Date of Graduation
Eberly College of Arts and Sciences
Geology and Geography
Joseph J Donovan
This study examines the water-level elevation history of selected flooding and flooded underground mines in the Pittsburgh coal basin of SW Pennsylvania from the time of closure until post-flooding pool-level reequilibration. Mines within this mining district developed pools with nearly steady-state groundwater flow within 10 to 50 years after closure. Equilibrated pool levels within each of the mines were controlled by various combinations of spillage to the surface or other mines, pumpage, and barrier leakage. In a study of flooding in the Clarksville, PA area, field water-level observations, mine geometry, barrier hydraulic conductivity, recharge rates, and late-stage storage gains were parameterized to match known pumping rates and develop a fluid mass balance. Vertical infiltration (recharge and leakage) estimates were developed using a depth-dependent model based on the assumption that most vertical infiltration is focused in areas with <75 m of overburden. A MODFLOW simulation of the nearly steady-state flow conditions was calibrated to hydraulic heads in observation wells and to known pumping rates by varying barrier hydraulic conductivity. The calibrated model suggests significant head-driven leakage between adjacent mines, both horizontally through coal barriers and vertically through interburden into an overlying mine. Calibrated barrier hydraulic conductivities were significantly greater than literature values for other mines at similar depths in the region. This suggests that some barriers may be hydraulically compromised by un-mapped entries, horizontal boreholes, or similar features that act to interconnect mines. These model results suggest that post-mining inter-annual equilibrium conditions are amenable to quantitative description using mine maps, sparse observation-well data, accurately-estimated pumping rates, and depth-dependent vertical infiltration estimates. Results are applicable to planning for post-flooding water control schemes, although hydraulic testing may be required to verify model results.;In a second study of a nearby area, three mines were mapped to determine mining type distribution (longwall, etc.) and these mining-type areas assigned typical porosity values based on industry-standard extraction ratios. The porosity estimates were plotted against coal-base elevation contours to model the hypsometric distribution of porosity. Using pumping rates from active operations and these hypsometric porosities, the approximate duration of flooding was estimated for two of the mines; these overestimate the actual (observed) flooding time by 200-275%. On the other hand, mine inflow rates estimated using observed water levels and the porosity model indicate temporal changes in the fluid mass balance for each mine that are consistent with spillage and/or barrier leakage between mines interpreted from water-level hydrographs. Results indicate that accurate prediction of the duration of mine flooding requires explicit consideration of groundwater conditions in adjacent mines and the potential for barrier leakage.
Light, David D. M., "Flooding and reequilibration of a series of Pittsburgh bed underground coal mines, 1980 to 2015" (2016). Graduate Theses, Dissertations, and Problem Reports. 6088.