The numbers are so vast, so enticing that they tantalize like a desert oasis.

Deep below the surface in Arizona – roughly a quarter mile underground – sit large volumes of water that are less salty than the ocean, but not easily used. At a depth of 1,200 to 1,500 feet, between 530 million and 700 million acre-feet fill this layer statewide.

If it were all pumped to the surface and purified, this brackish groundwater would supply Arizona’s water needs for a century or more. Problem is, it can’t all be pumped.

Though the numbers are legitimate – and detailed in an updated state assessment that was published in August – the reality for brackish groundwater, at this point, is more of a mirage. Exploiting this resource to satisfy the state’s demand for water in an arid climate is not as simple as drilling wells.

“This is not a new supply of water,” said Juliet McKenna, a hydrogeologist with Montgomery & Associates, the consulting firm that the state contracted for the brackish groundwater assessment. “This is physically groundwater and this is legally groundwater. And there are consequences and restrictions in both areas for trying to use this.”

McKenna, who managed the assessment, and other state water experts interviewed for this story explained that brackish groundwater has a slew of impediments – environmental, physical, financial, technical, regulatory, and legal – that limit its use, despite the efforts of enthusiastic backers in the Arizona Legislature who are looking for ways to counter the state’s declining Colorado River supplies.

“Brackish groundwater is still groundwater, right?” echoed Patrick Adams, water policy adviser to Gov. Katie Hobbs. “So its extraction impacts the aquifer as much as any other groundwater supply when it’s removed from storage. And really that needs to be considered – and its use needs to be considered –against that backdrop. Where’s the brackish groundwater located? What are the local groundwater conditions? What’s the health of the aquifer?”

Securing a reliable water supply is an existential question for high-growth Arizona and its desert economy. The Colorado River, a major source for central Arizona, has sputtered in the last two decades amid hotter, drier weather attributed to a warming climate. The state’s allocation from the river was whittled by at least 18% in each of the last three years. New operating rules that are under negotiation will likely extend or deepen those cuts past 2026, when current guidelines expire.

Water, as a result, is prominent in state policy debates.

Drilling into Arizona’s Brackish Supplies

A desire for more data on its water sources is why the Legislature inserted $50,000 for an updated brackish groundwater inventory in the 2023 budget. The Arizona Department of Water Resources then commissioned Montgomery & Associates to do the analysis.

Arizona is not alone in its quest to better understand its subsurface water. New Mexico is looking to expand its water supply by treating both brackish groundwater and the high-salinity, chemical-laden water that gushes out of oil and gas wells. To the east, the Texas Water Development Board has investigated and mapped the state’s brackish groundwater zones for the last 15 years. A $1 billion water fund approved by voters last year will include at least $250 million for marine and brackish water desalination.

The Arizona inventory identified 21 areas with brackish groundwater, four of which the state singled out for more detailed assessment. One focus area is the Little Colorado River Plateau, in the state’s northeast corner. About half of the assessed brackish groundwater is located there. (The assessment defined brackish groundwater as having total dissolved solids greater than 1,000 parts per million. Sea water, by comparison, is 35,000 parts per million.)

The other areas – Gila Bend, Ranegras Plain, and West Salt River Valley – are closer to the population centers in Maricopa County or to the Central Arizona Project canal that moves water across the state.

“We wanted it to be meaningful or useful,” said Ryan Mitchell, chief hydrologist for the state’s Department of Water Resources, about selecting the focus areas.

The discussions around brackish groundwater are as much about its limitations as its possibilities. McKenna pointed out several challenges. One, water in storage does not equal available water. The same physical drawbacks from pumping fresh groundwater also apply to brackish. As groundwater is pumped, the land above can crack and sink, damaging houses, roads, and other public infrastructure. The water table can drop and cause neighboring wells to go dry. Those outcomes can occur with relatively modest levels of pumping, let alone with a massive drawdown to access all the deep brackish groundwater assessed in the inventory. In an arid region, water at that depth is essentially non-renewable.

“If we dewatered those aquifers to 1,500 feet below ground surface, that’s an apocalyptic scenario,” McKenna said. “So we’re not pumping groundwater to those depths under any reasonable scenario. So the estimate of water that is there, in aggregate, does not translate to water that’s available for folks to use.”

Water is already used unsustainably in the study’s four focus areas. Each is currently operating at a groundwater deficit, McKenna said. More water is pumped out than is recharged.

Steep Challenges Remain in Using Brackish Water

Even if brackish groundwater is physically available, it is not necessarily desirable. Buckeye, one of the state’s fastest growing cities, sits within the Buckeye Waterlogged Area, located on the western outskirts of the Phoenix metro area. “Waterlogged” is a regulatory definition based on the area’s unique hydrogeology at the junction of three rivers: the Agua Fria, Gila, and Salt. Water pools here, and farmers have to pump it out so that their crops will grow. Due to salts in irrigation return flows, the water is brackish in places near the surface.

Buckeye, which pumps groundwater for its municipal supply, is surrounded by brackish groundwater, but Terry Lowe, the water resources director, says the city avoids it. For Buckeye, brackish groundwater is “not deployable,” as he puts it. Some of the Buckeye Waterlogged Area groundwater is between 3,000 and 4,000 parts per million of total dissolved solids, and the equipment and energy required to remove the salts is not cheap. “Treating that out is a waste of money,” he said.

What’s more, brackish groundwater has complications that involve waste disposal. Treating brackish groundwater produces a concentrated brine that must be handled delicately and expensively. Small quantities might be handled by a wastewater treatment plant. Large volumes are typically injected deep underground, but in Arizona that method is “effectively prohibited” without policy changes, a governor’s water council determined in 2022. The Arizona Department of Environmental Quality, the permitting agency for aquifer protection, said that no Class I deep injection wells operate in the state. Carollo, an engineering firm, concluded that cheaper brine disposal was essential for brackish groundwater to become an “economically viable water supply” in the state. Lowe also cited brine management as a reason his department shies away from brackish groundwater.

Then there are the legal and regulatory hurdles. The Legislature passed the Groundwater Management Act in 1980 in response to unsustainable use. It established Active Management Areas (AMA) to steward a finite resource. In practice, most users in the six AMAs need permission to pump and must replace a portion of their use. In the Phoenix AMA, which roughly corresponds with Maricopa County but also extends into neighboring Pinal, the goal is “safe yield” by 2025 – balancing groundwater extraction with recharge. It is not on track to meet that deadline. Incentivizing brackish groundwater use could put safe yield farther out of reach.

And one more headwind: Arizona restricts the movement of groundwater within the state. Five groundwater basins are designated as “transport” basins. Water in these areas can be pumped and exported to an AMA. Most other groundwater must be used in its basin of origin. Without a change in legal status, brackish groundwater would be stranded in place, able to be used locally but not moved to the areas of highest demand.

“For us it’s still considered groundwater,” Mitchell said. “It’s still regulated the same, it’s still accounted for and tracked and all the authorities are still in place, whether it’s brackish or fresh, it’s still treated the same.”

The Search for Water

To state Rep. Alexander Kolodin, these hurdles – physical, financial, regulatory – are obstacles that can be overcome. Kolodin, a Republican who represents northeastern Maricopa County, is the most enthusiastic booster of brackish groundwater in the Legislature. He sees the big number in the updated inventory and grows excited.

“Arizona is sitting on an absolute ocean of brackish groundwater,” he said. With the state’s take from the Colorado River declining, Kolodin wants to consider other sources of water to fill the gap. “I’m very interested in figuring out how we can tweak the law to utilize this resource’s maximum potential.”

Those tweaks at the state level, he said, would include reducing groundwater replenishment requirements in the AMAs for brackish water and relaxing the restrictions on moving groundwater out of its natural basin. “If you can’t transport it, you never really have much incentive to do it in rural areas because it’s still much more costly than our historical sources of water,” he said.

Kolodin advocated for $11 million in the state budget last year for a brackish groundwater pilot program. The Department of Water Resources published a request for information in October 2023. The pilot didn’t go much farther than that. Mitchell, who reviewed the submissions, said they read more like “qualifications packages” than a careful project plan. Due to a state budget shortfall this year, funding for the pilot was retracted.

Brackish groundwater boosters like Kolodin note the efforts in Texas, where the state government mapped its brackish reserves, estimated yields, required impacts analysis, and provided financing. El Paso has the country’s largest inland desalination facility, which has a production capacity of 27.5 million gallons a day. Mitchell, however, points out that the comparison is not one-to-one. Arizona has different hydrogeology, as well as more stringent legal and regulatory requirements.

The hunt for new water supplies is a longstanding feature of Arizona politics, extending back to the pursuit of the Central Arizona Project canal in the mid-20th century. In recent years, the prospectors have sought to turn salty water fresh.

A decade ago, under Gov. Jan Brewer, the state produced the Arizona’s Next Century report, which listed brackish groundwater as one of seven potential sources to augment the state’s supply.

Water augmentation was a major focus of Gov. Doug Ducey’s administration. In 2015, Ducey signed an executive order to establish the Governor’s Water Augmentation Council. In 2019, he signed another executive order that expanded the work to “investigate long-term water augmentation strategies for the state.” The Governor’s Water Augmentation, Innovation, and Conservation Council lasted until Gov. Hobbs was elected. In 2023, Hobbs formed the Governor’s Water Policy Council.

The Hobbs administration is less focused on brackish groundwater than her predecessors. The Governor’s Water Policy Council report, published earlier this year, does not mention it by name.

“Brackish groundwater development as a source for augmentation is not really at the forefront of where the Water Policy Council is focusing its efforts,” Adams, the governor’s water policy adviser, said.

For now, as more data is collected, brackish groundwater will remain just off center stage, with lingering questions about how and when it should be used.

“If it were to be utilized, it needs to be done so thoughtfully and mitigate impacts from pumping,” McKenna said. “It’ll be expensive, in terms of treating and permitting. But it is a supply that’s in our state, and like our other water supplies, I think we need to think about it and make thoughtful decisions about how to use it, if we want to use it.”