Beijing ? A dam temporarily blocked a toxic spill of cadmium from flowing downstream and reaching the country’s southern business center, Guangzhou, a news report said today. It was the second manmade disaster to hit a Chinese river in six weeks.

On Thursday, a slick of toxic benzene from the first accident in the north arrived in the Russian city of Khabarovsk, where worried residents flooded a telephone hot line.

The twin disasters highlight the precarious state of China’s water supplies for industry and homes. Regulators say its major rivers are badly polluted and millions of people lack access to clean water.

The accidents are an embarrassment to President Hu Jintao’s government, which has promised to clean up environmental damage from China’s 25 years of breakneck economic growth.

Authorities in southern China were dumping water from reservoirs into the Bei River to dilute the cadmium spill from a smelter. Today, the China Daily reported that a dam in the city of Yingde had temporarily stopped the spill from reaching Guangzhou, a city of 7 million about 60 miles downstream.

“Water in the lower stream is safe,” a local official, Wang Zhensheng, was quoted as saying.

Cadmium is a soft, bluish-white metal found in lead and zinc ores. Exposure to it can cause lung and prostate cancer, kidney damage and bone disease, according to the U.S. Occupational Safety and Health Administration.

Chinese army soldiers fill sandbags near the town of Fuyuan on the Amur River at the Chinese-Russian border. Remnants from a toxic Benzene spill reached the Russian city of Khabarovsk on Thursday as a second toxic spill from a Chinese factory, this time involving cadmium, was reported.

The government did not say when the smelter spill would reach Guangzhou, the heart of the region near Hong Kong with factories supplying China’s booming export industries. But the official Xinhua News Agency said city leaders were ordered to “start emergency plans to ensure safe drinking water supplies.”

Six weeks ago, a chemical plant explosion spewed benzene and other toxins into a northeastern river, disrupting water supplies for millions of people and straining relations with neighboring Russia.

On Thursday, the benzene spill flowed into the city of Khabarovsk in Russia’s Far East. Authorities there said the city of 580,000 people would keep supplying running water from the river because chemical levels were still within a safe range.

The smelter spill in the south already forced two cities upstream from Guangzhou to stop using river water, according to state media.