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<br />Figure E-1 in the Preliminary Stormwater Management Plan (Appendix A) of the <br />Preliminary Response Plan shows the Stormwater ditch system and the direction <br />of flow to the Stormwater retention pond. <br />The facilities area topsoil stockpile, which is located east of the facilities area, lies <br />outside the Stormwater ditch system. The ditch system consists of 2-foot-deep V- <br />ditches on either side of an access road. These ditches will provide adequate <br />protection of the topsoil stockpile from spills that could occur in the facilities <br />area. <br />3. The Groundwater and Surface Water Monitoring Plan for the Yankee Gulch <br />Sodium Minerals Project has been submitted. Changes specified in the DMG <br />letter to Paul Daggett dated April 12, 1999, and the DMG internal memo by <br />Harry Posey have been incorporated in this Plan. <br />4. The Subsidence Monitoring Plan for the Yankee Gulch Sodium Minerals Project <br />has been submitted. Included in the Plan is a description of Time Domain <br />Reflectometry (TDR) and mitigation action levels for subsidence detected by <br />TDR. <br />TDR is an accepted and effective method fur monitoring subsurface subsidence <br />in this type of application. Other nahcolite mining operations in the area are <br />currently using TDR with success under similar conditions. Considering <br />information acquired to date, we have no reason to believe TDR will not be an <br />effective technique for monitoring subsurface subsidence. <br />5. Because nahcolite occupies only 25 to 30 percent of the bulk volume of the rock <br />formation being mined, dissolution and removal of the nahcolite does not create <br />a void. The remaining 70 to 75 percent of the bulk volume is insoluble and <br />remains in the cavity. The remaining insolubles create a significant impediment <br />to characterizing the cavity shape. <br />Solution cavity characterization is addressed in Section 3.3 of the Subsidence <br />Monitoring Plan. American Soda will conduct an evaluation of current <br />technologies and methods to characterize cavity shape. If a technology or <br />method is found that has a high probability of success, it will be tested for <br />application in American Soda's operation. <br />The shape of a cavity is predicted to be near cylindrical, with an anticipated <br />maximum height of about 650 feet and an anticipated maximum diameter of <br />about 200 feet. The projected shape of the cavity is based on experimental work <br />1I: AppI.~11u. AJqua~ ~~ 3 <br />Y~nM LuL1~ SJium MmYrW pI1wY <br />Annc~n SdA LLI' <br />