Authorities in the southwestern Chinese province of Sichuan confirmed on Wednesday that a road accident had triggered the spillage of 38 tons of concentrated sulphuric acid into a tributary of the Yangtze River.
The spill happened on Sunday after two vehicles collided on a bridge where the Chaotian to Guangyuan section of the Guangdong to Shaanxi national highway crosses the Jialing River, the Southern Metropolis Daily newspaper reported.
It said the Guangyuan government had dispatched an emergency task force to handle the pollution in the river.
"After the incident, we reported it and went to the scene to deal with the problem," an official on the Guangyuan municipal environmental testing team said on Wednesday.
"After we took samples from the water, they were tested at the municipal testing station," he said. "There were no abnormalities in the water."
But he added that the water had shown some changes following the spill.
"At the time there probably was some [acidity] in the water, but after a couple of days, and once it was cleaned up, then it was fine," he said.
But a professor of environment and resources at Beijing's University of Technology told overseas media that the ecological damage from the spill was likely to be severe, as the acidity in the water would still be above normal levels, affecting the habitat for most species.
"Aquatic life can't exist under such conditions of acidity," the professor said. "We can definitely expect that this will cause a high degree of die-off among aquatic life."
The Guangyuan environmental official said that a report had been released, but that it said the water quality was within the normal range.
Zhejiang-based environmental campaigner Chen Faqing told RFA's Mandarin service that he had encountered similar situations in his work.
"In situations where there is clearly pollution, the test results from environmental protection agencies show a normal reading," he said.
"The environmental agencies alone get to say whether the water quality is good or bad," he said.
Chen said the authorities usually handled such incidents by partially admitting the truth.
"In cases where it's bad, they will say it's not too bad, and in cases where it's disastrous, they will admit there are some problems with water quality," he said.
"They are afraid that if they tell it like it is, they will cause panic in the local population."
Residents of Guangyuan said in interviews on Wednesday that they hadn't heard anything about the pollution in the Jialing River, the main water source for many households.
"We didn't know about this," said one resident. "Maybe they reported it but we didn't know because we didn't pay attention."
A second resident said he hadn't heard of any problems either.
"I don't know about this," he said. "I didn't pay attention [to the news] so I didn't hear about this."
Chen said many incidents of pollution, even quite major ones, along with subsequent protests by local people, go unreported across China.
He said several thousand people turned out in protest in the eastern province of Zhejiang in June after cancer rates skyrocketed among residents near a chemical plant in Deqing county.
"Around eight or nine thousand people blocked the gates of the factory and wouldn't let production continue," he said. "The police said they were creating a disturbance and arrested a few people."
"The government sent out work teams door-to-door to all the households in Zhongguan township to make sure they didn't protest or talk to the media," Chen said.
"They said they would close the plant, but the polluted environment is still there."
Last month, China pledged a clampdown on the dumping of toxic waste following a chromium pollution scandal linked to a river in its southwestern province of Yunnan which prompted food safety fears in Hong Kong.
The country has been rocked by a series of pollution scandals after years of lax enforcement of what environmentalists say are, on paper, high environmental standards.
Many of the poisonings have involved lead and various toxins from chemical and electronics factories, often affecting the health of local children.
Also last month, fish farmers in the southeastern province of Fujian blocked a major highway and clashed with police over large-scale pollution of the Min River, which they said was behind massive die-offs among their stock.
Top environmental officials have warned that the ecology of China's lakes and rivers has become unbalanced, and that water resources management in China faces "major challenges."
Reported by Fang Yuan for RFA's Mandarin service. Translated and written in English by Luisetta Mudie.