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COLORADO WESTMORELAND INC. SUMMARY OF SUBSIDENCE DATA JAMES V. ROBERTS, GEOLOGIST October 28, 1982 The implications of subsidence data collected suggest that total extrac- tion mining and attendant subsidence manifestations have a negligib]e im- pact upon groundwater occurrences of the Orchard Valley Mine. Amore de- tailed report of hydrologic conditions within the Orchard Valley Mine will be submitted with the 1982 Annual Hydrologic Report. Fault controlled mine inflows which are of an immeasurably small quantity appear to be related to long-term precipitation events, mainly spring runoff. These inflows dripped water during April and May 1982, and have subsequently stopped producing water (refer to mine map). Presently in the northern portions of the mine, damp roof conditions appear to be lithologically controlled. Meandering sandstone channel deposits of local extent drip water of immeasurably small quantity during the first few days of exposure, and subsequently dry up (refer to mine map). The groundwater occurrences within the present lease appear to be unaf- fected by mining practices. The spring located in the southeast quarter of the northeast quarter of Section 19, Township 13 South, Range 91 West is of colluvial origin (see Table #1). The approximate overburden thick- ness of 1130 feet and discharge characteristics of this spring seemingly preclude any disruption of flow. The other groundwater occurrence is a fault controlled spring located in the northeast quarter of the northwest quarter of Section 19, Township 13 South, Range 91 West (see Table #1). Pillar extraction and attendant subsidence in the 7th East Panel have created a "rubble zone far more permeable than the removed coal beds"1. The panel faced up to the fault and the increased storage capacity of the rubble zone will probably enhance the long-term discharge characteristics of this spring. The only subsidence related features located at the Orchard Valley Mine are cracks which lie between the 1st and 2nd West Panels. These features appear to be self-sealing due to natural slough age. A combination of factors contributed to the development of these cracks;l) shallow depth of overburden (less than 500 feet) and, 2) proximity to the outcrop-burn which created a cantilever effect. Other subsidence manifestations have not been discovered during periodic subsidence inspections performed by CWI staff (see Table #2). Surface subsidence is not expected to occur in the property formerly owned by R. Smith. The roof of the coal seam has slumped due to attenua- tion of coal thickness where the burn has been exposed. This situation creates support on the outcrop side of the area, which is different from the conditions present in 1st West. lUSBLM, 1978, Final West-Central Colorado Coal Environment Statement, Volume 1, p. 2 .