Among the many environmental issues facing Portland, storm water runoff into Casco Bay and other waterways is one of the most significant. A newly approved city plan to solve the problem is worthwhile, but local officials have to be careful they don’t create serious resistance that could further delay or even derail it.

The problem is that rain and snow don’t penetrate streets, parking lots, rooftops and other solid surfaces.

Precipitation falling there flows into sewers that are primarily designed to handle wastewater, causing them to exceed the ability of the city’s sewage treatment facilities to handle it. Relief valves prevent the system from backing up by diverting the overflow water away from treatment.

Instead, the sewage-laden storm runoff flows directly into Back Cove, Casco Bay and other waters, posing a threat to marine life — from clams to mussels to lobsters to fish — and potentially polluting what should be a pristine scenic and biological resource.

So, constructing a system to handle that runoff has been a federal, state and local priority for many decades. The city promised the federal Environmental Protection Agency in 1993 that it would deal with the problem.

Yet the sheer size of the project has been daunting to officials at all levels.

Now, the City Council has decided to be daunted no longer. It voted this week to move up the city’s deadline for completing the project from 25 years to 15, while adopting an alternative to the original plan that would reduce its overall cost from an estimated $500 million to $170 million.

The new plan, instead of creating a second parallel system of storm sewers, would hold the runoff in storage conduits until the treatment plants can catch up. That’s a good idea.

But it has a very unpleasant consequence: It would raise city sewage rates by 300 percent, from an average of $400 a year now to about $1,300 once the full cost of the bonds kicks in about eight years from now.

Here is where councilors should exhibit both caution and compassion.

Caution, because they don’t want to create a groundswell of resistance to the plan; and compassion, because hard-pressed homeowners and businesses in the city shouldn’t be asked to bear a financial burden that would create real hardship for many of them.

A separate committee is exploring financing options that could lessen the impact of the project. One idea would have the cost factored into property taxes, which are tax-deductible, where sewer fees are not. Another would be to figure a property’s “impermeable surfaces,” such as parking lots, into its rates, instead of merely using overall water use. Unless something creative is done, the higher fees could become a major issue in future council races — with unknown consequences both for councilors and for the plan itself.