SOUTH PORTLAND — Heavy equipment is at work in a section of the Mall Plaza’s parking lot. When the job is done, it won’t be obvious that a large stormwater treatment system is under the new pavement.

Arched chambers 3 feet below the surface will store stormwater runoff and release it at a slow pace. Ten treatment units filled with a sand-soil mix and wetland plants will help capture the pollutants that wash off pavement and other impervious surfaces.

The project at the shopping area at the corner of Maine Mall and Gorham roads is part of a larger plan to clean up the Long Creek watershed. Runoff from developments in the Maine Mall area has filled the creek with pollutants such as heavy metals, nutrients, chloride and hydrocarbons. It will take an estimated $14 million and 10 years to restore the watershed.

The U.S. Environmental Protection Agency is requiring more than 100 landowners to either participate in the effort run by the Long Creek Management District or clean up their properties on their own. The EPA intervened after it came under legal pressure from the Conservation Law Foundation.

Landowners in the collective effort will pay $3,000 per acre of impervious surface this year. They will pay less if they already have stormwater infrastructure such as detention and treatment systems.

With the landowners of more than 90 percent of the area’s impervious surface participating, the district is working with a $1.45 million budget.

The landowners’ assessments are funding the work under way at Mall Plaza to deal with runoff in front of Dick’s Sporting Goods and around Wendy’s and McDonald’s. A group from the National Association of Conservation Districts visited that site and others in the watershed Tuesday as part of a tour arranged by the management district.

An earlier phase of the Mall Plaza project — a large sand filter with a floor of about 30,000 square feet — was completed behind Dick’s and the Internal Revenue Service office last month.

The sand filter resembles an irregularly shaped basin divided into segments by low concrete block walls. Instead of running into Long Creek, stormwater will spread into the sections before soaking into an 18-inch layer of soil, sand and bark mulch and a 14-inch layer of stone product.

“It’s pretty simple in its components,” said Rob Woodman, an engineer with Deluca-Hoffman Associates.

Together, the two phases of the Mall Plaza project will treat stormwater from about 14 acres. They are expected to capture at least 1½ inches of runoff from a storm, said Tamara Lee Pinard, executive director of the management district.

Jeff Dennis, a biologist with the state Department of Environmental Protection, showed the tour group how large amounts of runoff carved a deep, wide channel on the main stem of Long Creek, near Darling and Philbrook avenues, making it inhospitable habitat.

“It goes from being really shallow like this to a torrent when it rains. That’s not a great place to live,” he said.

A zero-interest loan through the federal stimulus package paid for part of the Mall Plaza sand filter and smaller-scale projects on Darling and Philbrook avenues, where improvements around an office area included soil filters along a redesigned sidewalk and a landscape filter in the parking lot.

 

Staff Writer Ann S. Kim can be contacted at 791-6383 or at: [email protected]