NATURALLY ELEVATED ARSENIC AND OTHER TRACE ELEMENTS IN A SAND AQUIFER OF EASTERN AUSTRALIA

Aquifers comprised of unconsolidated sediments can be valuable resources due to their high groundwater yields. Much of eastern coastal Australia contains sand aquifers that are heavily exploited for drinking water and irrigation. Unless specifically targeted, many trace elements and metals have not been part of routine analyte suites in groundwater risk assessments in past decades. The recent widespread occurrence of arsenic in many sedimentary aquifers of the world prompted the analysis of dissolved arsenic in a drinking water supply aquifer at Stuarts Point, 600 km north of Sydney. Arsenic has been reported above the Australian drinking water limit of 7 ug L^-1 in 49% of groundwater samples analyzed. A detailed hydrochemical characterization of the aquifer revealed the source of arsenic to be natural, weathered from a mineralized arsenic-bearing stibnite (Sb₂S₃) deposit in the hinterland and transported downstream by fluvial processes.

Also reported above drinking water guidelines in this seemingly ‘pristine' and low contaminant risk environment were Cd, Hg, Mn, Pb and Se. However, spatial distribution is not widespread like dissolved arsenic distribution. Multivariate statistical analyses have linked these elevated elemental occurrences to specific zones in the aquifer. Such zones include shallow groundwater subjected to pyrite oxidation, groundwater flowing through fluvially deposited sediments or mineralized weathered bedrock clays, and groundwater impacted by seawater intrusion processes. Variation in chemical heterogeneity, particularly redox conditions and groundwater-matrix interaction, govern the sporadic distribution of these elevated elements.

These results can aid in determining optimal depths of groundwater withdrawal for various uses in the Stuarts Point aquifer. For example, Cd occurs in elevated concentrations at approximately 10 m below the ground surface. Bores installed primarily for irrigation of potato crops should therefore target shallow groundwater (less than 10 m) in order to reduce the possibility for Cd accumulation in potatoes. Distinct groundwater zones occur within a very limited depth (30 meters) in this aquifer, illustrating the importance of considering chemical heterogeneity in groundwater risk assessments for all aquifer matrices.