Abstract
This thesis examines sediment redox processes associated with organic matter degradation and their impact on the cycling of nutrients (N, P) and trace metals (Cd, Co, Ni, Pb, Zn). Our study site, Haringvliet Lake, is located in the Rhine-Meuse River Delta in the southwest of The Netherlands. This waterbody was
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formerly a tidal and brackish estuary but was separated from the North Sea in 1970 by a dam, as part of the Delta Works program. Currently, a partial restoration of estuarine conditions in the lake is planned, which can be achieved by altering the management of the sluice gates within the dam. A multi-component reactive transport model (RTM) is used to infer the rates of organic carbon mineralization pathways. The model is calibrated using field measurements of solid phases and dissolved concentrations, and experimental determinations of reaction rates. We find that the most important terminal electron acceptors for organic carbon oxidation are O2 (55%), NO3- (21%), and SO42- (17%). Model simulations conducted to approximate estuarine restoration show that the increased relative importance of sulfate reduction leads to a long-term conversion of sediment Fe(III) minerals to pyrite. The sediment nitrogen cycle was examined with both experimental techniques and reactive transport modeling. Potential nitrate reduction and denitrification rates were determined using flow through reactors. Experiments suggest that denitrification accounts for only half of the total potential nitrate reduction rate in Haringvliet sediment. Extraction results provide evidence that a reducible iron-phosphate mineral is a major pool for phosphorus in the sediment. Phosphorus diagenesis is added to the RTM which showed that the dissolution of the iron-phosphate phase and organic-P mineralization are important pathways for the release of PO4 in the sediment. Sediment trace metal concentrations are elevated, due to anthropogenic influences in the Rhine and Meuse Rivers. Results suggest that metals, which enter the sediment associated with oxides, become associated with sulfides. Extraction results show that Ni and Co ultimately associate with pyrite. Pore waters are generally saturated for trace metal mono-sulfides of Zn, Pb, Co, and Cd. Processes controlling the dissolved trace metal concentrations in the upper millimeters of sediment, such as trace metal scavenging by newly formed oxides, and sulfide oxidation, are important in controlling the diffusive release to the overlying water. The relative importance of sulfides in trace metal speciation is expected to increase following restoration, allowing for trace metal retention in the reduced sediment. The estimated change in sediment efflux is combined with the proposed restoration area and water flow rates to derive simple estimates of the changes in concentrations in the overlying water. The high flow rates at the site mean that under most conditions increased sediment effluxes will have minimal impact. Lower flow through the Haringvliet would decrease this dilution effect and also increase the chances of salinity stratification and bottom water anoxia.
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