Seasonal mobility of Arsenic and Antimony in the Macleay River 2016-2019

Water samples from the Macleay River were collected from Bellbrook bridge (30°49'10.58"S; 152°30'20.33"E) over a 35 month period (August 2016 to July 2019) at approximately 0.5 m below surface at mid-channel, directly into a clean (trace-grade HNO3- acid-rinsed), 1 L low density polyethylene container, pre-rinsed five times with sample water. A stratified sampling approach was employed, with samples collected every 5-7 days during ambient and low flow periods and daily (or multiple times daily) during high flow events. During high-flow periods when the bridge was submerged, samples were collected from the river bank using a 4 m extendable sample pole placed as far as possible into a turbulent part of the main flow. Water samples were filtered (0.45 µm; enclosed syringe filter) and filtered aliquots preserved using ultra-pure HCl, prior to analysis for As and Sb by ICP-MS (Perkin-Elmer ELAN-DRCe). Cl- and SO42- were analysed via via ion chromatography (Metrosep A Supp4-250 column, an RP2 guard column and eluent containing 2 mM NaHCO3, 2.4 mM Na2CO3 and 5% acetone, in conjunction with a Metrohm MSM module for background suppression). Mean daily river discharge was recorded at Georges Junction gauging station (NSW Water, station #206024, 30°45'6.71"S; 152°12'17.96"E). Mean daily water temperature in the Macleay River was recorded at Kempsey by automatic logger (NSW Water, station #206402, 31° 4'50.98"S; 152°50'33.05"E).