Microbial effects on the release and attenuation of arsenic in the shallow subsurface of a natural geochemical anomaly

- Very high As(III) concentrations were detected in the shallow groundwater.
- Arsenic is bound to Fe(III) (hydr)oxides, Fe(III) arsenates and newly-formed realgar.
- Reductive dissolution of Fe(III) and As(V) minerals by bacteria leads to mobilization of arsenic.
- Precipitation of realgar is constrained to anaerobic domains around and within organic particles.

Critical factors leading to arsenic release and attenuation from the shallow subsurface were studied with multidisciplinary approach in the natural gold-arsenic geochemical anomaly at Mokrsko (Czech Republic). The results show that microbial reduction promotes arsenic release from Fe(III) (hydr)oxides and Fe(III) arsenates, thereby enhancing dissolved arsenic in the shallow groundwater at average concentration of 7.76 mg/L. In the organic-rich aggregates and wood particles, however, microbial sulfate reduction triggers the formation of realgar deposits, leading to accumulation of As in the distinct organic-rich patches of the shallow subsurface. We conclude that precipitation of realgar in the shallow subsurface of soil/sediment depends on specific and non-trivial combination of water and rock chemistry, microbial community composition and spatial organisation of the subsurface zone, where speciation in saturated environments varied on a centimeter scale from reduced (decomposed wood, H2S and realgar present) to oxidized (goethite and arsenate minerals are present).