You don't need a weatherman to know which way the wind blows, says Bob Dylan, but if you want to know how water moves, Cath Hughes from ANSTO would be just the person to ask.

Along with colleagues from the Water Research Laboratory at the University of New South Wales (UNSW) and the Philippines Nuclear Research Institute, Cath has been working on several environmental management projects, including one at Manila Bay in the Philippines.

Australian scientists are helping the Philippines manage its stressed environment. 30 per cent of Manila's sewage, for instance, flows untreated into the Pasig River each day.

More than a quarter of the population of the Philippines (around 16 million people) live in the catchment area which drains into the bay. It is a major fishing ground, has a large shellfish industry, and is home to the nation's capital, Manila -- one of the busiest ports in south-east Asia.

"We were interested in the movement of contaminants through the bay, particularly those that could be a potential source of red tides," Cath said.

A red tide is a massive population explosion, or bloom, of certain types of algae which,
in this case, cause paralytic shellfish poisoning. They occur when the water is warm, still and loaded with nutrients.

The team from UNSW developed a computer model of the bay and Cath and her colleagues set out to see if it was a good fit with what was actually going on.

By releasing a radioisotope tracer into the bay and monitoring the plume of radioactivity, the team were able to map where the water went.

They used technetium 99m, a radioisotope more commonly associated with medical imaging, from a standard Gentech generator used in hospitals around Australia.

"As you'd expect from medical technology, it's convenient, easy to handle, and safe in the environment," Cath said.

The traditional way to see where water goes is to dye it a bright, sometimes fluorescent, colour.

"The problem with fluorescent dyes is that you need hundreds of kilos of dye to detect them, and if you're trying to study anywhere near polluted rivers -- which is just about all of them in this area -- their discharge is also slightly fluorescent and can become confused with the dye," Cath explained.

The upshot of this is that relying on the most widely used medical radioisotope -- technetium 99m -- to trace pollution in bays and estuaries has obvious advantages.

"Our technique has so much to offer," continued Cath. "With radioisotope tracers, a couple of millilitres is all you need. Although some people have concerns about radioactive materials in a way that they don't have about chemicals -- even though, scientifically, the reverse would often be more logical -- the use of a highly engineered medical radioisotope is reassuring."

Modelling water is a tricky business, but environmental managers rely on model predictions. Cath says working with radiotracers has shown her just how often the models we use are simply not up to scratch.

The good news is that UNSW's model did match the real world. "We were able to say, if there's a problem here, this how it will be spread," Cath said. "And there hasn't been a red tide since -- perhaps we've scared them off."