Portland recently took another step toward enacting a proposal to share the cost of treating the runoff from rooftops, driveways and parking lots in the city. And although it’s unrealistic to expect citizens to embrace an idea that would increase their bills, Portland’s stormwater fee proposal presents a worthy trade-off: an average $54 annual charge for most property owners, in return for lower sewer rates and the knowledge that they’re helping to keep local waterways clean.

The stormwater fee is needed to help fund $170 million in federally mandated upgrades to a system that now sends untreated sewage and storm runoff into Casco Bay during heavy downpours. Last week, a city task force presented its fee proposal to the City Council’s Finance Committee, and before year’s end, there could be a full council vote.

If the stormwater charge is approved, it would be based on the total square footage of rooftops, driveways and parking lots for each property, calculated from aerial photos. The fee would be partly offset by a lower city sewer rate, leading to an estimated $4.50 monthly increase in the average city homeowner’s water-sewer bill.

Without a stormwater fee, the city has two options for funding improvements. The first – increasing property taxes – would exclude large nonprofits like churches and hospitals. The second – raising sewer fees – would undercharge owners of property with extensive parking lots or other hard surfaces that shed rainwater into waterways after it’s had a chance to pick up oil, gas, antifreeze and litter.

The stormwater fee would hit big commercial and nonprofit property owners the hardest. The city government itself faces an estimated $365,000 in fees during the first year; Unum would pay $90,000; and Maine Medical Center’s fees would likely top $30,000.

But Maine Med, Unum or anyone else who wants to cut their stormwater fee can get credits by installing rain barrels or rain gardens. Rain barrels collect the runoff from rooftops, allowing it to be used for outdoor purposes and saving most homeowners about 1,300 gallons of water a summer, according to the U.S. Environmental Protection Agency. And research has shown that rain gardens, which use soil and plants to filter rainwater, can remove up to 90 percent of nutrients and chemicals from runoff before it flows into waterways.

The waters of Casco Bay are a picture postcard on a summer’s day. To help ensure that the bay is as lovely up close as it is from a distance, the city should pass this thoughtful, innovative stormwater fee proposal.