Trace metal release induced by chemo-lithotrophic denitrification in deeper aquifers of the Lower Rhine Embayment, Germany

Applied Geochemistry, vol. 205, pp. 106886

Abstract

Chemo-lithotrophic denitrification is a crucial process that mitigates high nitrate fluxes in suboxic aquifers; however, it bears the risk of increasing sulfate and trace metal concentrations, which are generally attributed to the pyrite oxidation involved. To assess trace metal release induced by this reaction, we link mineralogical-geochemical analyses to hydrochemical data. As an example, we investigate an area in the Lower Rhine Embayment, a Cenozoic sedimentary basin that hosts important groundwater resources in Western Germany. Geochemical analysis reveals that pyrite in the sediments contains arsenic, cobalt, copper, nickel, and zinc. The trace metals show a highly heterogeneous distribution between pyrite grains but also within individual pyrites. Analysis of groundwater samples indicates that chemo-lithotrophic denitrification results in the release of the aforementioned elements, except for copper. Subsequent oxidation and precipitation of a large proportion of the released iron produces protons, which are partly buffered by carbonate dissolution. Carbonate dissolution may be an additional trace metals source, especially for zinc. At the same time, precipitated iron(oxyhydr)oxides act as a sink for trace metals, particularly arsenic. The study demonstrates that trace metal release induced by chemo-lithotrophic denitrification depends on the trace metal content of pyrites, the extent of pyrite oxidation, the abundance and composition of additional mineral phases (e.g., carbonates), and the sorption and cation exchange capacity of the aquifer sediment.

Authors 6

1. Felix Fahrenbach corresponding Aachen Institute of Hydrogeology

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University, Institute of Hydrogeology, Aachen, Germany

2. Thomas R. Rüde Aachen Institute of Hydrogeology

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University, Institute of Hydrogeology, Aachen, Germany

3. Leonard Stoeckl

   Federal Institute for Geosciences and Natural Resources

   Affiliation as printed

   Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany

4. Georg J. Houben

   Federal Institute for Geosciences and Natural Resources

   Affiliation as printed

   Federal Institute for Geosciences and Natural Resources (BGR), Hannover, Germany

5. Sarah E. Wünsch Aachen Institute of Hydrogeology

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University, Institute of Hydrogeology, Aachen, Germany

6. Sven Sindern Aachen Institute of Applied Mineralogy and Economic Geology

   RWTH Aachen University

   Affiliation as printed

   RWTH Aachen University, Institute of Applied Mineralogy and Economic Geology, Aachen, Germany