our of the area’s largest suburbs have a $178 million disagreement with their water supplier. And Wednesday they took the “unprecedented” step of asking the state to intervene.

Garland, Mesquite, Plano and Richardson filed a petition with the Public Utility Commission seeking a review of their water rates. The cities buy water from the North Texas Municipal Water District, which has raised rates by nearly 70 percent since 2012. More increases are expected.

The cities say the existing "outdated" rate system places a burden on large, established cities compared to smaller, fast-growing ones.

"The rates set by the water district under the six-decade-old water supply contract are discriminatory, are inconsistent with water conservation and are not in the public interest," said Bruce Glasscock, Plano's city manager.

The cities described the current "take-or-pay" plan as unsustainable. The formula requires member cities to pay for as much water as they used in their record year.

Over the past 15 years, these four cities estimated they have paid $178 million for water they didn't use. In the 2014-15 fiscal year, Plano paid for 8.7 billion gallons of water it didn't use.

Representatives of these cities said those overpayments will continue into the foreseeable future without outside intervention.

Tom Kula, the district's executive director, said in a written statement he was disappointed with the cities' action this week.

"For the past year, the North Texas Municipal Water District (NTMWD) has been holding meetings with its 13 member cities to discuss potential alternatives for allocating the regional water system costs," he wrote. "We recognize the concerns of the four cities (Garland, Mesquite, Plano and Richardson), but the district must also consider the positions of the other nine..."

District officials have previously argued that that this system allows the district to pay for expansion as members' overall need grows in the coming decades. The district is working toward construction of Bois d'Arc Creek Reservoir in East Texas.

Also, district officials have pointed out that many of their expenses don't automatically decrease when water usage decreases. They said this isn't about paying for unused water but paying for a share of the system.

The district's statement this week said officials there have not "given up seeking a potential solution" and that this should be settled by the member cities. The district's board of directors and member cities would have to unanimously agree to alter the contract and its formula.

The 13 cities agreed in March to meet with consultants to try to find a solution. In the 2014-15 fiscal year, all member cities paid for water they didn't use.

City officials at Wednesday's news conference said an in-house solution didn't seem likely. Although the four coalition cities account for 750,000 ratepayers or 60 percent of the district's total, each city had just one vote.

Water exited a treatment basin in 2014 at the North Texas Municipal Water District in Wylie. ((Andy Jacobsohn / The Dallas Morning News))

The take-or-pay debate moved back on the district's agenda during the most recent drought, which led to permanent water conservation efforts. Those efforts combined with plentiful rain in 2015 and 2016 have severely reduced water use.

"The recent drought changed the way we used water," said Garland Mayor Douglas Athas. "And successful water conservation has changed the game. ... We were watering once every two weeks across all our cities. And the reward for our customers who worked so hard to conserve water was we raised their rates."

The combination of slower growth for those four cities and improved water efficiency means they are are unlikely to hit their maximum water volumes again, city officials said. Those peaks occurred between 2001 and 2006.

Meanwhile, some of the other member cities are growing so quickly that they often set new water usage records. Forney, Frisco, McKinney, Royse City and Wylie have all been on pace to set new water usage marks during the 2016-17 fiscal year.

The other members cities are Allen, Farmersville, Princeton and Rockwall.

Plano Mayor Harry LaRosiliere said the system's imbalance has led to some non-member, customer cities paying lower rates than some member cities.

“Plano has been voicing concerns about the rate methodology for over a decade,” he said. “This is unacceptable because we can no longer subsidize a system where we are paying for water that we’re not using.”