Denitrification coupled to pyrite oxidation and changes in groundwater quality in a shallow sandy aquifer

This study focuses on denitrification in a sandy aquifer using geochemical analyses of both sediment and groundwater, combined with groundwater age dating (³H/³He). The study sites are located underneath cultivated fields and an adjacent forested area at Oostrum, The Netherlands. Shallow groundwater in the region has high nitrate concentrations (up to 8 mM) due to intense fertilizer application. Nitrate removal from the groundwater below cultivated fields correlates with sulfate production, and the release of dissolved Fe²⁺ and pyrite-associated trace metals (e.g. As, Ni, Co and Zn). These results, and the presence of pyrite in the sediment matrix within the nitrate removal zone, indicate that denitrification coupled to pyrite oxidation is a major process in the aquifer. Significant nitrate loss coupled to sulfate production is further confirmed by comparing historical estimates of regional sulfate and nitrate loadings to age-dated groundwater sulfate and nitrate concentrations, for the period 1950-2000. However, the observed increases in sulfate concentration are about 50% lower than would be expected from complete oxidation of pyrite to sulfate, possibly due to the accumulation of intermediate oxidation state sulfur compounds, such as elemental sulfur. Pollutant concentrations (NO₃, Cl, As, Co and Ni) measured in the groundwater beneath the agricultural areas in 1996 and 2006 show systematic decreases most likely due to declining fertilizer use.     

Magnetic properties of hydrothermally synthesized greigite (Fe3 S4 )—II. High- and low-temperature characteristics

The magnetic behaviour of hydrothermally synthesized greigite was analysed in the temperature range from 4 K to 700 °C. Below room temperature, hysteresis parameters were determined as a function of temperature, with emphasis on the temperature range below 50 K. Saturation magnetization and initial susceptibility were studied above room temperature, along with X-ray diffraction analysis of material heated to various temperatures. The magnetic behaviour of synthetic greigite on heating is determined by chemical alteration rather than by magnetic unblocking. Heating in air yields more discriminative behaviour than heating in argon. When heated in air, the amount of oxygen available for reaction with greigite determines the products and magnetic behaviour. In systems open to contact with air, haematite is the final reaction product. When the contact with air is restricted, magnetite is the final reaction product. When air is excluded, pyrrhotite and magnetite are the final reaction products; the amount of magnetite formed is determined by the purity of the starting greigite and the degree of its surficial oxidation. The saturation magnetization of synthetic greigite is virtually independent of temperature from room temperature down to 4 K. The saturation remanent magnetization increases slowly by 20–30 per cent on cooling from room temperature to 4 K. A broad maximum is observed at ~10 K which may be diagnostic of greigite. The coercive and remanent coercive force both increase smoothly with decreasing temperature to 4 K. The coercive force increases from ~50 mT at room temperature to approximately 100–120 mT at 4 K, and the remanent coercive force increases from approximately 50–80 mT at room temperature to approximately 110–180 mT at 4 K.     

Changes in Magnetic Parameters After Sequential Iron Phase Extraction of Eastern Mediterranean Sapropel S1 Sediments

Iron is distributed over different minerals (i.e. silicates, pyrite, detrital oxides) that are present in a sediment sequence that formed under anoxic conditions. After post-depositional re-oxidation of the sediments pyrite is no longer present and diagenetic iron phases constitute an important portion of the iron in the oxidised part of the sapropel. They are very fine-grained making them amenable to analysis by means of sequential extraction and mineral-magnetic methods. The sequential extraction shows that besides iron in silicates, iron mainly occurs in amorphous oxides in the oxidised part of the S1 sapropel. Pyrite constitutes an important fraction in the still reduced part of the S1 sapropel. Some silicon is dissolved during the extraction for the amorphous oxides, suggesting that amorphous iron also occurs as ferro-silicate coatings. Mineral-magnetic analysis involved component analysis of the isothermal remanent magnetisation (IRM) and hysteresis loop measurements. Three coercivity phases could be identified in the IRM component analysis; these were interpreted as detrital magnetite, hematite, and biogenic magnetite. The diagenetically formed iron phases influence the parameters of the IRM components. Hysteresis measurements together with the IRM component analysis, indicate the importance of bacterial magnetite in the oxidised sapropel, particularly in the lower part of the active oxidation zone.     

Sulphur Enrichment in Organic Matter of Eastern Mediterranean Sapropels: A Study of Sulphur Isotope Partitioning

Sulphur isotope compositions and S/C ratios of organic matter were analysed in detail by combustion-isotope ratio monitoring mass spectrometry (C-irmMS) in eastern Mediterranean sediments containing three sapropels of different ages and with different organic carbon contents (sapropel S1 in core UM26, formed from 5–9 ka ago with a maximum organic carbon content of 2.3 wt%; sapropel 967 from ODP Site 160-967C, with an age of 1.8 Ma and a maximum organic carbon content of 7.4 wt%; and sapropel 969 from ODP Site 160-969E, with an age of 2.9 Ma and a maximum organic carbon content of 23.5 wt%). Sulphur isotopic compositions (34S) of the organic matter ranged from -29.5 to +15.8 and the atomic S/C ratio was 0.005 to 0.038. The organic sulphur in the sediments is a mixture of sulphur derived from (1) incorporation of 34S-depleted inorganic reduced sulphur produced by dissimilatory microbial sulphate reduction; and (2) biosynthetic sulphur with an isotopic signature close to seawater sulphate. The calculated biosynthetic fraction of organic sulphur in non-sapropelic sediments ranges from 68–87%. The biosynthetic fraction of the organic sulphur of the sapropels (60–22%) decreases with increasing organic carbon content of the sapropels. We propose that uptake of reduced sulphur into organic matter predominantly took place within sapropels where pyrite formation was iron-limited and thus an excess of dissolved sulphide was present for certain periods of time. Simultaneously, sulphide escaped into the bottom water and into sediments below the sapropels where pyrite formation occurred.