A protracted conflict over whether and how to protect fish from dying at desalination plants is clouding prospects for what would be California’s second large plant of this type – and for the future of desalination along the entire California coastline.

For years, a proposed Poseidon Resources desalination plant in Huntington Beach in Orange County has been kept in limbo. This has been due in part to disagreements over whether the plant should use conventional, surface-based intake pipes to take salt water directly from the ocean – intakes that already exist in a neighboring electric power plant – or use “subsurface intakes” that would suck in the saltwater underground.

The purpose of subsurface intakes is to prevent fish and other marine life from being trapped and killed in seawater flowing into the plant, a process known as entrainment. Another concern is impingement, in which fish are trapped against intake screens – juvenile fish typically can’t survive this for more than 24 hours. At least partly because of their higher construction costs, most large desalination plants worldwide don’t use subsurface intakes.

But now new California regulations governing desalination plants make subsurface intakes mandatory unless it can be shown they’re not economically and/or technically feasible. The new rules – the only such regulations in the world – are forcing additional delays for the proposed plant, which has been seeking state permits since 2001.

It’s clear that this case’s outcome will set a precedent for whether up to 15 proposed desalination plants are developed along the California coastline.

The new regulations were approved by the State Water Resources Control Board in May 2015 and ratified by the Environmental Protection Agency in April 2016. But they haven’t changed Poseidon’s longstanding position that subsurface intakes are not feasible at the Huntington Beach plant, in part because they would cost $1–1.5 billion to build, on top of the plant’s estimated $2 billion construction tab. The plant is expected to produce about 56,000 acre feet (69 million cubic meters) a year, or 50 million gallons (189 million liters) a day, of desalinated water.

Poseidon general manager Scott Maloni said he’s confident he can persuade the state that subsurface intakes aren’t feasible, but that if the state ultimately insists on subsurface intakes, the project will be dead: “Any way you slice it, subsurface intakes are a non-starter for our project.” He added that company officials have “no intention of building at any other site in Orange County, not after we spent 15 years to obtain a permit at this site.”

State officials state that at this point they can’t say if the subsurface intakes are feasible at either the Huntington Beach site or at any of the 13 other Orange County locations that are under review as potential alternative plant sites.

At the end of October, a regional water-quality control board based in Santa Ana – one of three agencies that must still issue permits for this project – sent the company a letter seeking 76 pages’ worth of additional information and said it may require Poseidon to hire a third party to review some of the information it submitted to the agency. The State Land Department and Coastal Commission must also approve this project, and the three agencies and Poseidon recently signed an agreement on how to streamline the final approval process.

Poseidon has yet to make its case in a way that meets the new rules’ requirements for proving infeasibility, said Kurt Berchtold, the regional board’s executive officer. While Maloni said he hopes his project can get final decisions in a year, Berchtold said it’s “tough to say” how long that will take.

“It’s a very broad question we are evaluating. We haven’t reached any conclusions yet,” Berchtold said.

If the company insists on building open ocean intakes, the new state rules will require it to prove a need for the project water – an issue that’s been tossed back and forth in Orange County for years. Environmentalists question the need for the project due to declining water use and falloffs in future use projections, but the plant’s supporters, including the Orange County Water District, say it’s still needed to insure reliability of water supplies during an ongoing drought.

If this project is eventually killed, “it is a signal to any other developer trying to build a desal plant that it is not going to happen,” said Robert Sulnick, executive director of a group supporting the plant known as O.C. Wise. “It’s too much time, too much money, too much hassle.”

In a staff report last year, the water board laid out what it saw as the environmental costs of not switching to subsurface intakes. Studies have shown that some 19.4 billion fish larvae were entrained in 2000–05 with cooling water taken into existing California coastal power plants. During the same time period, approximately 2.7 million fish weighing 84,000lb (34,000kg) in total were impinged annually at coastal power plants, along with marine mammals and sea turtles, the report said.

At Huntington Beach and other future plants, power plants will be taking in the ocean water at first, although that practice will have to end over the next few years under a separate set of state rules.

In California, subsurface intakes are used in small desalination plants in Sand City near Monterey and on Catalina Island, where – in addition to Long Beach – small pilot projects are also taking place. The largest of its kind worldwide has operated in Fukuoka, Japan, for 10 years, taking in barely a quarter as much water as Poseidon’s plant would take.

In 2013 Poseidon sought final approval of the Huntington Beach plant from the Coastal Commission, but the commission ordered a delay for a team of independent scientists to evaluate the feasibility of subsurface intakes there. The panel concluded in reports published in 2014 and 2015 that only one of nine subsurface intake designs is technically feasible at that site, and that no subsurface intake design is economically feasible.

The panel examined the possibility of what’s known as a seafloor infiltration gallery, lying about 3,400ft (10,000m) offshore. To build one, the claylike substrate beneath the ocean would be removed and replaced with sandier soils that allow seawater to infiltrate more easily. Wells would be installed beneath the new substrate to pump out the water.

The panel considered it unlikely that a desalination plant using the subsurface intake would find anyone to buy the water at its current construction cost. Maloni noted that the report also concluded that it could take up to seven years to build the subsurface intakes, and that excavation for their construction could have major environmental impacts, removing a lot of seafloor sands and damaging that coastal area’s benthic environment.

Sean Bothwell, policy director of the environmental group California Coastkeeper Alliance, said the panel’s report contained numerous critical errors and omissions. Most importantly, the group said the panel’s standard, that a water agency be willing to pay the higher costs of subsurface intakes, isn’t required by the new desalination rules.

“A project proponent can’t ‘self-select’ what’s economically feasible,” the group said. The state’s 2015 desalination regulations make it clear that “subsurface intakes shall not be determined to be economically infeasible solely because subsurface intakes may be more expensive than surface intakes,” Coastkeeper said in written comments about the project.

But a recent report commissioned by Poseidon argued that underwater intakes won’t help fish much – if at all. Written by Scripps Institution of Oceanography scientist Daniel Cartamil, the report said the studies which have asserted that these intakes reduce fish kills have no data to back up their claims.

“There is no … scientific basis for the conclusion that a [seafloor infiltration gallery] would be environmentally superior at Huntington Beach, since they have never been operated at a scale” like the Huntington Beach plant, Cartamil wrote.

Coastal Commission scientist Tom Luster countered that the report ignored most of the findings and legal requirements which caused the state to prefer subsurface intakes. The main reason there aren’t more studies documenting their success “is probably because there’s not that much to study,” he said. “A properly designed subsurface intake causes no more than an imperceptible effect on marine life in the water column and the seafloor above it.”

If this plant is approved without subsurface intakes, the alternative required by the new state desalination rules – and what Poseidon plans to install – is a series of wire-mesh screens at the open water intakes whose slots are no more than 1mm (0.04in) wide. They’re barely wide enough to fit a credit card through, said Poseidon’s Maloni.

In his 2015 report, Scripps scientist Cartamil predicted that the impacts on fish of the open water intakes would be very small, citing a 2011 report from a Poseidon consultant which concluded that the diversity of larval fish and shellfish living off Huntington Beach is less than in nearby coastal areas. At most, 0.01–0.23 percent of all fish larvae could be entrained in the intakes, that report said, and Cartamil concluded that the threat to these fish is much greater from overfishing, ocean acidification and pollution than from desalination plants.

But Coastkeeper’s Bothwell was critical of the screens, noting that a 2013 study done for the State Water Resources Control Board had concluded that they would reduce entrainment of all organisms in seawater by less than 1 percent. If the state is going to give the screens credit toward meeting the desalination plant’s mitigation requirements, that credit should also be no greater than 1 percent, that study said.