Six natural-gas producers agreed to install pollution-control equipment to comply with the Clean Air Act at their facilities in Utah, at a total cost of about $6.5 million.

The agreements mandate air-pollution reductions and conservation practices at the companies' natural-gas compressor stations, well heads and pipelines across the Uinta Basin, the Justice Department and Environmental Protection Agency said Friday.

The six companies listed in three settlements were Bill Barrett Corp. (BBG), Wind River Systems Inc. (WIND), XTO Energy Inc.(XTO), Dominion Resources Inc. (D), Whiting Petroleum Corp. (WLL) and Miller Dyer and Co.

In addition to the settlement, the companies are required to pay $632,000 in civil penalties and spend $200,000 on supplemental environmental projects.

According the complaints, the companies allegedly violated several provisions of the Clean Air Act, including emission standards for hazardous air pollutants and emissions-monitoring and reporting requirements. Dominion and Miller Dyer came forward and disclosed their violations under EPA's self-audit policy.

EPA Administrator Carol Rushin said the pollution controls would "reduce emissions of air toxics and greenhouse gases, while conservation measures will help return valuable natural gas to the marketplace."

The agency estimates the investment in the technology will reduce air pollution by more than 1,300 tons per year. The reduction in greenhouse gas emissions, including methane, is equivalent to the annual carbon sequestration of 9,400 acres of pine or fir forest or comparable to taking more than 7,600 cars off the road each year.

The natural gas conserved, meanwhile, is enough to heat approximately 1,080 homes annually.

-By John Kell, Dow Jones Newswires; 201-938-5285; john.kell@dowjones.com