Subsurface Water Chemistry in Mined Land Reclamation; Key to Development of a Productive Post-mining Landscape

In reconstructing the landscape during reclamation the sequence and methods of placement of overburden material determine the post-mining subsurface-water chemistry. The type of material at various places in the landscape, especially at and just below the land surface, determines the initial chemistry. The configuration of the land surface and distribution of permeability beneath the surface determine how the chemistry will evolve over time. The configuration of the land surface controls frequency and location of infiltration. The distribution of material beneath
the surface determines the rate of subsurface-water flow and thus the degree of flushing of the system. Interaction between the surface configuration and permeability distribution determines the shape of the water table, its proximity to the land surface, and the location and magnitude of groundwater discharge. The development of fractures in the cast overburden, as a result of differential settlement, can permit penetration of oxygen deep  beneath the surface. The resulting oxidation of large amounts of pyrite can produce, at least initially, very high levels of sulfate and salinity and low values of pH. Bulking of the cast overburden as it is emplaced will result in initially high values of hydraulic conductivity that may decrease over time. This would result in an increasingly sluggish flow system and a tendency for the water table to rise over a period of time.