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in the cavern diagram attached to the report. The cavern diagram depicts the projecte edian eter <br />of each production well for the upper zone, the lower zone, and a combined zone (upp <br />lower). Based on the map, it appears that the lower dissolution zone of wells 20-74, 20-31 and <br />possibly 20-33 aze in communication. It also appeazs that wells 29-29 and 29-68 the lower <br />dissolution zones may also be in contact. <br />Review of the American Soda permit file indicates that there is no permit restriction on the <br />maximum diameter for solution-mined cavities. This is because during original permitting it <br />was assumed, based on results at the test mine facility and thermal-economic modeling, that <br />even reaching 200 feet diameters in the cavities was unlikely. American Soda provided a report <br />prepazed by Agapito Associated, Inc. titled, "An Evaluation of Stability and Potential <br />Environmental Impact of the Commercial Solution Mine Plan." One of the commitments in the <br />report is the spacing between adjacent cavities. A minimum intact pillaz width of 50 feet must <br />be maintained between adjacent cavities to prevent cross communication of the mining fluids <br />and the potential for unacceptable amounts of cavity collapse in the direction of the dissolution <br />surface. The extent of cavity width variation that maybe caused, for example, by more rapid <br />dissolution of rich nahcolite beds, is the chief reason for the requirement to conduct cavity <br />chazacterization. <br />The purpose of the cavity chazacterization study was to better understand the size of the <br />cavities, and determine if a stable spacing for adjacent solution mining wells could be <br />established such that the minimum intact pillar width could be maintained while maximizing <br />resource recovery. Based on the communication between wells 20-74, 20-31 and possibly 20- <br />33 and wells 29-29 and 29-68, it appears that the current mine plan will need to be revised to <br />ensure of the 50 foot pillaz width is maintained. <br />Therefore, the Division requires that prior to any future mining at the Yankee Gulch Minerals <br />project American Soda will need to revise the mine plan using the we1128-21 cavity <br />characterization model to determine how other wells can be mined to establish proper spacing <br />for the wells and the maximum cavity diameters that can be developed. <br />If you have any questions, please do not hesitate to contact me at 303-866-4943. <br />S' cer ly, /~ <br />rica S. rosby <br />Environmental Protec ~ n ialist <br />cc: David Berry; DBMS <br />Carl Mount; DBMS <br />Allen Sorenson; DRMS <br />Paul Daggett; BLM <br />Dan Jackson; EPA <br />