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-40- Cumulative N drologic Impact AcrceaHnent -Ground Flate-' (COnt'd) Subsidence The impacts of subsidence due to mini~iu can be expected to vary in magnitude and extent. This variation will result from a combination of numerous natural phenomenon and from the effects induced by underground mining. Surface water and ground water can both be influenced by differences in timing of mining and the physical environment. It is assumed that all mining will result in subsidence at some future point in time, although surface manifestation may require many years to occur. Therefore, control and prediction of subsidence are critical, necessary measures, which should be implemented before and during any mining activity. Natural phenomena which influence subsidence are: - Depth of overburden; - Thickness of the coal seam; - Stratigraphy, lithology and structure of the overburden; - Topography; and - Lithology of the floor strata . Mining activities which affect subsidence are: - Size, shape, depth, location, and areal extent of workings; - Rate of development and extraction of resource; and - Mining methods. The possible impacts of subsidence include: - Modification of surface water hydrology - channel and watershed geometry and sediment erosion rates; - Modification of ground water hydrology - changes in spring discharge, aquifer characteristics and recharge; - Loss of water from surface impoundments due to disruption by surface cracking; and - Increase in surficial mass movement activity -landslides and rock falls. Natural physical influences upon subsidence can be controlled to the extent that mine plans can be designed to avoid obvious hazards (e.g. shallow openings beneath perennial streams, landslide bodies, impoundments, etc.), or to limit subsidence in accordance with the current state-of-the-art of ground control (engineered control of roof and pillar failures in mines). Subsidence prediction and ground control engineering are not definitive sciences. Design engineering concepts must be verified through continuous monitoring (surface topography, ground water hydrology and surface water hydrology) and observation. Evaluation of the impacts of subsidence upon hydrologic systems requires the collection of extensive monitoring data consisting of surface water and ground water monitoring (quantity and quality), precipitation gaging, evapotranspiration rates, geological mapping, topographic surveying and recording of mine inflow discharge and consumption rates. The monitoring data should be updated continuously as mining Frogresses to improve engineering control design parameters or to mitigate any unexpected subsidence occurrences. (Such procedures are necessary at each mine within the Somerset Coal Field in order to coordinate a comprehensive subsidence control plan.) This monitoring is now being required of operators thrc~~gh the Colorado Permanent Regulatory Program permitting process. It should be noted that no r.~o,~itoring is required at the Blue Ribbon :line due to the absence of structures or reno~~able resource lands.