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o area of stock ponds in the southwest part of Section <br />8 (T135, R91W). <br />To eliminate oc reduce the hazard of surface water <br />disruptions would require a change of the mining plans and <br />design of protective pillars or zones of lower recovery <br />beneath these areas as discussed in section 8.3. <br />Alternatively, surface modifications of the surface drainage <br />and ponds could be made in cases of any occurrence of <br />subsidence cracks. Such surface modifications would not <br />preclude a temporary disruption of the existing surface <br />water regime. <br />6.3 GROUND WATER <br />Total extraction (and, to a lesser extent, partial <br />extraction) of coal seams causes collapse, fracturing, bed <br />separation and bedding plane slip in the roof strata above <br />the seam. All of these disturbances of the overlying strata <br />can have a substantial impact on ground water if a major <br />aquifer is within reach of the disruption. The height of the <br />disturbed area depends on the thickness of the mined coal, <br />the mining method, the rate of the mining face advancement, <br />and on the geological characteristics of the overburden. <br />• The area of disturbance is generally divided into two <br />or three zones, based upon the extent of fracturing, as <br />follows: <br />o zone of primary caving :~~here [he caved rock is <br />completely disintegrated <br />o zone of secondary caving where a network of <br />horizontal and vertical cracks develops <br />o zone of bed separation where separation occurs <br />primarily along ire-existing bedding planes <br />The total thickness of first and second zones, where <br />the changes of permeability are substantial, typically <br />reaches 3 to 3.~ times (Ropski and Lama, 1973) and rarely <br />more than 10 times the height of the extracted seam <br />(lvardell, 1970) . <br />The height of the third zone, or the total height <br />where changes in permeability due to subsidence can occur, <br />is described by various auti~ors as 30 t (where t is the <br />fully extracted seam thickness), 58 t (Gviroman, 1977), 33.7 <br />t (Williamsor„ 1973) and 30 *_ (Wardell, '_97G). <br />• <br />- ~ `' - <br />