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

Abstract

Reservoirs are one of the primary water supply sources. Knowledge of the metal fluxes at the water–sediment interfaces of reservoirs is essential for predicting their ecological quality. Redox oscillations in the water column are promoted by stratification; turnover events may significantly alter metal cycling, especially in reservoirs impacted by acid mine drainage (AMD). To study this phenomenon, an experiment was performed under controlled laboratory conditions. Sediment cores from an AMD-affected reservoir were maintained in a tank with reservoir water for approximately two months and subjected to alternating oxic–hypoxic conditions. A detailed metal speciation in solid phases of the sediment was initially performed by sequential extraction, and pore water was analyzed at the end of each redox period. Tank water metals concentrations were systematically monitored throughout the experiment. The experimental results were then used to calibrate a diffusion-reaction model and quantify the reaction rates and sediment–water fluxes.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 (1–2mm) 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.