New breakthrough helps trace river pollution


5 August 2016 by Tom Marshall

Scientists are developing a new way to track the origins of phosphorus pollution in our rivers and understand how it behaves once it gets there.

Too much phosphorus in rivers is a big problem. It's one of the most important plant nutrients and excessive amounts can cause explosive growth of algae, killing fish and disturbing complex aquatic ecosystems. If it reaches lakes and the coastal ocean, it can also stimulate giant algal blooms.

But though we know many of our rivers have far too much phosphorus, we still have a limited understanding of exactly where it's coming from.

Dealing with this is vital if Britain is to meet the requirements of the EU Water Framework Directive, which says all rivers must be restored to 'good ecological status' - something we're still a long way from. To do this effectively, we need a detailed grasp of the sources of phosphorus - for example, in one river pollution might be coming from a sewage works, while in another the source might be agricultural fertiliser.

NERC-funded scientists have pioneered a new way of shedding light on this by analysing the chemical makeup of the oxygen that's joined with phosphorus to form phosphate ions. Phosphate is one of the main forms of phosphorus in waterways, and among the most readily available to aquatic organisms.

Like many elements, oxygen comes in different forms, known as isotopes. They all have the same number of protons and electrons in each atom - that's what makes them oxygen - but hold different numbers of neutrons.

In this case, the scientists adapted techniques used in other fields in order to look at two oxygen isotopes - 16O and 18O. The ratio between the amounts of these two isotopes present in phosphate varies depending on where it came from, forming a chemical signature that points to the source of a river's pollution, and lets researchers track it as it moves through the river ecosystem and is taken up by living things. They tested their approach on the River Beult in Kent, where nutrient pollution comes mainly from sewage plants and farms.

"This gives us a new and very precise way to understand the sources of phosphorus in rivers - whether that's sewage works, leaking water mains or fertiliser - and how it's taken up and cycled in the ecosystem once it gets there," says Professor Daren Gooddy, an environmental chemist at NERC's British Geological Survey who led the research alongside Dr Ben Surridge of Lancaster University. The technique can also enable researchers to understand how pollution levels in a river vary from season to season or even over the course of a single day.

Phosphate pollution in rivers comes from many sources, including outflow from sewage treatment works, fertiliser and slurry running off agricultural land and leaking mains water. Each source potentially has its own isotopic fingerprint, letting Gooddy and his colleagues understand the sources of a particular river's pollution problems. There are ways to address each of these sources - for instance, installing phosphate stripping at sewage works, or encouraging farmers to change how they manage their land. But these measures can be expensive, and if we don't understand all the sources in a catchment they may not be as effective as anticipated.

For instance, in some places the main source of phosphorus could be natural, coming from the underlying geology. In these cases the way in which farmland is managed may be less important for phosphorus pollution in rivers, and it would be better to target money elsewhere.

NERC provided the initial grant for the research, alongside parallel industrial funding from Yorkshire Water which was interested in the method's potential to improve understanding of the sources of river pollution. Several other potential users of the new method have since come forward, including the Environment Agency and several water companies. Wessex Water has just started funding a PhD student at the University of Bristol, supervised by Gooddy and Professor Penny Johnes, using this method to try to distinguish between natural phosphate sources and historic ones like legacy fertiliser application.

DOI: 10.1016/j.watres.2015.10.046.