On the nature of ballast tank sediments and their role in ship's transport of harmful marine organisms

A representative selection of 20 ballast tank sediments from iron ore, coal, salt and woodchip carriers sampled in 3 Australian ports were subjected to elementary chemical analysis, particle size analysis and microscopic observations. Chemical composition of ballast tank sediments was remarkably uniform between ship types, location within tanks, and also within sediment depth core sections taken from a selected tank. The ballast tanks did not accumulate and/or retain bulk biological material. Sediment organic content was usually as low as 1-6% of dry weight, with the exception of samples from a woodchip carrier containing wood fibres (9-13%). The majority of ballast tank sediments consisted of fine clays and silica (30-56% SiO2) with 80-90% < 25 μm (typical lower phytoplankton size) and 50% < 10 μm size, with the exception of samples containing larger wood fibres or rust (up to 16-18% Fe). Ballast tank sediments were typically much finer than harbour sediments, which are not normally entrained into ship's tanks. The incidence in ship ballast sediments of microscopically detectable biological organisms (such as diatom frustules or dinoflagellate cysts) is not an intrinsic property of ship ballast tank configurations but is a function of voyage and port biology.