Hydrogeology of reclaimed Gulf Coast lignite mines

Abstract

A theoretical study of the hydrogeology of reclaimed Gulf Coast lignite surface mines (less than 200 ft. deep) indicates relationships between spoil composition and spoil hydraulic conductivity, overburden stratigraphy and spoil hydraulic conductivities in a reclaimed surface mine, and depositional environment of the lignite and post-reclamation hydrogeologic conditions at a reclaimed surface mine. The hydraulic conductivity of mine spoil is a function of the composition of the spoil. A detailed knowledge of the overburden stratigraphy at a mine can be used to predict the composition of spoil and thus its hydraulic conductivity, as well as provide the means for extrapolating hydraulic conductivity data collected in one part of a reclaimed mine to other parts of the mine with similar stratigraphic conditions. The ratio of sand to shale in spoil will control its total volume and porosity increase upon being disturbed during mining, and its compressibility and porosity decrease with time and depth of burial in a reclaimed mine. Overburden consisting of overconsolidated shale will experience a relative increase in porosity and hydraulic conductivity upon mining. However, porosity and hydraulic conductivity will decrease with time after initial spoil placement and with increasing depth of spoil burial so that shale-rich spoil will become impermeable (K less than 10('-7) cm/sec) at depths greater than 30.5 m (100 ft). This will minimize the movement of mine water into deep aquifers. The hydraulic conductivity of sand-rich overburden will decrease upon being disturbed, thus lowering the transmissivity of reclaimed aquifers. Since the hydraulic conductivity of mine spoil is dependent upon spoil lithology and depth of burial, sand and shale isopach maps of the lignite overburden can be used along with isopach maps of the overburden thickness to determine the spatial variation of hydraulic properties that could occur at a site that is mined and reclaimed.