THE IMPACT OF VEGETATION ON MANGANESE BIOGEOCHEMISTRY IN ABANDONED COAL MINE SPOIL

Abandoned mine lands often contain piles of mine refuse (spoil) that continue to leach acid and heavy metals into groundwater and streams long after mining operations have ceased. Revegetation is a common reclamation technique that is used to stabilize mine waste against erosion and improve infiltration, but more research is needed to evaluate how vegetation impacts the mobility of metal(oid)s in the refuse and consequent metal(loid) leaching into groundwater and surface waters. In this study, we investigate the potential for forest vegetation that has developed on coal mine spoil to mitigate leaching of the metal manganese (Mn). We examine a forested watershed in the Appalachian region of Ohio where abandoned piles of coal mine spoil contain pyrite-bearing coal and black shale that continue to weather and release Mn into pore waters. We hypothesize that vegetation removes Mn from soil solution due to uptake, temporarily stores it in biomass, and then returns it to the soil surface in leaf litter where it is immobilized as Mn-oxides during litter decomposition. Geochemical analyses of soil, soil pore waters, and tree foliage are being used to evaluate changes in solid-phase Mn with depth in the soil, and to quantify annual cycling of Mn through vegetation versus loss of Mn in soil leachate. High concentrations of Mn in leaf litter (> 1,000 ppm) denote substantial uptake of Mn by trees; however, concentrations of dissolved Mn in pore water increase with depth in the soil and may leach from actively weathering zones below the rooting depth. Solid-phase Mn in the coal mine refuse is dominated by Mn-oxides that form a discrete Mn-rich layer below the rooting zone. This Mn-oxide-enriched layer overlies soil that contains a relatively large fraction of reduced material (organics and sulfide minerals) and may represent a redox interface. Our study provides insight into the impact of forest vegetation on the geochemical evolution of mine spoil and biogeochemical cycling of Mn in soils.