Why do lakes in China turn green? Report finds surprising new culprit.

When you think of pollution in China, you tend to think first of smokestacks and smog-bound factory landscapes and lead-poisoned children and fields coated in coal dust.

They all exist, of course. But a Chinese government report published Tuesday points out a little known fact: Nearly half of the pollutants that poison China’s waterways come from farms.

Agriculture, in one form or another, is to blame for 43 percent of China’s “chemical oxygen demand” or COD – a commonly used measurement for water pollution – according to a new census of pollution sources.

“This is the most significant finding of the report,” says Ma Tianjie, a spokesman at the Beijing office of Greenpeace. “These are largely unseen threats, invisible to the public.”

Every now and again they become visible – when a lake turns vivid green or bright blue for example, because the nitrogen runoff from fields has fed algae that bloom extravagantly.

Those algae also suck all the oxygen out of the water, and the fish suffocate. It happens quite a lot in China.

One reason we did not know the full extent of farmers’ responsibility for pollution is that until now Chinese environmental surveys have not included agricultural pollution. And one reason why that happened was that the Agriculture Ministry didn’t want them to.

But the new figures are indisputable. “To address water pollution in China at the root, we must make prevention and control of pollution from agricultural sources our top priority,” said Zhang Lijun, vice minister of environmental protection, as he released the audit – two years in the making – on Tuesday.

Villains of the piece include fish farms, intensive poultry farms, and livestock factory farms, whose raw sewage poisons rivers and lakes in many parts of the country.

The government is trying to counter this by encouraging the spread of biogas digesters and larger-scale methane plants, but they have a long way to go.

When will pollution peak?

It was not clear, though, from the press conference, how much further China has to go before its pollution levels start to fall. Richer countries have found that when gross domestic product per capita hits around $8,000 a year, pollution drops off. Mr. Zhang boasted today that China is so pollution-conscious that its poison levels would peak at the $3,000 point.

Unfortunately none of the journalists at the press conference had the presence of mind to point out that official Chinese figures have put per capita GDP above $3,000 for the past two years. But there is no sign of pollution levels falling, at least not from my window.