Diel cycling of Arsenic and Antimony in the Macleay River in October 2016

Diel sampling was conducted in a small pool in Bakers Creek (Macleay River), immediately upstream from the confluence with the main Macleay River trunk channel during October 2016. At the time of sampling, the pool was shallow (<0.7 m) and contained low-turbidity surface water overlying sandy-textured benthic sediments with abundant filamentous green algae (Spirogyra sp.) and a uniformly low flow (~0.01 m³/s). Water temperature, dissolved oxygen (DO), pH and redox potential were measured in situ at 5 minute intervals using a freshly calibrated EXO-2 multi-parameter sonde (YSI instruments). At each sampling interval (every 1.5-2 hrs), a 60 mL water sample was collected via a clean enclosed syringe and pre-cleaned PTFE tubing connected to a sampling port situated 0.1 m below the water surface and filtered immediately on-site (0.45 µm; enclosed syringe filter). Both syringe and tubing were pre-flushed (5x) prior to sample collection. Filtered aliquots were added to a 1,10-phenanthroline solution for subsequent photometric determination of Fe2+. Additional filtered sub-samples were collected and preserved for subsequent analysis of As3+ [via high performance liquid chromatography (HPLC; Perkin-Elmer Flexar) coupled with inductively coupled plasma mass spectrometry (ICP-MS; Perkin Elmer Nexion 350D), using a C18 (5 µm) column and mobile phase of 1 mM tetra-butyl ammonium hydroxide, 0.5 mM EDTA and 5% ethanol in de-ionized water (flow rate 1 mL min-1; 20 µL injection)] and Mn, Fe and Sb via ICP-MS.