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-23- <br />A. Impacts on Ground Water <br />The three underground mines and one surface mine active within the Canon City <br />Coal Field may impact ground water quantity by one or more of the following: <br />1) Dewatering of the coal, roof and floor with the advance of mining; 2) <br />Subsidence-induced flows into the mines; and 3) Degradation of water <br />recharging ground water aquifers. <br />1) When mine workings encounter saturated portions of the coal seams, <br />water will accumulate and have to be pumped out. The removal of this <br />water will lower piezometric surfaces in the coal seams and immediately <br />adjacent strata. The distance from the mine that this impact will <br />extend to is dependent upon the transmissivity and storage coefficient <br />properties of the strata as well as the lateral extent of the aquifer. <br />In most coal bearing strata, these properties are such that the impact <br />rarely extends more than a mile from the edge of the mine workings. <br />Only the Dorchester No. 1 Mine will be mining below the estimated <br />piezometric surface. This interpretation is confirmed from field <br />observations made by CMLRD personnel in the mines, where water was noted <br />to seep from the roof and floor only in the Dorchester No. 1 Mine. <br />2) Of the three underground mines, only the Dorchester No. 1 Mine will <br />be mining beneath a perennial stream or below the natural piezometric <br />surface. Subsidence-induced impacts of this mine are discussed in the <br />Dorchester No.l Mine Subsidence Section of this document (Section XXI). <br />The impacts of subsidence due to mining can be expected to vary in <br />magnitude and extent even within this one mine. This variation will <br />result from a combination of numerous natural phenomenon and from the <br />effects induced by underground mining. Surface water and ground water <br />can both be influenced by differences in timing of mining and the <br />physical environment. It is assumed that all mining will result in some <br />degree of subsidence at some future point in time, although surface <br />manifestation may require many years to occur. Therefore, control and <br />prediction of subsidence are critical, necessary measures, which should <br />be implemented before and during any mining activity. <br />3) Degradation of the ground water quality can be estimated by comparing <br />the area of potential impact to the total area of the synclinal basin. <br />Two mines that may potentially affect the quality of ground water are <br />the Dorchester No. 1 Mine and Twin Pines Mine. The Dorchester No. 1 Mine <br />permit area occupies 979 acres, or about 1.5 square miles. The Twin <br />Pines No. 2 mine occupies 120 acres or .19 square miles. The perimeter <br />of the synclinal basin encompasses about 47 square miles. Thus, the <br />permit areas occupy about 3.4% of the synclinal basin. If it is assumed <br />that data from Dorchester's monitoring well, MW-10, are representative <br />of water quality in the undisturbed portions of the Vermejo Formation <br />and data from the mine sump are representative of water quality from the <br />mined areas of the Dorchester No.l Mine, and water in the abandoned <br />Caldirola mine workings is representative of the Twin Pines No. 2 Mine <br />then mixing of the ground water from the mined areas of the Dorchester <br />No. 1 Mine and Twin Pines Mine with ground water in that coal seam <br />aquifer throughout the remaining area of the ground water basin would <br />