An independent pollution-control agency has rejected environmentalists’ claims that a planned landfill could desecrate possible burial grounds near the ruins of a once-thriving prehistoric city.

The Illinois Sierra Club and American Bottom Conservancy failed to show that Madison’s approval process for a landfill near the Cahokia Mounds State Historic Site was “fundamentally unfair,” the Illinois Pollution Control Board ruled Thursday.

The St. Louis suburb, which approved the landfill in February, would get roughly $1 million a year in fees from Houston-based Waste Management Inc., the nation’s largest garbage hauler.

Opponents on Friday said they were weighing whether to challenge the matter further.

“A municipality in search of revenue is going to choose revenue over any cultural or natural resources in the area. That’s what we see in so many of our cases,” said Bruce Morrison, an attorney with Great Rivers Environmental Law Center, which pressed the lawsuit.

Messages left Friday with John Papa, Madison’s attorney, were not immediately returned. Papa has said that the city followed appropriate steps to consider the landfill “and make a reasoned judgment on it.”

Bill Plunkett, a spokesman for the company, called the pollution board’s decision “the right one” and “supported by the evidence.”

Environmentalists say the expanded site would be within 2,100 feet of the Cahokia Mounds site, where as many as 20,000 people lived during its peak of 1100 to 1200 A.D. It was among the among the most complex, sophisticated societies of prehistoric North America.