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I?d 13ul.cr April fi, 20U2 Page 3 A Inter study by AAt in I995~ addressed the same concerns for higher extraction solution mining in Panel I secl:ing recovery of nuhcolitc above the I3oies tied (up to the dissolution surface). The engineering investigation delurmined that the R-C aquitard would elTectively isolate the upper strata Crum the effects of caving al the solution mining hot•izon. The study concluded that high-extraction solution mining (40% or more by volume) was viable with insigniGcattt adverse impacts to the overlying IJSDW aquifers a' Mahogany 7,ate resource. Panel I Cxpcriencc Conclusions from the 1995 study are corroborated by the successful solution mining experience in Panel 1. Available evidence supports the expectation that Che R-6 aquitard is sufficient to protect overlying groundwater and oii shale resources from caving at the solution mining horizon. Supporting technical data includes: • Aquifer water quality monitoring histories • Aquifer potentiometric water level monitoring histories • Well flow balance histories • Surface subsidence monitoring histories • Subsurface subsidence monitoring (time-domain reFlectomet•y (TDR) cable) histories • Cavity sonar surveys • Drilling experience within the active panel These data suggest that subsurface effects terminate near the solution mining horizon without consequence to higher overlying USDW aquifers or the Mahogany oil shale zone. Ground movement measurements in and above Cavity #4A provide the strongest evidence that overburden caving and fracturing is limited to the L-5 and lower R-6 units for vohunetric extraction ratios approaching the design limit (40%°). TDR monitoring in well WRNM 4A-SM and loss of production hibing in vertical recovery well WRNM 4A-6V indicate that roof caving extends above the dissolution surface, but terminates near the base of the R-6 aquitard, as illustrated in figure 3. The loss of approximately 84 ft of production tubing (two links) in well WRNM 4A-6V is attributed to roof fallout around the production string. F[owever, TDR monitoring in adjacent well WRNM 4A-SM shows that no subsurface movement has occurred 190 ft above the dissolution surface. 1'he estimated cavern span in the vicinity of the monitoring location is 85 to 130 ft, corresponding to a volumetric extraction ratio of 39% . Panel 1 cavity production estimates prepared by White River Nahcolite Minerals, LLC (WRN)S suggest that local volumetric extraction ratios as high as 55%4 (south Cavity #lA) have a Agapito Associates, Inc. (1995), "An Evaluation or hnpacts of High-Extraction Solution Mining on Overlying Aquifers and Mineabilily of Oil Shale," Report by M. P. Hardy and M.Lin to White River Nahcolite, Inc., November. White River Nahcolite Minerals, LLC (2002), "2001 Project Status and Annual Plan oP Development," Prepared for Bureau of Land Management, January 3l. ° Volumetric extraction ratio based on 220-ft cavity centerline spacing. s White River Nahcolite Minerals, LLC cavity production-volume allocation spreadsheets through January 2002 for cavities lA, 3A, and 4A, prepared by Steve Schram. Agapito Associates, Inc.