Guangzhou, China (CNN) -- On the banks of the Pearl River, vendors set up shop daily at the Luwei village market. Mr. Liu wanders through the stalls at dusk, selecting vegetables and fish from the local fishmonger for dinner. As the sun sets on the murky river, he marvels at the disturbing transformation of the waterway he calls home.
"The water has turned dark and black," he says.
"People used to swim in it," a cabbage hawker says across the market. "We know it's polluted, but what can we do?"
The Pearl River has sustained Chinese civilization for ages, but over the last few decades, civilization has not been kind to the river. It has become a dumping ground for debris, floating among massive algae blooms and even pig carcasses. Agricultural runoff is one of the river's biggest threats, next to industrial pollution.
The river is the lifeblood of the "world's factory floor," thousands of factories that produce the world's toys, mobile phones, computers, textiles and more.
It is also the blue jean capital of the world.
The township of Xintang, nestled in the northeastern part of the river delta, is an amalgamation of textile, denim and dyeing facilities. Inside, workers snap buttons on jeans so fast you can barely see their hands move.
The Chinese government estimates Xintang produces 200 million pairs of jeans per year including 60 different foreign brands. That is just under half of the 450 million pairs of jeans sold annually in the United States.
But what blue-jean clad consumers everywhere probably don't realize is the process by which denim is made may be poisoning China's water supply.
Satellite images reveal the part of the Pearl River adjacent to Xintang's blue jean factories indeed runs black. The nearby riverbank is piled with trash, including denim scraps.
Like most textiles, denim-making starts with plain white cotton. At the Jinxin Dyeing Plant, bales of it are piled up, then lowered into boiling vats of dye. The cotton emerges steaming, doused in that deep indigo blue color we all know well.
But the process releases tons of wastewater, a cocktail of dye, bleach and detergent. Foamy blue wastewater pools in a channel that winds around Jinxin factory property.
According to the factory boss, Li Zhongquan, most of the water is recycled. "If we didn't pay attention to the environment, the Communist Party would shut us down," Li said.
He later admits some of the wastewater is not recycled, but discharged, and claims he does not know where it goes.
You don't have to look far for a clue. Pipes at the edge of factory property lead directly into the Pearl River.
"The problem with those pipes is that they don't have labeling," said Greenpeace's China country manager, Edward Chan. "You don't know what is coming out from them. Some of it might be domestic discharge from the dormitories, but it could also include industrial waste."
Greenpeace's recent report "Poisoning the Pearl," indicates a fair share of factories may be flagrantly dumping their wastewater into the river.
The organization surveyed the contents of pipes from five different factories, including a textile factory and found that all five contained excessive amounts of heavy metals, organic pollutants and chemicals.
According to Dr. Tony Lu, Chief Medical Officer at Guangzhou's International SOS Clinic, these kind of toxins can be seriously hazardous to human health.
"If there are a lot of heavy metals, they are neurotoxic, carcinogenic, they disrupt the endocrine system," Lu said. "They cause cancer of different organs."
Greenpeace reported it discovered heavy metals like manganese, which can also be associated with brain damage.
But Lu said it was difficult to link industrial pollution to adverse health conditions along the Pearl River. The area has never had a documented outbreak of illness along the lines of "cancer villages" that have been discovered in other parts of China.
However, experts insist water pollution is a major challenge China has to confront -- or risk a massive threat to its water supply in the future.
"The number one problem (China) face(s) is water pollution," said Deborah Seligsohn of the World Resources Institute. "The textile industry is one of China's larger industries and one that uses a lot of water so it's traditionally had a lot of wastewater problems."
The Chinese government acknowledges it has a lot of work to do to clean it up. In February, the government revealed a detailed survey of water pollution indicating that it was twice as bad in 2007 as official figures suggested.
The Guangzhou Water Resources Bureau says it plans to spend $5 billion to improve wastewater treatment ahead of the Asian Games that will be held there this year.
In response to allegations of water pollution among denim producers, deputy director Wu Xuewei said: "If they're violating standards, we'll treat them as criminals and they'll be punished."
He added that the water department intends to implement and enforce a series of regulations, requiring companies to pass wastewater tests and imposing random inspections. He vowed factories will be fined, prosecuted or shut down if they exceed pollution limits.
But, when pressed, he claimed that regulation could be difficult due to the sheer volume of factories in the Pearl River delta.
"Of course we know what's in the pipes, every factory is supposed to register, but there are so many," he said. "What's exactly in the wastewater, I don't know."
Environmentalists say the biggest problem is that industrial pollution in a river as big as the Pearl can poison the entire ecosystem and put the people who live in it at risk.
In a fishing village along the Pearl River, a villager wades through a dirty pond and catches fish with his bare hands.
"It is a very good fish," he said. "I eat fish every day."
If he doesn't eat the fish for dinner tonight, he says he will send it to a local market where residents like Mr. Liu might buy it, not knowing whether or not it's contaminated.