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' December 23, 1999 ~ ~ Page a <br />' 4.0 REPORTING <br />' A subsidence report will be submitted to the BLM annually. The report will contain data <br />from surface monument measurements, TDR data, and borehole geophysical data. The annual <br />' subsidence report will include a map of surface monument locations, locations of wells where TDR <br />and borehole geophysics were conducted, a list of monument and well coordinates, and a summary <br />of pertinent observations during the reporting period. In accordance with CDMG requirements, if <br />surface subsidence is detected, it will be reported to the agencies within five working days. <br />' S.0 MITIGATION <br />No mitigation is currently planned for surface subsidence. Measurable surface subsidence <br />is not anticipated. Even in the event of the failure of several pillars between mature pillars, long- <br />term subsidence is expected to be less than three feet. A subsidence basin of such small magnitude <br />' over the size of a well field will be visually unnoticeable. <br />The TDR and borehole geophysics monitoring will be used for monitoring cavity roof <br />' integrity. The thickness of the strata between the cavity roof and the Dissolution Surface will vary <br />in thickness from 150 ft up to 300 ft. If one-half foot of separation or fracturing in the cavity roof <br />is detected within a distance of 50 ft of the Dissolution Surface, then the gas cap will be lowered so <br />that no fluid is in contact with the cavity roof, and cavern integrity warning systems (process <br />' monitoring) will be monitored closely for the life of the well. <br />Ifthe TDR system (described in Section 3.0) detects downward vertical movement (typically <br />' about 6 inches) at the Mahogany Zone in any solution mining well, American Soda will initiate the <br />following actions: <br />• Notify CDMG, BLM, and EPA that movement in the Mahogany Zone has been <br />indicated. <br />• Evaluate the integrity of the coaxial cable and the instmmentation associated with the <br />' TDR system at the particular solution mining well and evaluate the TDR <br />measurements in other solution mining wells in the azea. <br /> <br />' Agapito Associates, Inc. <br />