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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 (1-2 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. [ABSTRACT FROM AUTHOR]

Abstract: The discharge of acid mine drainage (AMD) into a reservoir may seriously affect the water quality. To investigate the metal transfer between the water and the sediment, three cores were collected from the Sancho Reservoir (Iberian Pyrite Belt, SW Spain) during different seasons: turnover event; oxic, stratified period; anoxic and under shallow perennially oxic conditions. The cores were sliced in an oxygen-free atmosphere, after which pore water was extracted by centrifugation and analyzed. A sequential extraction was then applied to the sediments to extract the water-soluble, monosulfide, low crystallinity Fe(III)-oxyhydroxide, crystalline Fe(III)-oxide, organic, pyrite and residual phases. The results showed that, despite the acidic chemistry of the water column (pH<4), the reservoir accumulated a high amount of autochthonous organic matter (up to 12wt.%). Oxygen was consumed in 1mm of sediment due to organic matter and sulfide oxidation. Below the oxic layer, Fe(III) and sulfate reduction peaks developed concomitantly and the resulting Fe(II) and S(II) were removed as sulfides and probably as S linked to organic matter. During the oxic season, schwertmannite precipitated in the water column and was redissolved in the organic-rich sediment, after which iron and arsenic diffused upwards again to the water column. The flux of precipitates was found to be two orders of magnitude higher than the aqueous one, and therefore the sediment acted as a sink for As and Fe. Trace metals (Cu, Zn, Cd, Pb, Ni, Co) and Al always diffused from the reservoir water and were incorporated into the sediments as sulfides and oxyhydroxides, respectively. In spite of the fact that the benthic fluxes estimated for trace metal and Al were much higher than those reported for lake and marine sediments, they only accounted for less than 10% of their total inventory dissolved in the column water. [Copyright &y& Elsevier]