KĪHEI — When South Maui is under water shortage restrictions, Rob Weltman resorts to collecting leftover shower water in pails and saving it for his most treasured plants.

Weltman never used to see water shortage declarations when he first moved to Kīhei more than two decades ago. But during the past few years, it’s become commonplace. And just last year, the island experienced its driest year on record. 

That’s why Weltman is supportive of Maui County’s plans to look into desalination, the artificial process that turns underground stores of brackish or saltwater into a potable supply.

“I personally am very much in favor of investing in technology so we’re ahead of the game, so we can do this in the future, because I don’t see any other alternative,” Weltman told the Hawai‘i Journalism Initiative.

But he and his neighbors still have a lot of questions about a project that is still in the very early stages of planning, and could cost anywhere from $119 million to $328 million depending on capacity.

“A path to desalination is a road trip, and at this stage, I would say we’re kind of still loading up the car,” said James Jensen, chief engineer with the Maui County Department of Water Supply. “Not knowing when a facility would actually be built, but part of this is to make sure we’re laying that groundwork.”

The expected costs are enormous. A brackish water desalination facility with a capacity of 0.5 million gallons per day and potential to expand to 2 million gallons per day could cost a total of $119 million, including $53 million just to build it. Other costs would come from engineering and permitting fees as well as an environmental assessment and mitigation measures, according to a 2024 feasibility study by the county. 

A saltwater desalination facility with capacity of 4 million gallons per day could cost $328 million, including $155 million just for construction. But even at top capacity, it’s still less than 10% of the expected islandwide water demand in the next decade.

Eva Blumenstein, planning program administrator for the Department of Water Supply, said that even with improved conservation and other solutions, desalination will probably be needed “at some point in the future.”

“Peak supply is getting dangerously close to capacity,” she said.

Maui’s population is projected to grow by 32% between 2015 and 2035, but water demand is projected to grow by 47% from 33.5 to 48.5 million gallons per day during that time, according to the Maui Island Water Use and Development Plan. South and West Maui are expected to see the biggest increases in demand in the coming years.

Maui relies primarily on groundwater and surface water, and the plan says the island “has more water than it needs to meet future demand.” Estimates put its existing water supply at 550 million gallons per day of groundwater, surface water and recycled water during normal time and 384 million gallons per day during drought constraints.

But much of that water is in remote areas of East and West Maui, “making it difficult and expensive to access.” And, East Maui is protected by a consent decree that requires any development of groundwater sources in the Ha‘ikū aquifer to undergo rigorous studies. 

Desalination of brackish well water or ocean water could serve as another sustainable source along with recycled water, rainwater catchment and stormwater, the plan said.

The concept isn’t new to Maui County, which first started talking about converting brackish water to use in Central Maui in 2006, but the idea never gained steam, Blumenstein said. 

In 2017, when the process of creating the Water Use and Development Plan kicked off, desalination was discussed as a strategy, but it “didn’t have much community support at the time, so it was kind of put on the backburner,” Blumenstein said.

It wasn’t until the South Maui Community Plan update process that the idea of desalination took on new life as the feasibility study was suggested and eventually completed in 2024. 

Across the state, it’s becoming more of a reality as the $204 million Kalaeloa Desalco facility that would produce 1.7 mgd is currently in the works on O‘ahu.

Earlier this month, the county held meetings in Kīhei and Lahaina, which are both home to brackish and saltwater aquifers that could serve as potential sources for desalination. Both areas have struggled with limited freshwater supply in recent years and faced shortage declarations at times.

In South Maui, most of the urban growth areas are in drier watersheds where there’s not enough groundwater to meet the demand, so water has to be imported from other areas. Blumenstein said that the ‘Īao aquifer in Central Maui has been “the workhorse that’s been supplying at least half the demand for a very long time” to South Maui.

“As we see climate changes, drier, longer droughts, we need to reduce pressure on ‘Īao aquifer,” Blumenstein said. 

The department’s Central-South Maui water system is expected to need another 6-10 million gallons per day by 2040. Capacity was at 26.8 mgd in 2025, with an average production of 23.9 mgd. By 2040, demand could rise to 37.1 mgd with no conservation measures or 29.5 mgd with progressive conservation.

In West Maui, historic low streamflows have led to a less reliable supply of surface water, and overall sustainable yield — the amount of water that can be reasonably taken without depleting the natural supply — is expected to decline.  

The department’s West Maui water system is projected to need another 3 to 4 million gallons per day by 2040. The system’s capacity was nearly 4.7 mgd in 2025, with an average production of nearly 3.8 mgd. By 2040, demand could reach as high as 8.7 mgd with no conservation measures or 6.8 mgd with progressive conservation.

Lahaina’s water permits also have been languishing in a stalled approval process since it was designated as a groundwater and surface management area by the state Commission on Water Resource Management in 2022.

Both areas are potential sites for desalination, with two brackish sources in Kīhei, one brackish source in Wailea, two saltwater sources in Honokōwai and two brackish or saltwater locations in Honolua identified in the county’s study. 

Solar is the recommended power source for both brackish and saltwater desalination plants.

While the study mostly focused on underground aquifers, it also found three potential options for ocean intake — Kahului Harbor, Mā‘alaea Harbor and north Lahaina in the vicinity of Hanaka‘ō‘ō Park.

Blumenstein said the county’s consultant, Honolulu engineering consultant firm Brown and Caldwell, looked at more than a dozen conventional and new technologies that could be used to convert the water with the primary goal of serving Central, South and West Maui. 

Brown and Caldwell’s Anita Selinger, the project manager and engineer for the desalination study, explained that the current plan is to use reverse osmosis to desalinate groundwater. It would be pumped up from the ground, go through treatment and screening, and then undergo reverse osmosis, which would result in a mix of 50% freshwater and 50% brine.

The concentrated saltwater would then be injected into deep wells along with the chemicals used to treat the water, such as antiscalants and coagulants that she said are “all certified for potable water use.”

Selinger said that a hydrologist would do a study to make sure wherever the saltwater is injected is safe, meaning “below the freshwater lens, below the brackish, below the saltwater, so it’s really deep.”

“We’re looking at more than a 50-year path to the ocean, and making sure that where we inject it is safe,” she said.

The state water commission would have to be part of the approval process for the saltwater injection, Selinger said.

Because desalination is still down the line, Blumenstein said cost estimates and the technology could change by the time the facilities are actually built. Potential funding could come from state or federal agencies. 

“There may be technologies that are more cost-effective and other uses instead of disposing of the brine,” Blumenstein said. “That’s what we’re really hoping for. … But that’s why we’re going really slow, making sure that we get the community’s input, and doing our due diligence to make sure that this is best for our community.” 

Brine disposal and energy costs are “the two biggest issues that need to be resolved” and are part of the reasons the idea didn’t get a lot of community backing in the past, she added.

Weltman says those are the top issues he wants to see addressed if the county moves forward with desalination. When it comes to energy, he’s hopeful that the growing cost-effectiveness of solar will help curb expenses in the future. But on the question of brine disposal, he worries about what will happen if the concentrated residue makes its way to the ocean. 

He wants to see that studied more carefully to make sure that whatever reaches the ocean won’t impact coral reefs or beachgoers, and he hopes that better technology and disposal methods will emerge once the county finally does turn to desalination. 

Ultimately, with declining rainfall and the growing demand for water, he sees the need.

“We’re doing a much, much better job at saving water in showers and irrigation than we were 20 years ago,” he said. “We’re still using more water, so we need to find another source, and I think that saltwater is really the only huge source we have. So I’m for the technology. I’m for investing in it.”

Kīhei resident Kimberly White said she’s also glad conversations are happening now about desalination but is concerned that the idea is still in the early stages two decades after it was first brought up. 

“We’re still not where we should be,” White said. “Even if we start today, 10 years from now, what are we going to do? So that’s what concerns me, and then you’re talking about families that want to live here. They’re going to need water.”

Overall, White thinks “it’s actually a great idea to look at desalination,” but like Weltman, she also wants to understand more about what will happen to the leftover brine.

Maui County Council Member Tom Cook, who holds the South Maui residency seat, told the Hawai‘i Journalism Initiative that desalination is “prohibitively expensive and not necessarily the easiest or best route.” However, he thinks it’s good for the water department to evaluate all options, especially if technology improves to the point that desalination becomes less expensive. 

“Investigating it is prudent, and I support it,” he said.

In the meantime, Cook said, he wants the county to keep pursuing solutions it can tackle in the present, such as shifting hotels to recycled water systems for irrigation and reducing their use of potable water. He said some of the South Maui hotels and Upcountry ranchers are interested in partnering on a system that could revegetate the mountain with recycled water and deliver it down the hill to the resort area.

In the last budget, his office included $500,000 for a preliminary report on expanding recycled water use south, and he plans to hold a town hall in September to update the community.

“I want to get much more aggressive in utilizing the R1 (recycled) water and freeing up the potable water,” he said. “And the hotels are a very obvious place to do it. … The infrastructure is expensive, but pays for itself, and it’s the type of things that a responsible government takes care of.”

Desalination is just one of the options the county is considering to address ongoing water shortages, along with recaptured stormwater, recycled wastewater and additional wells, Blumenstein said.

Maui County is also on the verge of acquiring Maui Land & Pineapple’s water system in West Maui that will put nearly all of West Maui’s drinking water under public ownership and add seven wells, two proposed well sites and a proposed reservoir site to the county’s supply.

Water conservation could also help slow demand. Earlier this year, the Maui County Council passed a bill that imposed new water conservation measures for all times, not just shortages. However, there’s no way to predict how successful the new law will be, and in the meantime, Maui will continue to grow and need more housing.

“We shouldn’t sit around and wait,” Blumenstein said. “We do the planning now. We’re not investing $300 million tomorrow. We’re just setting the stage so we’re ready when and if we need it.”