Research about McDowell County, West Virginia

USGS2017

Hydrogeology, groundwater flow, and groundwater quality of an abandoned underground coal-mine aquifer, Elkhorn Area, West Virginia

The Pocahontas No. 3 coal seam in southern West Virginia has been extensively mined by underground methods since the 1880’s. An extensive network of abandoned mine entries in the Pocahontas No. 3 has since filled with good-quality water, which is pumped from wells or springs discharging from mine portals (adits), and used as a source of water for public supplies. This report presents results of a three-year investigation of the geology, hydrology, geochemistry, and groundwater flow processes within abandoned underground coal mines used as a source of water for public supply in the Elkhorn area, McDowell County, West Virginia. This study focused on large (> 500 gallon per minute) discharges from the abandoned mines used as public supplies near Elkhorn, West Virginia. Median recharge calculated from base-flow recession of streamflow at Johns Knob Branch and 12 other streamflow gaging stations in McDowell County was 9.1 inches per year. Using drainage area versus mean streamflow relationships from mined and unmined watersheds in McDowell County, the subsurface area along dip of the Pocahontas No. 3 coal-mine aquifer contributing flow to the Turkey Gap mine discharge was determined to be 7.62 square miles (mi2), almost 10 times larger than the 0.81 mi2 surface watershed. Results of this investigation indicate that groundwater flows down dip beneath surface drainage divides from areas up to six miles east in the adjacent Bluestone River watershed. A conceptual model was developed that consisted of a stacked sequence of perched aquifers, controlled by stress-relief and subsidence fractures, overlying a highly permeable abandoned underground coal-mine aquifer, capable of substantial interbasin transfer of water. Groundwater-flow directions are controlled by the dip of the Pocahontas No. 3 coal seam, the geometry of abandoned mine workings, and location of unmined barriers within that seam, rather than surface topography. Seven boreholes were drilled to intersect abandoned mine workings in the Pocahontas No. 3 coal seam and underlying strata in various structural settings of the Turkey Gap and adjacent down-dip mines. Geophysical logging and aquifer testing were conducted on the boreholes to locate the coal- mine aquifers, characterize fracture geometry, and define permeable zones within strata overlying and underlying the Pocahontas No. 3 coal-mine aquifer. Water levels were measured monthly in the wells and showed a relatively static phreatic zone within subsided strata a few feet above the top of or within the Pocahontas No. 3 coal-mine aquifer (PC3MA). A groundwater-flow model was developed to verify and refine the conceptual understanding of groundwater flow and to develop groundwater budgets for the study area. The model consisted of four layers to represent overburden strata, the Pocahontas No. 3 coal-mine aquifer, underlying fractured rock, and fractured rock below regional drainage. Simulation of flow in the flooded abandoned mine entries using highly conductive layers or zones within the model, was unable to realistically simulate interbasin transfer of water. Therefore it was necessary to represent the coal-mine aquifer as an internal boundary condition rather than a contrast in aquifer properties. By representing the coal-mine aquifer with a series of drain nodes and optimizing input parameters with parameter estimation software, model errors were reduced dramatically and discharges for Elkhorn Creek, Johns Knob Branch, and other tributaries were more accurately simulated. Flow in the Elkhorn Creek and Johns Knob Branch watersheds is dependent on interbasin transfer of water, primarily from up dip areas of abandoned mine workings in the Pocahontas No. 3 coal-mine aquifer within the Bluestone River watershed to the east. For the 38th, 70th, and 87th percentile flow duration of streams in the region, mean measured groundwater discharge was estimated to be 1.30, 0.47, and 0.39 cubic feet per square mile (ft3/s/mi2

West Virginia↗

USGS2012

Use of modflow drain package for simulating inter-basin transfer in abandoned coal mines

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 3-year study was conducted in the Elkhorn area, West Virginia to better understand groundwater-flow processes and inter-basin transfer in above drainage abandoned coal mines. The study 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 in several ways, 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 reproduce the inter-basin transfer of groundwater from adjacent watersheds. 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 unmined strata and shifting the groundwater drainage divide up dip within the Pocahontas No. 3 coal seam several kilometers into the adjacent Bluestone River Watershed. Model simulations prior to use of the DRAIN package for simulating mine workings produced estimated flows of 0.32 to 0.34 m3/s in each of the similar sized Elkhorn Creek and North Fork Watersheds, but failed to estimate inter-basin transfer of groundwater from 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 m3/s for the Elkhorn Creek and North Fork watersheds respectively, which matched well measured flows for the respective watersheds of 0.47 and 0.26 m3/s.

West Virginia↗