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3.1.3 TDR Wells <br />Two wells with TDR cables are currently in use: 4A -5M south of the plant (Area P1) and 3M - <br />TDR south and east of the plant (Area B). Measurements from these devices will be <br />collected monthly by NS, and quarterly by a third party, during active mining and for three <br />years after cessation of mining within the areas south, and south and east of the plant <br />(Area P1 and Area B), or until they fail, whichever occurs first. If ground movement is <br />detected at 55 feet above the DS, NS will notify EPA, BLM, and DRMS and initiate an <br />investigation. If caving should occur 75 feet above the DS, NS will notify the EPA, BLM, and <br />DRMS and detail the remedial actions taken to slow or arrest further caving. If caving is <br />detected 95 feet above the DS, the EPA, BLM, and DRMS will be notified, and mining in the <br />area of the monitor well will stop until regulatory agencies approve alternatives. No more <br />TDR wells will be installed by NS. <br />3.2 Surface Subsidence <br />No surface subsidence has been detected to date. The purpose of the surface subsidence <br />monitoring is to determine if there is any surface movement resulting from solution mining. <br />No significant surface subsidence is anticipated. The surface monitoring plan has been <br />designed to ascertain surface subsidence over the solution -mined cavities. Surface <br />Subsidence monuments are constructed by cementing in place a section of rebar to <br />approximately 3 feet below the ground surface and/or into bedrock when encountered. <br />The coordinates and elevation of each monument are surveyed and recorded upon <br />installation. A surface subsidence control point is located at the Natural Soda plant and is <br />used to relate historic and newly collected data. Data collected from the Surface <br />Subsidence Monuments and monitoring are included in the annual report. The accuracy of <br />the surveys for the surface monument elevations will be commensurate with the <br />equipment standards in use by professional surveyors, typically within ± 0.5 feet. To date, <br />historical data has shown a typical survey variance of ± 0.2 feet.9 <br />3.3 Cavity Configuration <br />Cavity growth, size and geometry will be monitored throughout the operating life of the <br />wells according to the procedures listed below: <br />• A monthly mass balance calculation will be made of cavity volume based on daily <br />measurement of injection/recovery fluid parameters; flow, temperature, and <br />density. <br />• A mass balance leaching simulation model with volume calculation, to predict cavity <br />development with respect to shape and size will be used periodically. <br />Daub & Associates, Inc. <br />9 Revised: 9/18/2015 <br />Page 15 Natural Soda LLC 2015 EMP <br />