ARSENIC DISTRIBUTION IN A COASTAL AQUIFER AFFECTED BY SEAWATER INTRUSION

Dissolved arsenic occurs sporadically throughout the Stuarts Point coastal sand aquifer on the mid-North coast of New South Wales, Australia. Arsenic concentrations have been reported at more than 50 times the Australian Drinking Water limit of 7 ug L-1. In the absence of any dominant anthropogenic contamination, the arsenic within the aquifer is deemed to be naturally occurring. The source of arsenic to the aquifer is proposed as being derived primarily from the weathering of arsenic-rich stibnite deposits located in the upper reaches of the Macleay River catchment. Natural and anthropogenic effects contribute to current arsenic mobilisation in the groundwater. The heterogeneity of the aquifer sediments causes redox stratification to occur, which in turn governs arsenic mobility in the groundwater. The dominant geochemical processes governing arsenic mobility are reductive dissolution of iron oxides and the formation of arsenian pyrite under sulfate reducing conditions. Withdrawal of the aquifer's groundwater promotes seawater intrusion into the fresh water aquifer and coincides with increased concentrations of arsenic in groundwater. Average dissolved arsenic concentrations in the bulk of the aquifer generally do not exceed 15 ug L-1. However, dissolved arsenic begins to increase with increasing depth, suggesting an additional source of arsenic to the aquifer. Dissolved arsenic concentrations increase to more than 80 ug L-1 in groundwaters affected by seawater intrusion.

The increased concentrations of arsenic in seawater intrusion affected parts of the aquifer indicate that the seawater itself may be an additional source of arsenic to the aquifer. Current erosion of natural stibnite deposits, or release of arsenic from mine tailings in the upper Macleay River catchment, may be contributing to elevated concentrations of arsenic observed in the Macleay estuary and subsequent seawater intrusion groundwaters.