“…The influences of hydrogeochemical factors (e.g., redox conditions, pH, and competitive anions) on groundwater As enrichment have been well documented in shallow aquifers (Naseem & McArthur, 2018; Wang et al., 2018; Zhang, Ma, et al., 2017). Sedimentological factors, such as the amount of Fe(III)/Mn oxides (Gillispie et al., 2019), reactivity (Stuckey et al., 2016; Wallis et al., 2020), purity (Masue et al., 2007), and As/Fe molar ratio (Sø, Postma, Lan, et al., 2018) of Fe(III) oxides, and secondary Fe(II) precipitation (Kontny et al., 2021), are also important in controlling As source and fate. Upon biodegradable organic matter mineralization, high abundance of Mn oxides may buffer Fe(III) oxide reduction and As release, being mainly due to a clear redox sequence between Mn oxide reduction and Fe(III) oxide reduction (Gillispie et al., 2016, 2019).…”