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<br />Potential Hydrologic Impacts of Underground Mining
<br />Underground mine openings can intercept and convey surface water and groundwater. When excavated below the water
<br />table, mine voids serve as low- pressure sinks inducing groundwater to move to the openings from the surrounding
<br />saturated rock. The result is the dewatering of nearby rock units via drainage of fractures and water - bearing strata in
<br />contact with the mine workings. There is also the potential for impacts to more remote water - bearing units and surface
<br />water bodies depending on the degree of hydrologic communication. The extent and severity of the impact on the local
<br />surface water and groundwater systems depends on the depth of the mine, the topographic and hydrogeologic setting, ani
<br />the hydrologic characteristics of the adjacent strata. Additionally, the amount and extent of mine subsidence - related
<br />changes to the rock mass govern the impacts of underground coal mining on surface water and groundwater.
<br />In the flat -lying sedimentary rocks of southwestern Pennsylvania, underground mining is routinely accompanied by rock
<br />fracturing, dilation of joints, and separation along bedding planes. Rock movements occur vertically above the mine
<br />workings and at an angle projected away from the mined -out area. Mining - induced fracturing within this angle can result in
<br />hydrologic impacts beyond the margins of the mine workings. The zone along the perimeter of the mine that experiences
<br />hydrologic impacts is said to lie within the "angle of dewatering" or "angle of influence" of the mine. Angle of influence
<br />values of 27 to 42 degrees have been reported for the coalfields of northern West Virginia and southwestern Pennsylvania
<br />(Carver and Rauch, 1994; Tiernan and Rauch, 1991).
<br />These changes to the rock mass can change the water transmitting capabilities of the rock by creating new fractures and
<br />enlarging existing fractures. This typically results, at least temporarily, in detectable changes in permeability, storage
<br />capacity, groundwater flow direction, groundwater chemistry, surface- water /groundwater interactions, and groundwater
<br />levels. Depending on the ratio of overburden to seam thickness and the type of mining, measurable surface subsidence
<br />may occur. As previously discussed, this surface movement ranges in type from broad troughs approximating the area of
<br />coal extraction (typical of longwall mining) to complete collapse of overburden from the mine to the surface, e.g., sinkhole
<br />subsidence (generally associated with shallow room - and - pillar mining).
<br />•. The various underground mining techniques have distinctly dissimilar impacts on local water resources. In short, the
<br />impacts of room - and - pillar subsidence tend to be localized, irregular, and often long delayed; whereas those of longwall
<br />subsidence are immediate, pervasive, systematic, and ultimately predictable (Booth, 1997).
<br />The following sections review some general aspects of mining - induced impacts to water resources. However, the impacts
<br />of mine subsidence on surface and groundwater flow quantity and quality are not easily generalized.
<br />"...The enhancement of the overburden hydraulic conductivity due to mining is neither uniform nor well- defined. Predicting
<br />impacts is difficult and there is no such thing as a 'typical' hydrogeologic setting or mine site." (Parizek and Ramani, 1996)
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<br />Potential Impacts on Streams and Surface Waters
<br />The impacts of underground mining on surface waters can range from no noticeable impact to appreciable diminution,
<br />ponding, and /or diversion. The formation of subsidence- induced cracks, surface depressions, and /or sinkholes at the
<br />bottom of, or adjacent to, surface water bodies, such as streams, ponds, and lakes can lead to complete or partial loss of
<br />water due to leakage to the underlying strata. The resultant changes in surface slope can adversely impact drainage along
<br />irrigated fields, canals, sewers, and natural streams (Bhattacharya and Singh, 1985).
<br />Room - and - pillar mining is generally less disruptive to nearby surface waters than high - extraction methods. Individual
<br />openings have only minimal localized draining impacts due to self- supporting roof members which span the opening to
<br />form a compression arch, with the support pillars serving as abutments. This "pressure arch" limits not only the
<br />deformational, but also the hydraulic influence of the opening (Booth, 1986). As additional entries are driven, the resultant
<br />network of intersecting drains act as a planar underdrain inducing downward leakage from overlying units. However, due tc
<br />its built -in system of support pillars and limited mining- induced fracturing, significant drainage is typically limited to near-
<br />mine units.
<br />• Many detrimental impacts of room - and - pillar mining take years or even decades to occur as weak coal pillars deteriorate
<br />over time (Sgambat, 1980). Deteriorating or under -sized pillars that fail over time result in vertical extension of mine-
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