Natural attenuation of arsenic by ferric hydroxide in a coastal aquifer

Groundwater in a coastal sand aquifer is enriched in arsenic as a result of historical discharges from an industrial facility on the central NSW coast. Subsequent investigations also identified arsenic in the soil within the saturated zone down-gradient of the source area. Arsenic concentrations in soil and groundwater decrease markedly along a narrow flow path towards a tidal river some 300 m from the source. Despite high arsenic concentrations in groundwater, measured concentrations at the expected discharge location did not exceed the ANZECC/ARMCANZ (2000) guideline values similar to concentrations elsewhere in the river. Given the mass of arsenic in the aquifer, relatively high groundwater flow velocities in the aquifer and strong gradients of arsenic concentrations in soil and groundwater, naturally occurring processes within the aquifer are thought to retard the migration of arsenic towards the river. A conceptual site model based on adsorption of arsenic to iron minerals was developed to account for the attenuation of arsenic within the aquifer. Under the moderately oxidising groundwater conditions in the aquifer, ferric hydroxides (amorphous iron) were considered likely to provide sufficient adsorption sites to bind arsenic anions. The conceptual site model was assessed by geochemical modelling (PHREEQC) and a program of laboratory testing. The laboratory testing included sequential extractions to quantify exchangeable, amorphous and crystalline proportion of total iron within the aquifer. Arsenic adsorption capacity and the desorption properties of the aquifer materials were assessed using column tests. The laboratory program verified that ferric hydroxide associated with the aquifer materials has the capacity to adsorb the arsenic present in the aquifer. Modelling of the field and geochemical data confirmed the key process that attenuates arsenic within the aquifer is adsorption by naturally occurring ferric hydroxides.