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APPENDIX D <br />SPECIFIC RETENTION AND ITS CONSEQUENCES TO WATER MOVEMENT <br />IN THE AREA OF THE NORTHFIELD MINE <br />Specific retention of a soil or rock is the ratio of the volume of water retained after saturating and <br />then draining by gravity, to its own volume. The water retained in the materials is held by <br />capillary forces against the pull of gravity. <br />Before a coal seam such as the Ocean Wave in the Vermejo Formation is mined, all pore spaces <br />are filled with water, except for possibly a minimal amount of entrapped air. When underground <br />excavation of the coal begins, water is allowed to drain freely into the resultant mine cavity (see <br />Figures 5 and 6 in text). That water which remains in the formation, above the mine floor level, <br />is specific retention. Similarly, any water left within lumps of removed coal is specific retention. <br />After excavation of a coal seam has started, the ground water system is altered. The saturated <br />level of the aquifer is lowered to the floor level of the mine, or to the free water level in the mine <br />• if the mine is flooded. Coal removed from the face of the mine is above the saturated level and <br />any water left in the mined coal is specific retention. <br />Water removed with the raw coal from the Northfield Mine, specific retention1, estimated to be <br />8.33 percent by wet weight of the raw coal, is "mined" water, or water which is not an active part <br />of the surface water system. The removal of this water in the coal does not bring about a decrease <br />in water flow downstream, since this water is held by the coal as specific retention in the altered <br />environment of the mine. Meanwhile, any free water in the immediate vicinity of the excavation <br />continues to flow into the mine cavity to ultimately become part of the surface water system. <br />' A laboratory estimate of specific retention is obtained by placing a thoroughly wetted core sample of coal <br />in a desiccator for one day with 97 percent relative humidity and 30 pounds of vacuum pressure. The moist <br />weight is then recorded, and the sample is oven dried at 104°C for four days. The difference between the <br />• moist weight and the oven-dried weight is an estimate of the amount of water which would not drain by the <br />force of gravity alone. <br />