Animal husbandry is contaminating China’s water and has been linked to turning lakes bright green, a phenomenon known as eutrophication

The farm, located at the end of a narrow dirt path, announces its presence with a piercing stench. At first, the caretaker of the collective facility in Kunming says the farm recycles all the animal waste into manure fertiliser. But later, he sheepishly points behind the pigsty.

There, hordes of flies swarm above a festering field of grey-black dung. A few times a month, Cai shovels the steaming excrement produced by some 100 swine owned by local families into a nearby creek, where a mile downstream, villagers fish on the rocky shores of a small lake.

The hilltop fishing spot feeds into Dianchi Lake in south-western Yunnan province, a major tourist attraction and one of China’s largest fresh water bodies. Dianchi is famous for a number of reasons: its sheer size (nearly 40km long), its distinctive “crescent moon” shape and the surrounding scenic hills and rock gardens. But it has another distinguishing feature; for several decades, each summer the surface of Dianchi has turned bright green from algae blooms caused by excess nutrients such as nitrogen and phosphorus – a largely manmade phenomenon known as eutrophication.

Facebook Twitter Pinterest Polluting pig farm by Dianchi Lake. Photograph: Joanna Chiu

Eutrophication plagues areas around the world where rapid urbanisation and agricultural growth has taken place near bodies of water. When it rains, chemicals from cities, factories and fertilisers or untreated animal waste on farms can wash into lakes, streams and rivers. The extra nutrients feed algae on the surface of the water that can proliferate enough to block light. This deprives organisms of the light they need to photosynthesise. In the UK, for example, Tamar Lakes in south-western England suffered intense eutrophication pollution between 1975 and the 2000s, and the phenomena is also widespread in estuaries and coastal areas in the US.

Polluted water is a chronic problem in China, with citizens increasingly speaking out in frustration and even suing the government. In early 2013, some 16,000 dead pigs (including corpses infected by porcine circovirus) floated to Shanghai along the Huangpu river – a grisly sight that raised public concern about both unethical agricultural practices and water contamination. Some local governments have resorted to digging deeper wells to reach safe water, following an official survey finding as much as 80% of groundwater in major river basins is unsafe for human consumption.

Animal husbandry is a major contributor to contamination of China’s vital drinking water and seafood sources. The extensive survey of the impact of China’s agricultural transition published in Environmental Research Letters in 2016 found that in 2000, when the country was systematically expanding the size of farms, 30% to 70% of manure was dumped directly into rivers. Before 1970, when farms were mostly owned by single families, only 5% of manure was dumped into rivers. Later, a field test by Anhui Agricultural University showed that agricultural activity around Chaohu Lake in central Anhui province was the primary cause of pollution, according to a 2013 state newspaper report.

Facebook Twitter Pinterest Residential houses by polluted Dianchi Lake. Photograph: Fabio Nodari/Alamy

China’s surging appetite for meat has compounded the problem. Pork is by far the most popular type of meat consumed nationally, and the humble pig enjoys an exalted status in Chinese culture as a lucky creature symbolising prosperity and peace.

But environmentalists say the average citizen in China does not connect their love of pork to water pollution.

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In Dianchi Lake the algal bloom is so heavy that when speedboats pass near the boardwalk, they spray a green-tinged wake. On the beaches of the north-eastern coastal city of Qingdao, the algae is so thick in summers that tourists cannot even wade through so they make the best of the situation by burying themselves in the algae for pictures.

Some argue that the problem may be particularly acute in Yunnan because of the high concentration of pig farms in the province, which is partly fuelled by the fact locals eat a lot of pork. Last year, pork production reached 6.1m tonnes and comprised 77% of all meat production in the province, according to industry statistics. This means Yunnan residents consumed 127kg of pork per capita in 2017 – twice the national average.

Cai cannot afford to eat much meat. He comes from a destitute part of the neighbouring province of Guizhou and has only lived in Kunming for two years. He sleeps in a shed next to the pigsty and has never heard of Dianchi Lake, let alone eutrophication.

The joint family farm he manages is part of a local government drive launched in 2013 to consolidate small farms into bigger ones to help alleviate poverty. Officials provide caretakers with information on breeds, technology, disease prevention and sales but have not enacted regulations on waste treatment procedures.

Facebook Twitter Pinterest Green algae blooms at Dianchi Lake. Photograph: VCG/Getty Images

The underlying issue is economic, says Prof Rachel Stern, who specialises in environmental law in China at UC Berkeley. “The local government is reluctant to impose expensive pollution reduction requirements on small farms that can ill afford them.

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“In other pollution hotspots, local governments have typically solved similar issues either by providing pollution reduction subsidies, or coming up with an economic re-development plan to re-employ those who lose their livelihood,” she says.

It is a similar sight at another small pig farm around 50km away from Dianchi Lake, where a plot of agricultural land stands next to apartments towers. Animal faeces is piled up beside a small pigsty.

Facebook Twitter Pinterest The polluting pig farm in Kunming, Yunnan province. Photograph: Joanna Chiu

“Generally the way the government tries to solve water pollution is to shut down small animal farms and to make them into bigger factories,” says Wang Jing, head of Greenpeace East Asia’s food and agriculture programme.

“But this disrupts the pre-existing loop of agriculture in China which is to recycle animal waste into fertiliser using traditional methods. Some of the biggest farms do have industrial converters on site but they’re disincentivised to use them because they’re not legally obliged to and the cost of running the machine costs more than the value of the fertiliser produced,” Wang says.

One of the country’s biggest pig farms in Kunming, Yunnan Southwest Tianyou Animal Husbandry Technology Co, received a ministry of agriculture award earlier this year for being one of China’s “most beautiful” pig farms.

Protected by high brick walls and a pack of guard dogs, it houses some 10,000 pigs for supply to the national market. Its website boasts the farm uses top agricultural technologies. Photographs in local media show pigs in pristine pens.

“We opened here in 2008 and in 2010 we started using an industrial machine to convert waste into fertiliser. It was an important thing to do to protect the water and the soil,” says Ge Tao, a sales representative for the company.

“Some farms have it but some don’t. It’s not a government regulation to have it,” Ge says, adding that he wasn’t sure how waste was processed before 2010.

Reflecting on the green sludge in Dianchi, Ge says even though it is caused by various factors, privately owned farms should do their part to reduce pollution. However, the Guardian was not permitted to tour the company’s grounds because of quarantine policies, and could not confirm whether the fertiliser converter was being operated.