Macropore flow on a reclaimed surface mine: infiltration and hillslope hydrology

Surface coal mining in central Pennsylvania alters the physical and hydrological properties of soil from its undisturbed state and changes the surface hydrologic response to precipitation. Newly reclaimed minesoils exhibit low, steady-state rates of infiltration (1–2 cm/h) and produce runoff dominated by infiltration-excess overland flow. Within 4 years after reclamation, rates of infiltration on some minesoil surfaces approach pre-mined rates (8 cm/h). As a greater percentage of rainfall infiltrates these reclaimed minesoils, little change occurs in the total volume of surface storm runoff on an event basis. This indicates that infiltrated water contributes to storm runoff by the processes of throughflow and return flow. As the rate of infiltration increases with time, the hydrologic response of the minesoil is controlled by rapid macropore throughflow within the unsaturated minesoil. Application of fluorescent dyes on the reclaimed surface reveals preferred flowpaths through macropores surrounding the numerous, large rock fragments in the minesoil. Development and integration of a macropore network on a reclaimed, surface-mined watershed has a significant, direct effect on the contributions to surface storm runoff from infiltration-excess overland flow, throughflow, and return flow. As rates of infiltration increase through time and slightly delayed macropore return flow increases, the peak rate of surface storm runoff is reduced, therefore reducing the potential for severe gully erosion on the reclaimed site. In addition, throughflow moves predominantly within the minesoil horizon and has limited contact with potentially acid-producing backfill. Better understanding of the process of macropore flow in reclaimed minesoils will help investigators evaluate past strategies and develop new reclamation techniques that will optimize the long-term effluent and ground water quality while minimizing the short-term surface erosional effects of mining and reclamation.