Use of Modflow Drain Package for Simulating Inter-Basin Transfer of Groundwater in Abandoned Coal Mines

Abstract

Simulation of groundwater flow in abandoned mines is difficult, especially where flux to and from mines is unknown or poorly quantified, and inter-basin transfer of groundwater occurs. A study was conducted in southernmost WV to better understand inter-basin transfer of groundwater in above-drainage abandoned coal mines. The Elkhorn area was specifically selected, as all mines are located above the elevation of tributary receiving streams, to allow accurate measurements of discharge from mine portals and tributaries for groundwater model calibration. Abandoned mine workings were simulated initially as a layer of high hydraulic conductivity bounded by lower permeability rock in adjacent strata, and secondly as rows of higher hydraulic conductivity embedded within a lower hydraulic conductivity coal aquifer matrix. Regardless of the hydraulic conductivity assigned to mine workings, neither approach to simulate mine workings could accurately estimate interbasin transfer of groundwater from the adjacent Bluestone River Watershed. To resolve the problem, a third approach was developed. The MODFLOW DRAIN package was used to simulate seepage into and through mine workings discharging water under unconfined conditions to Elkhorn Creek, North Fork, and tributaries of the Bluestone River. Drain nodes were embedded in a matrix of uniform hydraulic conductivity cells that represented the coal mine aquifer. Drain heads were empirically defined from well observations, and elevations were based on structure contours for the Pocahontas No. 3 mine workings. Use of the DRAIN package to simulate mine workings as an internal boundary condition resolved the inter-basin transfer problem, and effectively simulated a shift from a topographic-dominated to a dip-dominated flow system, by dewatering overlying un-mined strata and shifting the groundwater drainage divide up dip within the Pocahontas No. 3 coal seam approximately five kilometers into the adjacent Bluestone River Watershed. The simulation of mine entries and discharge using the MODFLOW DRAIN package produced estimated flows of 0.46 and 0.26 m 3 /s for the Elkhorn Creek and North Fork watersheds respectively, which matched measured flows for the respective watersheds of 0.47 and 0.26 m 3 /s.

Natural and Anthropogenic Processes Affecting Domestic Groundwater Quality within the Northwestern Appalachian Basin

“…Higher helium concentrations (mean = 1.57 × 10 –1 ccSTP/L, SD = 2.51 × 10 –1 ccSTP/L), higher 4 He/ 20 Ne (mean = 394, SD = 522), and lower average (mean = 0.331 RA, SD = 0.544 RA) ratios of 3 He/ 4 He trending toward crustal production ratios support the hypothesis that samples in C3 represent groundwaters that have undergone extensive interactions with aquifer materials over comparatively long flow paths that likely transit the core of hillslopes before discharging near valley bottoms . Within this region, these flow paths are theorized to occur laterally along permeable formations and downward through near vertical fractures and cleats in a stairstep pattern. ,,− …”

“… 83 Within this region, these flow paths are theorized to occur laterally along permeable formations and downward through near vertical fractures and cleats in a stairstep pattern. 55 , 57 , 86 − 88 …”