PORTLAND — The tide of stormwater fees flowing to the city since Jan. 1 has exceeded expectations.

While the $2.9 million collected is almost $356,000 more than budget projections through June 30, another $377,000 is overdue for the first six months of the program, city Assistant Finance Director Suzanne Knight said in a July 14 email.

The monthly fee of $6 per 1,200 square feet of property was instituted Jan. 1 as a way to help fund improvements to the city stormwater and wastewater collection systems. The fees are assessed to all private and public property owners in the city and on Peaks Island for properties larger than 400 square feet.

Municipal roads, railroad tracks, runways at the Portland International Jetport, and other Casco Bay islands are exempt.

Anne Bilodeau of the Finance Department said 21,000 property owners have been billed, with 18,900 receiving quarterly statements, 1,400 billed monthly, and 700 billed annually.

Compliance rates for monthly payment accounts began at nearly 95 percent in January, then declined throughout the first half of the year. In June, almost 85 percent of owners had paid invoices on time.

The increase in outstanding monthly bills means that revenue peaked at almost $336,000 in January; June revenues were just under $293,000. The rate for quarterly accounts was 87 percent in the first quarter of the year, and slipped to 77 percent in the second quarter, with revenues falling from $564,000 to $502,000.

More than 86 percent of accounts billed annually were paid by June 30, generating almost $6,500 in revenue.

Revenue from the fees pays for projects to meet a state mandate that the city reduce the flow of solid waste and wastewater into Casco Bay. The order from the Maine Department Environmental Protection is 25 years old, and city officials have projected about $170 million of improvements still need to be done by 2030 to meet the consent agreement terms.

After study by a task force, which reported to the City Council in 2012, the stormwater fee was determined to be the fairest way to help pay for projects. Included are the separation of stormwater and wastewater mains and making improvements to the Portland Water District wastewater treatment plant near East End Beach.

The task force included city staff and businesses such as Oakhurst Dairy.

Since 1993, overflow volumes at the treatment plant have been reduced by more than 42 percent, and plans call for reducing overflows by storing 15 million more gallons of stormwater and wastewater while more mains are separated.

Prior projects had been funded solely through sewer fees, meaning property owners not using city sewer services avoided sharing the financial burden of the upgrades. Property owners can also apply for credits based on measures taken to reduce the flow of stormwater or the amount of impervious surface on their land.

When the stormwater fee was enacted, sewer rates were reduced from the budgeted $9.70 per hundred cubic feet to $8.20 per hundred cubic feet.

The reduction in sewer rates was designed to generally keep the new stormwater fee from increasing overall fees to homeowners, Ian Houseal said as the stormwater fee was still being discussed in 2014.

Houseal is now the Housing Safety Program coordinator, but took the lead in implementing the stormwater fee councilors approved last year.

Both the sewer and stormwater fees are projected to increase annually, with rates reaching $11.46 and $8.20, respectively, by 2019.

David Harry can be reached at 781-3661 ext. 110 or dharry@theforecaster.net. Follow him on Twitter: @DavidHarry8.

Fernald Street in Portland, seen July 18, was resurfaced after work to add stormwater lines flowing to Back Cove.