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.lwgiat 1991 Gypna 7LC •Arulyrir of.Lrbridencr Efferu on Hydrdogy • 3R • The potential oaurrence of rubsidence rocks is another feature that could disrupt rurface and shallow groundwater regimes. Recent experience from Ohio and West Virginia, rupported by other experience gained from mines in the Somerset and Paonia, Colorado areas, indiates that rubsidena cndu may develop in area of low overburden thickness and on bedrock outcrops. Ongoing subsidence monitoring for the Foidel Creek Mine did not indiate any visible rurface uadss in the undermined area. The occurrence of subsidence sacks on the surface may not necessarily rerult in mine inflows when a surface water body is disrupted. Studies by various authors such u Orchard (1969, 1975) in Great Britain, Williamson (1978) in Australia, and Hydro-Geo Consultants, Inc., in West Virginia (1991) and Ohio (1988), indiate the rubsidence cracks are limited in depth by the development of compressive stresses repladng the tensile stresses that cause surface fracturing. The depth of rurface rubsidence cracks due to tensile strew does not typially ezceed 50 feet. Depth, extent, and size of surface tension a~acks would be limited in the unconsolidated sediments of the Fish Creek AVF. Subsidence cracks occurring across a stream dtannel could result in disruption of streamflow. Due, however, to the presence of thick, fino-grained sedimenu in and underlying the Fish Creek streambed within the study area, signifiant surface cracking is not antidpated, and any sacks which do develop should be reduced u subsidence movement relieves assodated stresses. Fractures in the fine-grained sediments will collapse and fill due to the weight of overlying materials, and settlement and consolidation of partides deposited in fracture voids by natural surface and groundwater action. Given these considentioas, no significant flow impacts are antidpated. • 2.2 IMPACTS ON SURFACE WATER QUALITY Impacts on surface water quality due to rubsidence an develop for the following reasotu: • Changes in the land rurface within the drainage area and changes in stream gradients Change in groundwater quality in cares of gaining streams Changes in land surface and stream gradient can affect weer quality by inaeuing sediment load. Significant increases in stream gradient may rerult in increased flow velodties and, therefore, increased erosion potential. The potential for surface water quality changes is ezpressed by the Universal Soil Loss Equation (Wisdtmeier and Smith, 1978) which is relatively insensitive to minor loalized changes in rurface slope of the magnitude projetxed for rubsidena areas. The potential for impacts on surface water quality due to dtanges in groundwater quality is insignificant, espedally in cues where only a small part of the s[reamflow is being redtarged from the bedrock adjacent to the alluvial valley floor. This is believed to be the case in the study azea. 3.0 IMPACTS ON GROUNDWATER Longwall mining (and, to a lesser eztent, partial extraction) of coal seams cruses collapse, • fracturing, bed separation, and bedding plane slip in the roof straw above the seam. All of these impacts on the overlying strata can result is dtartges to surface and groundwater if a major water resource is within the limits of the disturbance. The venial limit of disturbance depends ~' 0'6''`„„ M A CZ /nt. • P.O. Boz 771018 • Sttambart Springr, Colorrdo 80!77 • (303J879d260