Nitrate contamination due to agricultural activity represents a worldwide threat to groundwater resources. Denitrification is the major pathway of removal of nitrate from groundwater systems. This anaerobic process may be coupled to the oxidation of organic matter or to that of pyrite. Coupled denitrification - pyrite oxidation may lead to mobilization of trace metals and sulfate production, and may, therefore, cause degradation of groundwater and surface water quality. Relatively little is known about the quantitative role of pyrite oxidation for nitrate removal and factors controlling this process in groundwater systems. We propose an integrated combination of experimental, field and local/regional reactive transport modeling to (1) elucidate the key factors controlling the rate of denitrification plus the relative contribution of organic matter and pyrite as electron donors at two study sites (these factors include pH, temperature, nitrate concentration, the surface chemistry of pyrite and reactivity of the organic matter), (2) determine the impact of denitrification coupled to pyrite oxidation on local and regional groundwater quality, and (3) assess how the natural capacity of the groundwater systems for denitrification will evolve with time at the two locations. We will focus on two sites in the Netherlands (Oostrum in Limburg and the Kempen region in Noord Brabant) where preliminary field results indicate substantial agricultural nitrate pollution, nitrate removal through denitrification coupled to pyrite oxidation, and trace metal mobilization. Particular innovative aspects of this study are (1) the combined use of S and N isotopes to assess the sources and transformation of nitrate and sulfate, and (2) the reconstruction of present-day and historical groundwater quality and the prediction of its future evolution on a regional scale. The results of this project will provide much needed data on crucial biogeochemical processes that control groundwater quality, as well as new modeling tools to assess the impact of nitrate contamination on groundwater resources.

Freshwater systems have recently been recognized as an important component of the global carbon cycle. They can act as a carbon sink (via burial) or source (due to heterotrophic processes) depending on the carbon flows within food webs. Heterotrophs in freshwater systems rely on three major sources of organic matter/energy: (1) local primary production, (2) terrestrial inputs from plants and soils and (3) methanotrophic biomass. Unraveling the relative importance of these carbon sources and their fate is crucial for understanding inland organic carbon cycling. Here we propose to combine stable isotope and organic geochemical approaches to elucidate food web functioning in freshwater systems and to trace carbon flows. Natural abundance isotope signatures of carbon, nitrogen and hydrogen will be used to unravel the relative importance of autotrophic and heterotrophic pathways. Organic molecules characteristic for specific organisms or a specific source (e.g. soil carbon) will be used as a complementary tool to trace carbon flows within lakes. Deliberate tracer experiments in which either primary producers or secondary bacterial producers are labeled will be used to directly quantify carbon transfer between organisms.

Iron (Fe) is an essential nutrient that plays a key role in biogeochemical cycling in aquatic systems. The bioavailability of other essential nutrients, such as phosphorus and silicon, as well as trace metals are intimately coupled to Fe cycling through adsorption and co-precipitation processes in the water column and sediment. Consequently, the dynamics of Fe mineral transformations in sediments regulate the magnitude and timescale of removal of bio-available forms of these elements from water bodies. Laboratory studies have demonstrated that the structure and stability of Fe minerals is affected by the chemical conditions under which they form. However, limited understanding of how this may affect sedimentary Fe cycling under changing environmental conditions remains a critical knowledge gap. This is of particular relevance in light of the growing number of aquatic systems that suffer from eutrophication and hypoxia, which severely alter sediment geochemistry. Here, I propose to study how the dissolution/transformation dynamics and nutrient sorption behavior of major Fe minerals are affected by the sedimentary conditions under which they form. The research will focus on the impact of key dissolved species that show changes in availability in response to eutrophication, such as the nutrients phosphorus and silicon and other important solutes (bicarbonate, iron, sulphate, and sulphide). I will synthesize Fe minerals under different conditions, and investigate (long-term) Fe mineral dissolution and transformation under laboratory and field conditions. I will utilize advanced analytical techniques for characterization of dissolved and solid phase species, in particular diffusive gel techniques and X-ray absorption spectroscopy. These will provide detailed, high-resolution information on Fe mineral dynamics. Quantitative geochemical modeling of mineral (trans)formation and sorption processes will complement the experimental findings. This novel and thorough approach will provide crucial insight into the feedbacks between Fe mineral dynamics and environmental change.

Iron oxyhydroxides are ubiquitous reactive constituent of soils, sediments and groundwater environments. They exhibit large surface areas which bind trace metals, nutrients and organic molecules. At oxic to anoxic transitions, iron oxyhydroxides reductively dissolve via a number of abiotic and microbial pathways. In particular, they may serve as terminal electron acceptor for the oxidation of organic matter by heterotrophic bacteria. The geochemical and ecological importance of enzymatically-mediated reduction of iron oxyhydroxides in subsurface systems is now widely recognized. However, a complete quantitative (and predictive) understanding of the environmental and physiological variables controlling the activity of iron reducing microorganisms in these systems is still lacking. To remediate this situation we propose to establish the relationships between the bioavailability of natural iron oxyhydroxides and their chemical redox reactivity. Furthermore we will assess the responses of the activity and community structure of natural consortia of iron(III) reducing microorganisms to changes in the nature and abundance of iron oxyhydroxides and organic substrates. The experimental results of the study will be used to develop, calibrate and test a quantitative model capable of predicting the rate of enzymatic reduction of iron oxyhydroxides under the range of conditions encountered in the subsurface. Control Analysis will be used to estimate the elasticity of microorganisms to changes in substrate concentrations and control of functional groups of microorganisms to electron fluxes through the system. The proposed research will be based on an interdisciplinary approach combining experimental and theoretical methods from geochemistry and microbiology. The expected results will provide new, mechanistic insight into the reactivity and bioavailability of natural iron oxyhydroxides, the microbial ecology of Fe(III) reducing communities, and the kinetics of enzymatic Fe(III) reduction.

Dit is gekoppeld aan Open Programma project 817.01.015 The Arabian Sea represents the iconic text book example of an open ocean oxygen minimum zone (OMZ). Low oxygen concentrations (down to less than 2 µM ) are found between 200 and 1000 meters water depth. This makes the northern Arabian a perfect natural laboratory to test the impact of oxygen depleted bottom waters on sediment organic matter processing, early diagenesis and the validity of proxies. Although the existence of the Arabian Sea OMZ is known for at least a century, the biological communities and the biogeochemical functioning of this system are still poorly understood. Low bottom-water oxygen concentrations have consequences for the organisms living at and within the sediments and consequently for the biogeochemical processes taking place. Here we propose to investigate the impact of oxygen depletion on organic matter processing and preservation and the recording of proxies by obtaining a set of multi cores along a transect through the OMZ on the slopes of a sub-marine mountain, the Murray Ridge and deploying sediment traps within and below the OMZ. Key objectives of the proposed research are: 1) to assess the impact of water column and bottom water oxygenation on organic matter degradation and transformation, in particular on specific biomarkers, 2) to identify the key organisms involved in organic matter processing under low-oxygen conditions and to link this to foraminiferal distribution patterns in order to evaluate and refine foraminiferal based proxies, 3) to investigate the impact of bottom water oxygenation on sedimentary nitrogen and phosphorus regeneration and trace metal cycling, 4) to assess the consistency and accuracy of diverse proxies (foraminifera, various organic biomarkers and trace metals) in recording bottom-water oxygen conditions.