Thousands of soldiers and police sealed off several miles (kilometres) of the Luo River after a truck loaded with 11 tonnes of highly toxic sodium cyanide tumbled into a brook leading to the river near Luoyang on November 1, Luoyang's City Morning Post said.Troops and farmers had also built two dams downstream within hours of the spill to block the contaminated water, the newspaper said.
Hundreds of tonnes of chemicals were then poured into the river to neutralise the toxin, it said.
"By now all our 11 checkpoints down the river show no signs of further contamination or danger," an official at the Environmental Protection Bureau in Luoyang, capital of Henan Province, told Reuters by telephone.
"We can say the alarm is over."
One farmer who was poisoned in the accident was recovering at a local hospital and several head of livestock were killed, local newspapers said.
The Luo River is a branch of the Yellow River, the main waterway in northern China.
"We have detained six people in connection to the spill," a police officer in Luoyang said, but declined further comment.
Local media said the drivers of the truck fled after the crash without warning authorities and the local government learned of the spill only after receiving reports of dead cattle.