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1'.d 13ukcr April 8, 2(1(12 Page 5 distances above the solutiun mining horizon than demonstrated in Panel I, raising concerns that the R-6 aquitard may prove too thin to adequately protect the aquifers or Mahogany Zone. Specilic adverse impacts of issue are (I) increased hydraulic conductivity causing leakage and/ot' contamination of aquifers, and (2) rock mass damage rendering the oil shale resource unminable by conventional underground methods. USDW Aquifers Figure 8 conceptually describes the evolution of overburden disturbance above an ever- widening solution cavity. The sequence shows that roof collapse will first occur at some limiting span (Stage 3}, followed by progressive caving upward into the overbw~den (Stage 4). Eventually, caving is arrested by the bulking of collapsed material (Stage 5). 'this point def nes the upper extent of the "cave zone," as discussed in the 1995 AAI reportz. Above the cave zone, strata are subjected to severe bending, producing a zone of fracturing. The degree of bending and fracturing diminishes further into the roof (Stage 6). At this cavity span, a stable arch exists near the top of the ti•achue zone. At wider spans, the height of the fracture zone increases, disturbing a greater volume of overburden sU~ata (Stage 7). Finally, beyond some "critical width" all overburden strata are Cully disturbed and maximum surface subsidence is achieved (Stage 8). Above the fracture zone, overburden strata lie within the "zone of continuous deformation," remaining effectively intact and unfractured. The risk of disturbing overlying aquifers depends upon the height of the cave and fracture zones. It is estimated that solution mining in Panel 1 has not progressed beyond Stage 4 or 5 (Figure 8) due to limited extraction. However, there is a reasonable likelihood that combined Panel 1-2 mining will approach very high to near complete extraction at some locations (i.e., continuous dissolution of intervening cavity pillars). While it is unlikely that "critical widths" for subsidence will be achieved, it is not unreasonable to expect Stage 6 or even Stage 7 (Figure 8) disturbance within the range of economic mining. Extensive investigation by the Chinese Central Coal Mining Research Instituteb shows [hat the potential for altering hydraulic conductivity in the overburden is generally limited to the cave and fracture zones, while above the fracture zone there is no signifcant change in conductivity. Using the running-water technique, the Lrstitute has successfully measured the "hydraulic" fracture zone heights for more than 1301ongwall panels in 45 mines. The technique requires a surface hole be drilled into the gob over a fully caved longwall panel. Static water levels and water consumption during drilling are monitored. The fracture and cave zones are identified and defined in terms of drilling water consumption q, where: `Chinese Central Coal Mining Research Institute, [3eijing Coal Mining Institute (1981),Surface Morwnen(s, Overbao-dcn Faihu~e nncf Its Applicntions, Liu, T.Q. (editor), Coal Industry Publishing Company, China, December, 369 pp. (in Chinese). ~ Peng, S. S. (1992), Surface Subsidence Engineering, Published by Society of Mining, Metallurgy, and Exploration, Inc., Littleton, Colorado. Associates, Ittc.