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~'"'~~4'~. UNITED STATES ENVIRONMENTAL PROTECTION AGENCY <br />D ~ REGION 8 <br />~~' 999 18r" STREET -SUITE 300 <br />~ DENVER, CO 80202-2488 <br />http://www.epa.gov/region 08 <br />Ref: 8P-W-GW <br />ADDENDUM <br />TO THE <br />STATEMENT OF BASIS <br />MINOR PERMIT MODIFICATION No. 6 <br />EPA FINAL AREA PERMIT <br />No. C030858-00000 <br />Class III SOLUTION MINING WELLS <br />YANKEE GULCH PROJECT <br />Rio Blanco County, Colorado <br />As a result of on-going discussions with American Soda regazding review and <br />interruption of the quarterly monitoring reports relating to possible cavity instability, American <br />Soda undertook a complete review of their cavern monitoring system. The purpose of this <br />review was threefold: 1) to look at the possible causes of an out of a reported out-of-balance <br />situation in cavity 20-26; 2) to evaluate the various monitoring pazameters and their relationship <br />to potential cavity failure; and 3) to come up with a process and criteria for determining if there <br />may be a cavity failure. As a result of this review, a meeting between EPA and American Soda <br />representatives was held on January 29, 2003, to discuss the well stability issue and possible <br />failure. It was agreed at this meeting that the following general relationships between <br />temperature, pressure, flow and concentration/density existed in the nahcolite solution mining <br />wells during normal mining operations: <br />1) the inlet pressure increases as the concentration ofnahcolite in the production <br />fluid increases; <br />2) the volumetric flow rates of the injection fluid and production fluid are roughly <br />equal (after accounting for additional fluids having been left in cavity as it grows); <br />3) the cavity temperature increases as a result of an increase in injection fluid <br />temperature; <br />4) the concentration of nahcolite in the production fluid increases as cavity <br />temperature increases; and <br />5) cavity pressures remain stable. <br />Page 1 of 3 <br />~Pdnted on Recycled Paper <br />