Arsenic release and speciation in a degraded fen as affected by soil redox potential at varied moisture regime

Soil surveys have demonstrated arsenic (As) contents of up to 1600 mg kg− 1 in surface horizons of degraded fens in the Bavarian Molasse basin, Germany. Ground water from the Tertiary aquifer seems to be the primary source of As. Yet, the cause of its accumulation in the topsoil is unclear. Focussing the influence of redox processes on As redistribution, we conducted soil column experiments with the A (716 mg As kg− 1), Ag (293 mg As kg− 1) and 2Ag (37 mg As kg− 1) horizons of a Mollic Gleysol (pH 7.2). The fixed beds were equipped with redox electrodes and suction cups and subjected to a saturation–drainage–saturation cycle. Water table fluctuations were simulated by defined pressure heads applied to the lower column boundaries via a digitally controlled vacuum system. After water saturation, the redox potential (EH) dropped to minimum values of around 0 mV in the A/Ag and−400 mV in the 2Ag horizon. Soil drainage resulted in a quick return to oxidising conditions. Both in the A and Ag horizons total aqueous As concentrations were low (up to 20 μg l− 1) and not related to EH. In contrast, aqueous As concentrations of the 2Ag horizon were between 5 and 140 μg l− 1 and increased as the EH decreased. However, the As species distribution showed no clear trend with EH since both As(III) and As(V) were detected under reducing conditions. High release of As in the 2Ag horizon is consistent with low contents in Fe (hydr)oxides. In the A and Ag horizons, pedogenetic enrichment of sesquioxides contributes to a comparably lower sensitivity of EH to water saturation and favours As retention. Thereby, As released under saturated conditions in the 2Ag horizon may be stabilised in the topsoil following capillary rise. Thus, surface horizons may act both as historic and as recent sinks for geogenic As at the site.

Research Highlights►This soil column study focussed on redox processes induced by water table fluctuations as a possible route of As enrichment in the topsoil of a Mollic Gleysol in Upper Bavaria. ►Lowest EH values and highest dissolved As concentrations were observed under water saturated conditions in the most frequently waterlogged 2Ag horizon. In contrast, the EH of the A and Ag horizons was higher and dissolved As concentrations were low in spite of high As contents in bulk soil. ►Pedogenetic enrichment of Fe (hydr)oxides may buffer the EH of the topsoil during prolonged waterlogging and at the same time constitute an important sink for As. ►Our results strongly corroborate the hypothesis that As accumulation in the topsoil is driven by water table fluctuations at the site followed by capillary rise of dissolved As.