A city task force is proposing a fee program that addresses the root cause of sewer overflow pollution. The idea will be unveiled Monday and deserves serious consideration by the public.

The problem is that during heavy rains, Portland’s sewer system overflows, and untreated sewage winds up in Casco Bay. This contaminates the water, promotes the growth of bacteria that can make swimmers sick and violates the federal Clean Water Act. The city has been put on notice that it has to treat storm water runoff, a project expected to cost $170 million.

The challenge has been how to pay for it without overburdening homeowners and other taxpayers, or by increasing sewer bills to unreasonable levels.

The task force proposes charging a fee to the owners of properties where impervious surfaces, such as parking lots and large roofs, create lots of runoff.

The approach makes a lot of sense. It is not residential property owners who make the sewers overflow, but they are the ones paying most of the sewer bills. Large buildings surrounded by large paved parking lots may not use much water (which is how sewer bills are determined) but they produce the runoff that overloads the sewers and pollutes the bay during heavy rains. It makes sense that they should pay their fair share of the cost.

This approach has another benefit: It encourages good behavior by property owners. They can avoid fees assessed for impervious surfaces by putting in plants, rain barrels or collection ponds. That also reduces demand on the sewer system, making it function better.

The plan comes at an awkward time. The city is struggling to come up with an explanation of why it failed to bill Shipyard Brewery for 15 years of sewer fees. That is the subject of an outside review by an attorney, but those questions should not stop the city from moving forward with a plan to fairly distribute the costs of the overflow project.

Given the options, a fee for impervious surfaces makes a lot of sense.