Biogeochemical Cycling of Nutrients and Trace Metals in the Sediment of Haringvliet Lake: Response to Salinization

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 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

Biogeochemical cycling of nutrients and trace metals in the sediment of Haringvliet lake: reponse to salinization

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 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

Potential nitrate removal in a coastal freshwater sediment (Haringvliet Lake, The Netherlands) and response to salinization

Nitrogen transformations and their response to salinization were studied in bottom sediment of a coastal freshwater lake (Haringvliet Lake, The Netherlands). The lake was formed as the result of a river impoundment along the south-western coast of the Netherlands, and is currently targeted for restoration of estuarine conditions. Nitrate porewater profiles indicate complete removal of NO-3 within the upper few millimeters of sediment. Rapid NO-3 consumption is consistent with the high potential rates of nitrate reduction (up to 200 nmolNcm-3h-1) measured with flow-through reactors (FTRs) on intact sediment slices. Acetylene-block FTR experiments indicate that complete denitrification accounts for approximately half of the nitrate reducing activity. The remaining NO-3 reduction is due to incomplete denitrification and alternative reaction pathways, most likely dissimilatory nitrate reduction to NH+4 (DNRA). Results of FTR experiments further indicate that increasing bottom water salinity may lead to a transient release of NH+4 and dissolved organic carbon from the sediment, and enhance the rates of nitrate reduction and nitrite production. Increased salinity may thus, at least temporarily, increase the efflux of NH4+ from the sediment to the surface water. This work shows that salinity affects the relative importance of denitrification compared to alternative nitrate reduction pathways, limiting the ability of denitrification to remove bioavailable nitrogen from aquatic ecosystems

Geochemistry of trace metals in a fresh water sediment : Field results and diagenetic modeling

Concentrations of Fe, Mn, Cd, Co, Ni, Pb, and Zn were determined in pore water and sediment of a coastal fresh water lake (Haringvliet Lake, The Netherlands). Elevated sediment trace metal concentrations reflect anthropogenic inputs from the Rhine and Meuse Rivers. Pore water and sediment analyses, together with thermodynamic calculations, indicate a shift in trace metal speciation from oxide-bound to sulfide-bound over the upper 20 cm of the sediment. Concentrations of reducible Fe and Mn decline with increasing depth, but do not reach zero values at 20 cm depth. The reducible phases are relatively more important for the binding of Co, Ni, and Zn than for Pb and Cd. Pore waters exhibit supersaturation with respect to Zn, Pb, Co, and Cd monosulfides, while significant fractions of Ni and Co are bound to pyrite. A multi-component, diagenetic model developed for organic matter degradation was expanded to include Zn and Ni dynamics. Pore water transport of trace metals is primarily diffusive, with a lesser contribution of bioirrigation. Reactions affecting trace metal mobility near the sediment–water interface, especially sulfide oxidation and sorption to newly formed oxides, strongly influence the modeled estimates of the diffusive effluxes to the overlying water. Model results imply less efficient sediment retention of Ni than Zn. Sensitivity analyses show that increased bioturbation and sulfate availability, which are expected upon restoration of estuarine conditions in the lake, should increase the sulfide bound fractions of Zn and Ni in the sediments

Organic matter mineralization in sediment of a coastal freshwater lake and response to salinization

Solid phase and pore water chemical data collected in a sediment of the Haringvliet Lake are interpreted using a multi-component reactive transport model. This freshwater lake, which was formed as the result of a river impoundment along the southwestern coast of the Netherlands, is currently targeted for restoration of estuarine conditions. The model is used to assess the present-day biogeochemical dynamics in the sediment, and to forecast possible changes in organic carbon mineralization pathways and associated redox reactions upon salinization of the bottom waters. Model results indicate that oxic degradation (55%), denitrification (21%), and sulfate reduction (17%) are currently the main organic carbon degradation pathways in the upper 30 cm of sediment. Unlike in many other freshwater sediments, methanogenesis is a relatively minor carbon mineralization pathway (5%), because of significant supply of soluble electron acceptors from the well-mixed bottom waters. Although ascorbate-reducible Fe(III) mineral phases are present throughout the upper 30 cm of sediment, the contribution of dissimilatory iron reduction to overall sediment metabolism is negligible. Sensitivity analyses show that bioirrigation and bioturbation are important processes controlling the distribution of organic carbon degradation over the different pathways. Model simulations indicate that sulfate reduction would rapidly suppress methanogenesis upon seawater intrusion in the Haringvliet, and could lead to significant changes in the sediment’s solid-state iron speciation. The changes in Fe speciation would take place on time-scales of 20–100 years

Modeling nitrogen cycling in a coastal fresh water sediment

Increased nitrogen (N) loading to coastal marine and freshwater systems is occurring worldwide as a result of human activities. Diagenetic processes in sediments can change the N availability in these systems, by supporting removal through denitrification and burial of organic N (Norg) or by enhancing N recycling. In this study, we use a reactive transport model (RTM) to examine N transformations in a coastal fresh water sediment and quantify N removal rates. We also assess the response of the sediment N cycle to environmental changes that may result from increased salinity which is planned to occur at the site as a result of an estuarine restoration project. Field results show that much of the Norg deposited on the sediment is currently remineralized to ammonium. A rapid removal of nitrate is observed in the sediment pore water, with the resulting nitrate reduction rate estimated to be 130 lmol N cm–2 yr–1. A model sensitivity study was conducted altering the distribution of nitrate reduction between dissimilatory nitrate reduction to ammonium (DNRA) and denitrification. These results show a 40% decline in sediment N removal as NO3 – reduction shifts from denitrification to DNRA. This decreased N removal leads to a shift in sediment-water exchange flux of dissolved inorganic nitrogen (DIN) from near zero with denitrification to 133 lmol N cm–2 yr–1 if DNRA is the dominant pathway. The response to salinization includes a shortterm release of adsorbed ammonium. Additional changes expected to result from the estuarine restoration include: lower NO3 – concentrations and greater SO4 2– concentrations in the bottom water, decreased nitrification rates, and increased sediment mixing. The effect of these changes on net DIN flux and N removal vary based on the distribution of DNRA versus denitrification, illustrating the need for a better understanding of factors controlling this competition