A Field, Laboratory and Modeling Study on Reactive Transport of Arsenic in a Coastal Aquifer

Enrichment of arsenic in sedimentary aquifer is wide spread in the world, but studies that combine field, laboratory and modeling approach to investigate reactive-transport of arsenic are few. Speciation data of Fe and As in pore water and sediment collected from a coastal aquifer discharging to the Waquoit Bay, MA, shed light on sorption and redox reactions occurring along the flow path of a terrestrially derived groundwater plume. The discharging reducing groundwater is bracketed by an upper and a lower redox interface, indicated by dissolved Fe(II) and sedimentary FeII/Fe profiles. As the fresh groundwater plume advects toward the bay, a decrease of dissolved total As concentration from ~0.19 to ~0.03 µmol/L was accompanied by an increase in the proportion of the As(V) from ~15% to > 90%. The immobilization of dissolved arsenic occurs at the redox boundaries during discharge, with enrichment of sedimentary As to ~0.45 mg/kg, compared to depleted sedimentary As of ~0.1 mg/kg within the reducing groundwater plume. Oxidation of arsenic contributed to immobilization because > 90% of phosphate extractable As from the sediment is As(V).

Batch adsorption experiment using sediment samples with a range of FeII/Fe ratios revealed that most of the As(III) was oxidized over 2 weeks of experiment, and that the distribution coefficient (Kd, 30~83 L/kg) and sorption capacity (1515~4762 µg/kg) increase with decreasing sediment FeII/Fe ratio, suggesting that Fe(III)-oxyhydroxides are capable of trapping more As than reduced Fe(II)-minerals.

To better understand the chemical and hydraulic properties controlling the reactive transport of arsenic, a three-dimensional reactive multi-component transport model, PHT3D simulated the oxidation and sorption of Fe and As in the aquifer, using observed hydraulic gradient and experimentally determined Kd. Heterogeneous distribution of hydraulic gradient or distribution coefficient results in significant spatial and temporal variability of As flux in a subterranean estuary.