout the commercial operating life of the Yankee Gulch Project. <br />Results ofthe subsidence monitoring (both surface and subsurface) will be checked periodically, as <br />needed, to verify and calibrate numerical models used in the design of the well field layout. <br />Surface monitoring will be conducted by surveying monument locations. Subsurface <br />' monitoring will be conducted by downhole instrumentation over cavities and borehole geophysical <br />' logging of the overlying strata. <br />' 2.0 SURFACE SUBSIDENCE MONITORING <br />The purpose of the surface monitoring is to determine surface movement as a response to <br />' solution mining. The surface monitoring plan has been designed to ascertain the shape and extent <br />of a subsidence basin should one develop over the well field. <br />' 2.1 Monument Locations for the Initial Mining Area <br />Figure 1 shows the approximate locations of surface subsidence monuments over the initial <br />' mining area (0-5 year panel). A total of approximately 51 monuments, including 8 section or <br />quarter-section corners, 20 ground surface locations, and 23 monuments affixed to production well <br />' casings, will be used. The 8 section orquarter-section comers and the 20 ground surface locations <br />will be established prior to the start of commercial operations. The well-casing monuments will be <br />' located on wells to be developed on 600-ft spacing planned for the first pass of solution mining. <br />' These monuments will be established upon construction of the respective wells. <br />It should be noted that the use of monuments affixed to well casings may cause two <br />' problems. First, the reclamation of well pads may have to be decayed until after monitoring ceases, <br />or the monument elevation may have to be adjusted to accommodate reclaimed topography. Second, <br />' both upward and downward movement resulting from thermal expansion and contraction of the <br />casing could contribute to misinterpretation of subsidence movements. <br />' The subsidence monument arrangement in Figure 1 is designed to characterize the shape and <br />extent of a subsidence basin should one develop. The geometry of the monument locations is <br />' Agapito Associates, Inc. <br />