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CHONGQING, Jan. 23 (Xinhua) -- Authorities in southwest China's Chongqing Municipality have successfully capped an abandoned well where leaking gas led to the evacuation of more than 3,500 people on Sunday.

The capping operation, led by experts from China National Petroleum Corporation, began at 2:50 p.m. on Tuesday, when concrete mixers poured concrete down more than 700 meters into the well.

A team of firemen stood by throughout the 40-minute session.

Ai Yang, vice secretary-general of the Chongqing municipal government, declared the operation was successful at 3:30 p.m..

More than 2,000 evacuees who live beyond a 500 meter radius of the well were allowed to return home Tuesday morning, as the local environment monitoring station confirmed that air and water quality were normal and no toxic or combustible gas had been detected, Ai said.

He said another 1,500 evacuees living closer to the well are likely to be given the green light to return home before 6:00 p.m.

The gas leak occurred at 8:00 p.m. Sunday at an abandoned well in Tongqiao town of Shuangqiao district on the outskirts of Chongqing. No casualties were reported.

The local government immediately evacuated all residents within a radius of one kilometer. Some were relocated to local schools and hotels while others stayed 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. After three months of drilling, the company deserted the well it failed to tap into a hot springs.

Experts say the well was not properly sealed, and pressure in the well caused the gas 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.