We have developed a method to calculate ground‐ water recharge rates using the mass‐balance equation, water‐ table elevation data, estimates of hydraulic conductivity, and aquifer thickness data, and have applied this method to produce a map of the recharge and discharge patterns for a ground‐water basin in central Wisconsin. This recharge mapping method is simplified using a modified computer program, the USGS Modular Groundwater Flow Model (McDonald and Harbaugh, 1984). The modeled recharge pattern compares favorably with a recharge map based on field observations. Because recharge rates are extremely sensitive to hydraulic conductivity, the magnitudes of the calculated rates are less reliable than the patterns of recharge and discharge areas. However, introducing stream discharge data constrains the model to produce net recharge rates averaged over the basin which agree with estimates of the basin yield. Because the method is insensitive to the position of lateral boundaries, it can be used to map recharge over areas within basins that are not physically bounded. Recharge maps made with this method can be used to design ground‐water monitoring networks and as frameworks for interpreting geochemical or potentiometric data.

Abandoned up-dip drift mines in high-sulphur coal are a major source of acid mine drainage (AMD) in Appalachia. Studies of mines in the Monday Creek watershed of southeastern Ohio show that mines are recharged by surface runoff into subsidence features that dilate the natural stress-relief fracture system. The direct connection between the ground surface and the mines leads to a rapid response in the hydrograph, with a one- to four-day lag between precipitation and corresponding peak mine discharge. Subsidence occurs in topographic depressions where overburden is presumably relatively thin. Subsidence features drain 20–36% of the surface area. Unsaturated storage appears to be volumetrically insignificant, so that far more recharge occurs than the 5% often assumed for this region. Mine storage can change rapidly due to subsidence recharge. Hydrologically, mines with subsidence features behave like karst systems, with meteoric ‘quickflow’ representing more than 50% of the total flow. Mine discharge concentrations are relatively uniform through time, suggesting either equilibrium controls on chemistry or drainage of a well-mixed pool, or both. Evidence of dilution by high flows is slight. The first high flows after a baseflow period show only a slight increase in concentrations, attributed to flushing of stored reaction products. Loadings (concentrations x flow) depend on volumetric discharge and as a consequence are highly variable. The Eh/pH environment in up-dip drift mines indicates that mine waters are in contact with the atmosphere at least part of the time, unlike a true groundwater. Iron buffering partly controls pH, which clusters around values of 3.6–5.0.

An abandoned underground coal mine complex in southeastern Ohio was hydraulically sealed in 1980 by a 300-m long subsurface clay dike and mine-entry seals near the down-dip coal outcrop. Clay plugs also were emplaced in separate entries at a higher elevation than the dike. The intent was to flood the mine to decrease acid mine drainage. A few months after construction, an entry-seal clay plug blew out at an elevation corresponding to 85-90 percent inundation, and drains in natural materials supporting the dike began flowing, indicating leakage through or around the dike. Given these early setbacks, the objective of this study was to assess the sealing project after 20 years. Seasonally, inundation can reach 85-90 percent, but high water levels are transient because of leakage. Nevertheless, mine water chemistry indicates partial suppression of pyrite oxidation. Since sealing, mine water pH increased from 2.7 to 5.3, conductivity decreased from 2700 to 600 mu S/cm, and DO is <2 percent saturation. In a stream receiving the mine drainage, acidity and metals concentrations are highest at low flow due to release of stored mine water, but acid and metal loadings are lowest at those times. The improvement in water quality due to sealing is distinguished from annual variance or natural attenuation by using a nearby unsealed mine as a control. Factoring out pre-sealing differences between the two mines, decreases in acidity loading due to sealing are significant, and the sealing project has been beneficial.

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Understanding flow-system dynamics of underground coal mine complexes is essential to designing in-situ remediation. Such complexes can be studied by applying time series analysis to the precipitation series and well and mine discharge hydrographs. The Corning mine complex discharges acid mine drainage into Sunday Creek in Ohio, USA. Analysis of the Corning discharge shows that the aquifer has a short (9 days) response time and has little capacity for long-term storage. A time lag of 3-4 days occurs between precipitation and discharge, which corresponds to pressure pulse propagation displacing stored mine water, rather than actual advective flow of water. A gradual decline in hydraulic head is observed from the unsaturated recharge area to the fully flooded discharge area. All well hydrographs show a similar seasonal trend, with mine water levels peaking in early summer, and reaching their lowest levels in early fall, consistent with seasonal recharge patterns. A relatively isolated and fully saturated portion of the mine is sensitive to diurnal barometric pressure, as is typical of confined aquifers, with well water level declining as pressure increases. Other portions of the mine behave as unconfined aquifers, insensitive to barometric pressures. The results also demonstrate the important spatial heterogeneity of the aquifer and indicate that the mine does not behave as a single pool.