By Alister Doyle
OSLO (Reuters) - Pollution levels in many Chinese lakes have declined somewhat from high levels in the past decade, helped by billion-dollar investments in urban sewers and waste water treatment, scientists said on Monday.
Concentrations of phosphorous fell by a third from 2006 to 2014 in 862 freshwater lakes around China, although they remain above clean water levels, according to the report about the chemical element in the journal Nature Geoscience.
Phosphorous is vital to life, but high concentrations can trigger blooms of toxic algae that choke fish and other life. Man-made sources of phosphorous include waste water, livestock farming, aquaculture and chemicals.
"The current decline in the most populated areas is due to improved sanitation facilities" such as pipelines, waste water treatment plants and improved rural toilets, said Yan Lin, an author at the Norwegian Institute for Water Research.
The findings could guide other developing nations seeking ways to clean up vital freshwater resources, he told Reuters.
The United Nations says that 2.4 billion people, about a third of the world population, lack access to basic sanitation services. Almost 1,000 children a day die because of diarrhoeal diseases linked to polluted water or sanitation.
The study, the first to have common measurements of phosphorous across China's lakes, showed the median level fell to 51 micrograms per 1 liter (1.76 UK pints) in 2014 from 80 in 2006.
That is still high, however: A level of 25 is considered good quality water in European legislation.
"A long time is to be expected before Chinese lakes could reach good ecological status," according to report by a team of scientists in China, Canada and Norway.
Phosphorous builds up in lake sediments and lingers long after sources of pollution have dried up.
Despite overall declines in most parts of China, phosphorous levels had risen from low levels in the sparsely populated northeast, it said.
The rise could be linked to climate change, which is causing more downpours and erosion of soils, Lin said. A study last year of rivers and lakes in the United States also found rising levels of phosphorous, perhaps linked to changing rainfall washing more phosphorous into the water.
(Reporting By Alister Doyle; Editing by Hugh Lawson)