Processes controlling geogenic arsenic distribution in soils formed from iron-rich sedimentary rocks

Soils formed from Fe-rich sedimentary rocks can contain elevated As contents. Although the geogenic origin of As in these soils is recognized, the processes controlling its distribution within soil profiles and its mobility in topsoils are still unclear, limiting effective prediction of soils with potentially hazardous As contents for human health/ecosystems. We investigated 10 soil profiles (0-85 cm) formed from the As- and Fe-rich Aubange Formation in Belgian Lorraine. Sampling sites were chosen to represent the diversity of soil profiles associated with these rocks, with soil material collected every 5-cm for detailed As distribution assessment. Chemical and scanning electron microscopy analyses revealed that As is consistently associated with Fe oxyhydroxides variably enriched in K, Ca, Al and Si. X-ray diffraction identified goethite as a primary phase. The As-containing Fe oxyhydroxides originated from the soil parent material. They likely formed in response to fluctuating redox conditions during paleo-weathering of the Fe-rich sedimentary rocks and remained stable during pedogenesis. The As content varies widely within and across soil the profiles (23-753 mg ⋅ kg^-1) and in topsoils (29-165 mg ⋅ kg^-1), reflecting the chemical heterogeneity of the parent material. Additionally, exogenous processes, including colluvial transport and deposition of soil particles during and/or before pedogenesis, influenced As distribution within and across the soil profiles. Despite exceeding soil quality standards, As in the Fe-rich Aubange topsoils has low bioaccessibility (3-6 %) due to its strong association with the low-solubility Fe oxyhydroxides. Our findings emphasize the need for tailoring soil quality guidelines to the local pedo-geological context.