FORT COLLINS — A final vote is scheduled Tuesday by the Fort Collins City Council on whether to approve a hydraulic fracturing ban in the city, a move that could trigger a lawsuit by the state.

City councilors gave initial approval to a fracking ban last month. Fracking involves pumping a stew of water, sand and chemicals into a drilled hole to free oil and natural gas trapped in shale.

Fort Collins residents concerned about fracking's effects on air quality and health have encouraged councilors to pass the ban. The Colorado Oil & Gas Association and Gov. John Hickenlooper have suggested separately that they would take legal action against the city if it outlawed fracking.

Cities that ban fracking have little chance of success, according to a review of Colorado case law. State law pre-empts oil and gas drilling prohibitions.

The city's proposed ordinance bans fracking as well as storage of drilling waste in the city unless councilors approve a specific agreement with an individual oil company. Another version of the ordinance would ban oil and gas operations in addition to fracking unless the city reached an agreement with a company.

Under new state rules that increase buffers between oil and gas wells and buildings, fracking cannot take place in 90 percent of land in the city. The so-called setback rules take effect Aug. 1.