Phosphorus from human and household waste, rather than fertiliser run-off from farming, is the main source of river pollution, according to recently published findings.
A ten-year study of nine rivers including the river Thames used another chemical, boron, found in washing powders, to help identify household waste as the main source of phosphorus.
The study, led by Professor Colin Neal, from the Centre for Ecology & Hydrology, in collaboration with scientists from Bangor and Durham University, has now been published in the journal Science of the Total Environment.
Dangers of phosphorus
Excess levels of phosphorus in water contributes to the process known as eutrophication, whereby certain species, for example algae, thrive and rapidly begin to dominant the river at the expense of other species, including fish. When the algae die, their decomposition removes vital oxygen from river waters.
Farming - in particular the over- and mis-timed application of pig, poultry and dairy slurry, has previously been blamed for phosphorus run-off and pollution of water supplies.
As recently as 2002, Defra had estimated that agriculture was responsible for about 50 per cent of phosphorus inputs to surface waters in the UK, with human and household waste responsible for some 24 per cent.
However, scientists now say agriculture's contribution has been exaggerated and that it is likely to be the source of just 20 per cent of phosphorus pollution, with household waste contributing 73 per cent.
The Environment Agency, commenting after the publication of the findings, said it agreed with the analysis and believed sewage effluent accounted for 60-70 per cent of the total phosphorus entering rivers in England and Wales. However, it said that for lakes, agriculture was still seen as the main source with household sewage second.
Impact of sewage
The study found the impact of sewage on river ecology was greater because its highest levels coincided with spring and summer growing periods.
'The critical time for biology is during the growing period when flows are relatively low and the effluent inputs are diluted the least. So, when biological activity is high phosphate concentrations are highest due to effluent inputs,' said Professor Neal.
'During the winter there may well be a high agricultural input of phosphorus, but this is not the critical period for biology and in many cases the phosphorus in agricultural runoff is in particulate rather than dissolved form - it is the dissolved form that is critical for eutrophication,' he said.
Professor Neal said their study did not argue that farming should be ignored and that high-risk areas of intensive livestock production should still be targeted. But he said that in agricultural areas greater emphasis should be placed on identifying effluent sources from houseolds, including septic tanks and local drains.
At the national level, he said their studies indicated that sewage should now be seen as the 'prime target for phosphorus remediation in rivers.'
However, Professor Neal warned that simply removing phosphorus might not provide the solution to good river ecology.
Analysis of targeted phosphorus removal on the River Kennet, off the Thames, found eutrophication to still be a major problem.
'The Kennet example shows that the problem cannot be solved just by removing phosphorus - the whole ecosystem needs fixing. We have to think more about how to make our rivers clean and how to restore the ecosystem back to what was before.
'We need to address far more than phosphorus concentrations in rivers, such as flow, habitat, and water resources which requires new science that looks at the complex relationships between hydrology, biology, chemistry and habitat, as well as our interactions and needs,' said Professor Neal.
Centre for Ecology and Hydrology
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