The twin disasters highlighted China's chronic environmental problems and the precarious state of its scarce water supplies.
The latest spill occurred on the Bei river in Guangdong, China's most densely populated province with more than 100 million people and a center for its export-driven manufacturing industries.
Running water in Shaoguan, about 150 miles north of Hong Kong, was shut off Tuesday from about 9 a.m. to about 5 p.m., according to employees of three downtown hotels.
"Today, everything is back to normal," said a woman who answered the phone at the city's Hotel de Royce. She would give only her surname, Li.
The interruption came after the government said a smelter in Shaoguan dumped toxic chemicals into the Bei, causing levels of the heavy metal cadmium to jump to 10 times acceptable levels.
It wasn't clear how many people were affected in Shaoguan, which has about 520,000 people in its urban center.
Officials who answered the phone at the Shaoguan city government and water company and refused to give their names denied there was any disruption of water supplies.
Downstream from Shaoguan, the major city of Guangzhou — China's southern business center — was ordered to make emergency plans to ensure safe water supplies, the official Xinhua News Agency reported Wednesday night. The Bei flows into the Pearl River, which passes Guangzhou and empties into the South China Sea west of Hong Kong.
Xinhua didn't say what Guangzhou was told to do, or how many people there rely on water from the river.
It said Foshan, a manufacturing center nearby, also was told to make emergency precautions even though the toxins weren't expected to threaten either city.
It said 20 monitoring stations had been set up along the Bei.
In Yingde, a city north of Guangzhou and about 50 miles downstream from Shaoguan, Xinhua said water to 100,000 urban residents might be halted. It said the city used 15 fire trucks and other vehicles to deliver clean water to urban areas.
But officials contacted at the Yingde city government, water company and environment bureau said supplies were normal. Local officials in China often are reluctant to confirm the existence of industrial accidents or other disasters without clearance from the central government.
Officials in Yingde were dumping water from a suburban reservoir into the river to dilute the toxins and were building a pipe from the reservoir to bring clean water into the city, Xinhua said.
China has suffered a series of such disasters, often blamed on lack of required safety equipment or officials' refusal to enforce environmental rules that might hurt local businesses.
The accidents are an embarrassment to the government of President Hu Jintao, which has promised to clean up environmental damage from China's 25 years of breakneck economic growth.
The government says China's major rivers are badly polluted with such industrial chemicals. It says millions of people live in areas without adequate supplies of clean drinking water.
Last month, a chemical plant explosion in China's northeast spewed 100 tons of benzene, nitrobenzene and other toxins into the Songhua River, a key water source for millions of people. The major city of Harbin shut down running water to 3.8 million people for five days.
The Songhua flows into the Heilong River, which carried the toxins into Russia.
On Wednesday, the slick was nearing Khabarovsk, a city of 580,000 people in Russia's Far East. Authorities there shut down water supplies to 10,000 people as a precaution against possible contamination.
A top regional environmental official warned all Khabarovsk residents not to drink tap water.