The density of major pollutants in a river in southwest China's Sichuan Province has dropped after the river was contaminated by waste chemicals from a local manganese plant.

As of Thursday morning, the density of manganese and ammonia nitrogen in the Fujiang River, the main source of water in the city of Mianyang, has dropped sharply, according to a monitoring report released by the Mianyang environmental protection bureau.

However, the density of the pollutants is still higher than the national standard, the report said.

Heavy rains that fell Wednesday night diluted the pollutants, said Lu Liangjun, director of the emergency management office of the Mianyang municipal government.

The river's water quality is expected to reach acceptable levels soon, as more rain is predicted for Thursday evening. However, local residents should avoid drinking tap water until an inspection report by the city's environmental protection bureau and center for disease control is released, he added.

Waste chemicals from the Xichuan Minjiang Electrolytic Manganese Plant were washed into the river by heavy rains last Thursday, according to a notice posted on the city government's official website.

Mianyang residents resorted to buying bottled water after local authorities reported the contamination on Tuesday.