Abstract
Groundwater is an essential resource for humans and its quality is often threatened by leaching of contaminants from surface soils. In agricultural areas, nitrate from fertilizer or manure is one of the most common pollutants. Natural attenuation of nitrate in groundwater systems mainly occurs through denitrification, a process in which
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the nitrogen is lost to the atmosphere as nitrogen gas (N2). In aquifers, the electron donor required for this microbially-mediated reaction is supplied by dissolved or solid organic matter or inorganic compounds such as the iron-sulfide mineral pyrite (FeS2). The aim of this thesis is to increase the understanding of the biogeochemical dynamics of nitrogen and sulfur in groundwater with specific attention to the role of pyrite oxidation in removal of nitrate. The research focuses on a field site at Oostrum, the Netherlands. Shallow groundwater in this region has high nitrate concentrations (up to 8 mM) due to intense fertilizer application. Geochemical analyses of the sediment and groundwater, including groundwater age dating, indicate that denitrification coupled to pyrite oxidation is a major process in the aquifer and that it leads to release of trace metals such arsenic (As) and nickel (Ni) to the groundwater. Systematic differences between time series of historical inputs of nitrate and sulfate for the period 1950 to 2000 and measured groundwater concentrations confirm that the removal of nitrate goes hand in hand with production of sulfate in this aquifer. Decreased fertilizer inputs over the past decades have led to an improvement in groundwater quality beneath the cultivated land but not at a downstream forest site due to lateral inflow of groundwater. The results of multi-isotope analyses (d15N-NO3-, d18O-NO3-, d34S-SO42-, d18O-SO42-, d34S-pyrite) confirm that pyrite is the main electron donor for denitrification at this location and that this process can explain 70% of the sulfate present in the zone of denitrification. Consistent with the geochemical analyses, 16S rRNA sequencing revealed the presence of bacteria capable of sulfide oxidation coupled to nitrate reduction. Laboratory incubations of pyrite-bearing sediments confirm that autotrophic denitrification with pyrite has the potential to efficiently remove nitrate from groundwater also in aquifers with no prior contamination with nitrate. A reactive transport model (PHT3D) was used to integrate and analyze the biogeochemical and isotopic dynamics in this nitrate-polluted aquifer (Oostrum) over the last 50 years. Model simulations were constrained by measured concentration depth profiles and age dating results as presented in the field studies. Model results illustrate that denitrification largely prevented the discharge of nitrate to surface waters, while the discharge of sulfate increased, peaking around 25 years after agricultural practices were changed. At this site, the discharge of both nitrate and sulfate are expected to decline further over the next decades
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