STABLE ISOTOPE AND MICROBIAL INDICATORS OF SULFUR CYCLE PATHWAYS IN AN AMD TREATMENT SYSTEM

Acid mine drainage (AMD) discharges associated with abandoned coal mines pose environmental challenges in the Midwestern U.S. At one of these sites, the Tab-Simco mine near Carbondale, Illinois, a passive-treatment system, which includes a sulfate-reducing bioreactor and an oxidation pond, was constructed in 2007 to remediate AMD characterized by low pH (~2.5) and high average concentration of dissolved ions: SO₄ = 3500 mg/L, Fe = 600 mg/L, Al = 150 mg/L, and Mn = 40 mg/L. Hydrogeochemical, microbiological, and stable isotope data have been used over the past five year to assess the efficiency of the treatment system.

We report small temporal variations in the sulfur isotope values of dissolved sulfate (δ³⁴S_SO4) in the effluent from the mine’s main seeps and observation wells. In contrast, significant δ³⁴S_SO4 differences between the passive bioreactor inflow and outflow were recorded. In the bioreactor outflow, a general temporal decrease in δ³⁴S_SO4was observed at the same time as increased sulfate & metal concentrations and decreases in pH were measured. Compared to δ³⁴S_SO4 in the bioreactor outflow, contrasting δ³⁴S_SO4 trends were measured in the post-treatment oxidation pond, with increasing δ³⁴S_SO4 values during 2007 and decreasing δ³⁴S_SO4 values afterwards.

To investigate microbial contributions to the sulfur cycle in the system, molecular techniques targeting the 16S rRNA and dsrAB gene sequences were employed. Phylogenetic analysis of the 16S rRNA gene sequences in the post-treated waters indicated the presence of a sulfur-oxidizing bacterium most closely related to Sulfuricurvum kujiense as the predominant phylotype. In conjunction with this finding, there was a low diversity of sulfate-reducing bacteria detected in the post-treatment waters with sequences most similar to the slow-growing Desulfobacca acetoxidans being the most prevalent.

The significance of these trends is likely related to the declining ability of the treatment system to produce alkalinity which affected both bacterial sulfate reduction processes in the bioreactor and oxidation pond and the sulfate mineral precipitation throughout the treatment system. Our results are being used to optimize the bioremediation strategies of the Tab-Simco treatment system.