Pollutants - are fine sediments the real culprits?

Thursday, 03 March, 2005

Studies have shown that significant percentages of heavy metal and organic pollutants in major harbours such as Sydney's are bound up in fine sediments.

This is due to a range of events. Soluble contaminants are eventually flushed out by tidal cycles and storms in the catchment area. These contaminants are more soluble in the fresh water of creeks and canals, but when mixed with seawater, the contaminants will adsorb onto fine suspended sediments and eventually settle to the bottom of the waterway.

And for those contaminants that like to bind to solid surfaces, such as heavy metals and other hydrophobic (organic) pollutants, fine sediments have the largest surface area-to-volume ratio for solids within the harbour.

If fine sediments are the major 'sink' for contaminants in harbours such as Sydney's, does this mean that surrounding populations are progressively contaminating these harbours?

Or are the harbours and bays actually cleaning themselves?

Generally speaking, major harbours around the world are highly urbanised and industrialised catchment areas, often with long histories of settlement and contamination. In most cases, urban and industrial runoff is widely regarded as the major non-specific source of total contaminants to these harbours.

Because heavy metals and other organic pollutants (organo-chlorides) have a distinct tendency towards solid-phase partitioning, this means that the majority of contaminants in the catchment areas ultimately become linked to sediment particles, with some dissolving under certain conditions.

However, unlike many organic contaminants, heavy metals do not degrade - they remain in the environmental system. Therefore, where high levels of metals persist in the sediments, it is important to understand where these contaminants move to, and what processes, such as dredging, construction or erosion, might disturb them and make them available.

To work out the ultimate fate of this sort of contamination, the movement of fine sediments within these waterways must be tracked. It is here that ANSTO's unique nuclear capabilities, isotopic techniques and experienced researchers are providing new answers on how best to deal with contaminated sediment without creating other problems.

Nuclear techniques have distinct advantages over conventional methods in this area. Highly accurate and specific information on physical, chemical and biological processes can be obtained using a proxy isotope with appropriate properties, called a 'tracer'. Tracers may be naturally occurring isotopes, characteristic of the particular process under investigation, artificial radioisotopes introduced as analogues of natural products, or non-radioactive labels on substances retrieved from nature.

Non-radioactive tracers are used in urbanised areas. These are applied to trace the movement of sand or sediment and the geochemical cycling of elements in the water.

Artificial sand tracers have been used to show the movement of sand during storms, where mega-rips cause beach erosion on the east coast of Australia. This work has shown that future climate change could magnify the sand loss. These tracers are also applied to studies of offshore sewage or pollutant plume dispersal.

And the news isn't all bad.

While fine sediments do contain a lot of unpleasant contaminants, within limits most do not cause environmental problems because they are not bioavailable. They do not enter the food chain and poison harbour-dwelling fish (and ultimately us!).

However, some contaminants are bioavailable - for example, seagrasses and animals living in the sediment ingest them or pass sediment water over their gills. Thus, it becomes important to determine the degree of bioavailability so that environmental managers can make risk assessments on those areas which need to be cleaned up or which need to be avoided or excluded from activities such as fishing.

Isotopes can help here as well. Most metal contaminants have radioactive isotopes. At ANSTO, very small quantities of these isotopes are being used in biological studies to work out the degree of bioaccumulation and the biological half-lives of the contaminants in the affected animals and plants.

Article reproduced from the May 2004 issue of the BusinessANSTO newsletter, with permission from ANSTO.

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