Vertical distribution of As(Ⅲ) and As(ⅴ) in a coastal sandy aquifer: Marine clays, a potential reservoir for arsenic in coastal groundwater systems

摘要

Arsenic species were measured in a bundled-piezometer installed in the Holocene barrier sand deposits of the Stuarts Point coastal sands aquifer, Northern New South Wales, Australia. Vertical distribution shows two peaks of elevated arsenic concentration. At a depth 10-11 m below ground surface (bgs), As~(Tot), As(ⅴ) and As(Ⅲ) are in the range of 52-85, 38-67 and 14-18 μg/L respectively and the ratio of As(ⅴ)/As(Ⅲ) is well above 1 at 3.7-2.7. The second peak at a depth 24-26 m bgs shows the highest concentrations of As~(Tot), As(ⅴ) and As(Ⅲ). At 25 m bgs As~(Tot), As(ⅴ) and As(Ⅲ) are at 337, 125 and 212 μg/L respectively and the ratio is below 1 at 0.6-0.7. High As~(Tot) and As(ⅴ) concentrations in shallower depths are associated with acidic conditions and very low concentration of all ions. Desorption of As from Al hydroxides and As enriched Fe oxyhydroxides are the possible mechanisms releasing As into the aquatic system. The high concentrations of As~(Tot) and As(Ⅲ) at 25 m are concordant with high pH values and high HCO_3~- and Cl~- concentrations. The high concentration of As at this depth is possibly attributed to HCO_3~- competition with HAsO_4~(2-) which is reducing the possible extent of As sorption on Fe and Mn oxyhydroxides. The hydrochemistry of As at this depth shows its source to be potentially influenced by the mixing of saline water leaching from adjacent Holocene marine clay deposits into the adjacent groundwater system.