Texas regulators issued an environmental permit Thursday for the Port of Corpus Christi to build what could become the state’s first seawater desalination plant — but the U.S. Environmental Protection Agency may refuse to accept it.

The state permit for a desalination plant on Harbor Island represents the culmination of years of business strategy, political maneuvering and lawyering effort on behalf of the port, which wants to build a large-scale facility to convert seawater from the Gulf of Mexico into freshwater. The marine desalination plant is expected to cost at least half a billion dollars to construct; an estimate provided to the Texas Water Development Board puts the cost at more than $800 million.

Environmental groups have fought the project for four years on the grounds that wastewater from the plant could harm sensitive coastal ecosystems.

The dispute centers on what type of permit is required: The EPA believes the desalination plant needs a “major” environmental permit — which requires EPA review — while TCEQ says the facility should be considered a minor project, which does not require federal review.

“If the TCEQ issues [the permit] without responding to the EPA … the EPA’s position will be that it is not a validly issued [permit],” Earthea Nance, regional administrator for the EPA’s Dallas-based Region 6, wrote in a Sept. 2 letter to TCEQ Commissioner Jon Niermann. Some experts speculated that the EPA may sue the TCEQ to determine whether Texas is legally obligated to consult the EPA on such seawater desalination permits.

TCEQ commissioners on Thursday seemed to dismiss the EPA’s concerns. Commissioner Bobby Janecka said he considered the federal agency’s objections but called them “outside our window of decision” on whether to issue the permit.