Benthic metal fluxes and sediment diagenesis in a water reservoir affected by acid mine drainage: A laboratory experiment and reactive transport modeling

Under oxic conditions, pH, Fe and As concentrations decreased in the tank due to schwertmannite precipitation, whereas the concentrations of Al, Zn, Cu, Ni, and Co increased due to Al(OH)3 and sulfide dissolution. The reverse trends occurred under hypoxic conditions. Under oxic conditions, the fluxes calculated by applying Fick's first law to experimental concentration gradients contradicted the fluxes expected based on the evolution of the tank water. According to the reactive transport calculations, this discrepancy can be attributed to the coarse resolution of sediment sampling. The one-cm-thick slices failed to capture effectively the notably narrow (1-2 mm) concentration peaks of several elements in the shallow pore water resulting from sulfide and Al(OH)3 dissolution. The diffusion-reaction model, extended to the complete year, computed that between 25% and 50% of the trace metals and less than 10% of the Al that precipitated under hypoxic conditions was re-dissolved during the oxic period. In contrast, only 22% and 9% of the Fe and As, respectively, that precipitated under oxic conditions were re-dissolved under hypoxic conditions. Overall, the results of the integrated annual balance, which also took the solid phase fluxes into account, demonstrated a net transfer of all studied elements to the sediment. In this AMD-affected reservoir, the sulfur cycle is the predominant control on the reservoir redox system; SO4 is the major oxidant of organic matter and S(II) is the major O2 consumer, each accounting for nearly 90% of the total process in both cases.