Terrestrial sedimentary pyrites as a potential source of trace metal release to groundwater - A case study from the Emsland, Germany

- Pyrite from heavy mineral fraction of aquifer sediment analyzed for trace metal content.
- Pyrites contain significant concentration of trace metals, such as nickel, arsenic, cadmium.
- Trace elements are released by autotrophic denitrification.
- Reactive transport model predicts small zone of trace element accumulation.
- Release of trace elements strongly dependent on nitrate content of groundwater.

Pyrite is a common minor constituent of terrestrial freshwater sediments and a sink for trace elements. Different amounts and morphological types (framboids and euhedral crystals) of sedimentary pyrites were found in the heavy mineral fraction of cores obtained from several drillholes located in the Emsland region, NW Germany. Their trace element contents were investigated to assess their potential for groundwater contamination after oxidation, e.g. induced by dewatering or autotrophic denitrification. Nickel, arsenic and cadmium were found in significant concentrations in pyrite. Geochemical modeling showed that elevated trace metal concentrations in groundwater, potentially exceeding drinking water standards, should preferentially occur in a less than 1 m thick zone situated around the depth of the redoxcline, where nitrate is reduced by pyrite. This was confirmed by depth-specific groundwater sampling in the Emsland and by previously published studies. The absolute concentration of released trace metals depends on their content in the pyrite but also strongly on the nitrate load of groundwater.