Iron and Manganese Biogeochemistry in Forested Coal Mine Spoil

Abstract

Abandoned mine lands continue to serve as non-point sources of acid and metal contamination to water bodies long after mining operations have ended. Although soils formed from abandoned mine spoil can support forest vegetation, as observed throughout the Appalachian coal basin, the effects of vegetation on metal cycling in these regions remain poorly characterized. Iron (Fe) and manganese (Mn) biogeochemistry were examined at a former coal mine where deciduous trees grow on mine spoil deposited nearly a century ago. Forest vegetation growing on mine spoil effectively removed dissolved Mn from pore water; however, mineral weathering at a reaction front below the rooting zone resulted in high quantities of leached Mn. Iron was taken up in relatively low quantities by vegetation but was more readily mobilized by dissolved organic carbon produced in the surface soil. Dissolved Fe was low below the reaction front, suggesting that iron oxyhydroxide precipitation retains Fe within the system. These results indicate that mine spoil continues to produce Mn contamination, but vegetation can accumulate Mn and mitigate its leaching from shallow soils, potentially also decreasing Mn leaching from deeper soils by reducing infiltration. Vegetation had less impact on Fe mobility, which was retained as Fe oxides following oxidative weathering.

Biogeochemical Controls on the Potential for Long-Term Contaminant Leaching from Soils Developing on Historic Coal Mine Spoil

“…The primary source of Fe and Mn in coal shales is typically sulfide phases [22,75], and the small amount of Fe in the oxidizable fraction, which tracks with the LOI values in the mine spoil soil, is likely primarily associated with the residual pyrite identified (Figure S5). More recently, deposited spoil pile at the same site had greater proportion of Fe in the oxidizable fraction, consistent with less weathering [9].…”

“…The enrichment of Mn in the shallow surface could be due to biocycling which is driven by the accumulation of litter-derived organic matter in shallow soils [88,89]. Aqueous Mn concentrations and the abundance of solid-phase Mn consistent with Mn oxides was also high below the deep organic-rich layer, from which we infer that aqueous Mn at depth was leached from Mn-bearing pyrite in the organic-rich layer [9]. Higher amounts of exchangeable Mn soils developing on shale can also be the result of a high abundances of clay minerals which can leach Mn [90]; however, there was no evidence for significant differences in clay mineralogy between the mine spoil and high wall soils.…”

“…The lithology of the watershed consists primarily of a coal-shale (of the Pennsylvanian Allegheny group) as well as siltstone, sandstone, thinly bedded limestone, and 0.3-1.5 m thick coal seams [18,19]. The region's biome is defined as Temperate Deciduous Forest [20], and previously reported data on the climate of the watershed, and contaminant transport potential and remediation activities of the Huff Run Watershed can be found in [9,21,22]. With respect to the mining history of the watershed, underground coal mining in the region began as early as 1810, and surface mining became dominant in the late 1940s [23].…”

“…Loss on ignition (LOI), as a proxy for organic matter content, was performed with 1 g of milled sample added to a ceramic cup and heated to 550 • C for four hours in a muffle furnace and then re-weighed. Given the soil pore water pH and XRD results (discussed below), it is unlikely that significant amounts of carbonate were present, and only 2-4% of additional mass loss at 950 • C in similar soils has been reported [9], therefore loss on ignition at higher temperatures was not performed.…”

“…Recent work has shown that µm-scale pyrite and other metal sulfide grains can be present in soils developing on coal mine spoil even after decades since emplacement [8]. The potential for continued oxidative dissolution and leaching of metal(loid)s from historic coal mine spoil could represent a long-term source of slowly diffusing contaminants into surface and groundwater [9]. However, the actual flux of contaminants through soils developing on coal mine spoil and the relationship between current mineral weathering reactions and solute release has not been fully examined.…”