Controls of organic matter bioreactivity on arsenic mobility in shallow aquifers of the Hetao Basin, P.R. China

Abstract

Organic matter (OM) is widely accepted to be the trigger for arsenic (As) mobilization from sediments into groundwater. Identifications of reactivity and sources of organic matter responsible for driving As liberation remain unresolved. To address these issues, we provide the extensive characterization of OM in shallow groundwater, surface water, and sediments near a permanent wetland and a dried wetland in the Hetao basin with high As groundwater and low As groundwater, respectively, using fluorescence spectroscopy and parallel factor analysis (PARAFAC). Dissolved organic matter (DOM) of shallow groundwater and sediments near the permanent wetland had higher biological index (BIX), and more protein-like components and microbially-derived components relative to that near the dried wetland, showing higher bioreactivity. Fingerprint of water stable isotopes and higher water levels of wetland than shallow groundwater pointed to the recharge of wetland water into shallow groundwater. Laboratory incubations of the sediment with more bioreactive OM obtained near the permanent wetland mobilized more As. Both DOM properties and laboratory incubations indicate that OM with high bioreactivity promoted As mobility mainly by fuelling microbial respiration of Fe oxide reduction. Recharge of wetland water into shallow groundwater introduced bioreactive DOM into shallow aquifer. Groundwater DOM near the permanent wetland was typically sourced from both wetland DOM and sedimentary OM with high bioreactivity. However, sedimentary OM was the major source of groundwater DOM near the dried-wetland. It suggested that downward infiltration of surface-derived OM and perturbations of aquifer sediments by changing groundwater flow regimes increase bioreactivity of groundwater DOM and therefore enhance As mobility.