| Drainage from abandoned coal mines has resulted in severe water quality problems. The oxidation of sulfide minerals in coal and associated rocks releases iron-rich, acidic solutions that damage vegetation and aquatic ecosystems. The objective of this study was to characterize the sediment column of an established compost wetland constructed for the treatment of acid mine drainage to gain insight into biogeochemical processes that might impact treatment efficiency. To do this, mineralogy and geochemical stability of ochreous sediments were examined, spatial and seasonal trends in porewater chemistry were measured, and bacterial community composition profiled. The mineralogical composition of the ochre portion of the sediment column was a mixture of schwertmannite [Fe8O8(OH)4.8(SO4)1.6] and goethite (alpha-FeOOH). Initial drainage conditions favored the precipitation of schwertmannite, which transformed at a rate of 10--30 mol/m 3/yr to goethite. The sulfide minerals, pyrite (FeS2) and greigite (Fe3S4), were identified along with magnetite (Fe3O4) in the compost layer of the sediment. Vertical gradients in porewater chemistry were similar throughout the wetland system and, with the exception of dissolved sulfide concentration, no consistent seasonal trends were detected. Dissolved sulfide was elevated in the compost relative to the ochre and in June compared to February. Porewater pH ranged from 3 to 7 and increased with depth; whereas, the Eh ranged from 110 to 750 mV and decreased with depth. Both pH and Eh changed abruptly near the interface between the ochre and compost layers. Dissolved Fe occurred primarily as Fe(II) and peaked within the interface region. Concentrations of other major elements (Al, Ca, K, Mg, Mn, and Na) in the pore waters showed some variation between cells and sampling dates, but vertical gradients generally reflected wetland stratigraphy. Terminal restriction fragment length polymorphism analysis (T-RFLP) of 16S rRNA genes was used to profile bacterial community composition. Bacterial diversity was found to be similar throughout the sediment profile; however, bacterial communities clustered together and could be correlated to sediment properties. Many terminal restriction fragment's (TRF's) consistent with bacteria relevant to wetland treatment efficiency were found. These included multiple TRF's consistent with eight genera of sulfate-reducing bacteria as well as iron-reducers like Shewanella and Peleobacter. |
