Authorities in southwest China's Chongqing Municipality are making last-minute preparations to cap an abandoned well where leaking gas led to the evacuation of more than 3,500 people on Sunday.

Evacuees who live beyond a radius of 500 meters from the well were allowed to return home Tuesday morning, as the local environment watchdog confirmed that air and water quality were normal and no toxic or combustible gas had been detected, said Ai Yang, vice secretary-general of the Chongqing municipal government.

He said the gas leak had almost stopped, but an emergency task force led by experts from China National Petroleum Corporation will cap the well Tuesday afternoon to prevent future leakage.

Company sources said the capping operation would start at around 3:00 PM, and if successful, all the evacuees will return home within the day.

The gas leak occurred at 8:00 PM Sunday at an abandoned well in Tongqiao Town of Shuangqiao District on the outskirts of Chongqing.

The local government immediately evacuated all residents within a radius of one kilometer. Some were relocated to local schools and hotels while others were taken to stay with relatives.

A preliminary investigation showed the well was drilled by a local tourism company in March 2005, with the intention of building a spa. The company deserted the well three months later when no hot springs were found.

Experts say the well was not properly sealed, and pressure in the well caused the gas to leak. The gas was confirmed to be a mixture of sulfurated hydrogen and methane.

A deadly gas leak in Chongqing's Kaixian County three years ago killed 198 people and hospitalized more than 9,000.

Last month, another gas leak in Xuanhan County, about 500 kilometers northeast of Sichuan's provincial capital Chengdu, lasted six days and led to the evacuation of 12,000 people.

(Xinhua News Agency January 23, 2007)