Pleistocene sands of the Mississippi River Alluvial Aquifer produce the highest groundwater arsenic concentrations in southern Louisiana, USA

•Arsenic concentrations up to 637 µg kg−1 measured in Pleistocene sands of the MRAA.•As(V) is the predominant form (ca. 84%) of the total dissolved As measured in MRAA.•Highest As, Fe, and Mn concentrations in wells located near the riverbank.•Spatial correlation are stronger between concentrations of As & Fe than As & Mn.•Labile DOC from the river may fuel microbial reduction of Fe(III) and Mn(IV)