Sedimentary controls on arsenic distribution in meander-belt deposits of the Po Valley, Italy

Geogenic arsenic in soils and aquifers is a threat to public health, which can be mitigated by improving our understanding of arsenic distribution in natural environments. In alluvial plains traversed by meandering rivers, solid- and aqueous-phase arsenic concentrations tend to vary across sedimentary deposits accumulated by different processes linked to river morphodynamics: mud-prone and commonly organic-rich abandoned-channel fills arising from meander cut-off act as local sources of arsenic, which can be transferred to adjacent point-bar deposits related to meander growth. Meanwhile, spatial variability in arsenic contamination may also arise from variations in sediment provenance across a fluvial landscape, and from inherent downstream changes in a fluvial system. Yet, the relative importance of facies and provenance as controls on arsenic concentrations in fluvial sediments still needs to be assessed. Through integrated analyses of geomorphological, sedimentological and geochemical data, this study examines the spatial variability in arsenic distribution from deposits of the late Holocene channel belt of the Po River, Italy. Three study areas were investigated along a >100 km stretch of channel belt to evaluate the possible roles of downstream changes in river behaviour and sediment supply from variably arsenic-rich catchments. Sedimentological controls on solid-phase arsenic concentrations are recognized at the scales of both elementary lithologies (facies) and depositional sub-environments, highlighting the roles of morphologically recognizable abandoned-channel fills as sources of arsenic that can be mobilized via groundwater flow and become trapped in point-bar elements. Although solid-phase arsenic concentrations are dominantly related to the presence of organic matter and clay, local arsenic enrichment may be related to arsenic advection across meander-belt sediments, a process that is itself controlled by petrophysical heterogeneity. No evident relationship is seen between arsenic concentrations and point-bar facies distributions related to styles of meander morphodynamic evolution. Yet, limited variability across the study areas suggests that the facies control remains dominant over a potential provenance control related to catchment integration. The results help elucidate the role of sedimentary heterogeneity in the distribution of arsenic in sediments, soils and aquifers, in the Po Valley and other analogous fluvial environments.