Using k0-NAA to assess sediment quality in Causeway Lake (Queensland, Australia)

Causeway Lake is a popular recreational area for water sports. It was created when a bridge, a weir and a causeway were built across the Mulambin estuary in 1939. The current water depth varies from 0.5-1.3 m, versus the original 2-3 m depth. The Mulambin area has recently seen residential and other developments. The sedimentation rates in the lake were estimated recently at 1000 m3/y transported into the lake by tidal currents and 1500 m3/y from the catchment.This study aimed to determine metal concentrations in Causeway Lake sediments using instrumental neutron activation analysis (INAA) and the k0-method of standardisation, and to assess the sediment quality using the ANZECC sediment quality guidelines. NAA is attractive because of its minimal sample preparation requirements and multi-element detection capability; furthermore a single monitor is sufficient for the k0-method to determine the concentration of all detectable elements in a sample. To check the reliability of the k0-NAA method, BCSS-1 (estuarine) and PACS-2 (marine) certified sediment reference materials were analysed. The k0-NAA anlyses of the Causeway Lake sediments were also compared with ICP-OES or ICP-MS analyses.The study used the new 20 MW OPAL research reactor operated by ANSTO. At the INAA irradiation positions used the neutron flux was around 1.5×1013 cm-2 s-1 and was well thermalised (thermal/epithermal flux ratio >1000). Irradiations were carried out in both the short and long residence time facilities to maximise the number of quantifiable elements. The neutron flux monitor for k0-NAA was the reference material IRMM-530R which contained 0.1 % gold. Gamma-ray spectra were analysed using HyperLab and elemental concentrations calculated using Kayzero for Windows.The ratios of the k0-NAA data to the reference sediments’ certified values were generally 0.9-1.1 indicating reliability of the method. At least 30 elements (excluding Ag, Cd, Cu, Ni and Pb, but including rare earths) were detected in the Causeway Lake sediments. However only eight elements were compared because of the limited ICP-OES/ICP-MS dataset. The table (ratio of k0-NAA to ICP-OES/ICP-MS data) shows comparable average concentrations for surficial sediments and core slices (particles <60 micron), except for Cr and Sb. No significant metal contamination was indicated, although As and Cr appeared enhanced in some sediments.