Application of neutron activatable tracers (NATs) for cohesive sediment transport studies in contaminated estuaries

ANSTO and the University of NSW Water Research Laboratory (WRL) are investigating the migration of contaminants associated with cohesive sediments in Homebush Bay, Sydney. The study area is a highly urbanised and industrialised catchment with a long history of contamination Until 1890, when an ocean outfall was commissioned, domestic and industrial waste was discharged directly into Sydney Harbour. Heavy metals and other hydrophobic pollutants have a distinct tendency towards solid phase partitioning. This means that the majority of heavy metals in the estuary are linked to particulates rather than occurring in the dissolved phase. Hence, in order to assess the impacts of the pollution and develop a scientific basis for remediation it is necessary to understand processes that resuspend and disperse the contaminated sediments. The study approach involved the evaluation of the numerical model of the processes using activatable tracer techniques. An ideal tracer binds to the material of interest with high integrity and is detected with high sensitivity and selectivity. Tracers can be used to study sediment transport over extended periods and are therefore ideally suited to observing the impact of extreme weather events on sediment mobilisation by monitoring the distribution of the label before and after the event. The tracer must not only adhere to the cohesive sediment with high integrity but must be detectable with high efficiency, high sensitivity and relatively low cost. Identification of the optimum activatable tracer involved an assessment of the nuclear and sorption properties. The implementation of the tracer study involved (a) labelling sediment from the study area with indium-115 in the laboratory and equilibrating for 3 weeks; (b) choosing a site where bathymetric surveys indicated significant recent accretion; (c) injection of the labelled sediment into an accurately located site in Homebush Bay; (d) undertaking three surveys over the subsequent months; (e) analysis of samples via irradiation in the Fast Access Neutron facility in ANSTO's research reactor, HIFAR and gamma counting on a High Purity Germanium detector; and (f) data processing, where the tracer concentrations were contoured using the Surfer routine and interpreted in terms of advective and dispersive transport using a Gaussian approximation. Information on vertical transport was obtained by coring. These results are being used to evaluate a three-dimensional finite element model of the study area. Estimates have been made of the aerial dispersion coefficients, of the surficial mixing due to bioturbation and of advective transport. This paper will focus on the optimum choice of the tracer for cohesive sediment transport studies and some early results