The country's mayors are convening in Washington, D.C., this week to discuss their growing frustration with federal orders to clean up their sewer systems -- sometimes at a cost of billions of dollars.

It's a situation that Indianapolis Mayor Greg Ballard says is one of the most pressing challenges facing cities. "It's a huge issue," he says. "It may not be for the general public, but it's a national issue."

In the 1970s, cities got sizable federal grants to clean up wastewater systems that were polluting lakes and rivers. That money has largely dried up, but the U.S. Environmental Protection Agency -- through a tool called "consent decrees" -- can still mandate cleanups. And mayors say it's happening more frequently. "The mayors are frustrated with the goal line being moved on compliance," Ballard tells Governing.

Hundreds of cities are trying to follow federal environmental regulations to clean up their wastewater systems, Ballard says. While mayors emphasize that they don't oppose clean water, they also say that at a time when they're still struggling with decreased revenue, it's a cost that puts a strain on their budgets. Meanwhile, they say, passing the costs onto customers through utility rate hikes disproportionately affects low-income residents. (There's also a political motive: Those rate hikes -- like taxes -- are viewed unfavorably by voters.)

As Governing wrote earlier this year:

In recent years, cities across the country have increasingly been saddled by "consent decrees" from the EPA that order them to overhaul their waste water systems, often at substantial cost. In Omaha, Neb., the price tag is $1.7 billion. In Kansas City, Mo., it’s $2.5 billion, and in St. Louis, Mo., it’s $4.7 billion.

Ballard says new EPA guidelines may encourage regional offices to be more flexible in their approach to ordering cities to improve their systems. The EPA has said it may reduce the cost of compliance for cities.

"EPA has worked closely with many local governments to identify innovative and cost-effective solutions to these issues, including additional use of green infrastructure, that acknowledges the budgetary challenges they face while ensuring that every community's water is clean and safe," the agency said in a statement earlier this year.

EPA officials are scheduled to attend the mayors' meeting Friday.