GEOCHEMICAL EVALUATION OF WEATHERING PROCESSES IN COAL MINE SPOIL

Coal has been a primary resource for energy production throughout the United States, negatively impacting many environments through acid mine drainage (AMD) from underground mines and the weathering of abandoned mine spoil (waste rock) on the land surface. At the Huff Run Watershed in Ohio, ongoing remediation efforts aim to treat point source pollution from legacy mining operations; however, it is not well understood how nonpoint source pollution from the weathering of mine spoil contributes to AMD and how it affects metal cycling in the surrounding vegetation. Our objective was to use flow-through column experiments to model the influence of forest vegetation on weathering reactions in mine spoil collected from Huff Run Site 25 (HR-25). Mine spoil was reacted with either a rain water solution or rain water enriched with citrate, representative of organic acids that vegetation release from their roots. Effluent solutions from the columns were analyzed to quantify concentrations of metal cations (e.g., Fe, Mn), anions (e.g., SO₄^2-, Cl^-), and dissolved organic carbon (DOC). In the rain water column, soluble salts were dissolved and leached out in one pulse near the start of the experiment. DOC initially decreased and then became constant, while effluent pH remained constant at 6.5 over the course of the experiment. In contrast, effluent solutions from the citrate column contained elevated concentrations of solutes that indicate active weathering of the spoil and potential oxidation of sulfide minerals. Additionally, the pH of the citrate effluents increased from 6.5 to 8, and DOC concentrations were constant and higher than in rain water effluent, though lower than the influent citrate solution. We propose that citrate may be complexing metal cations in the column and reducing iron oxyhydroxide minerals, e.g., Fe(OH)₃, to produce mobile Fe²⁺ and OH^-, thus raising the effluent pH. Citrate oxidation coupled to Fe(OH)₃ reduction would release CO_2, causing there to be less DOC in the effluents than in the influent solution. The column data also reveal that the reactions in the citrate column may reach steady-state over time. Our results indicate that vegetated mine spoil may experience increased metal mobilization relative to non-vegetated mine spoil due to enhanced chemical weathering by organic acids.