SALEM — The Department of Transportation is using a University of New Hampshire design to clean stormwater as part of the Interstate 93 widening project.

The gravel wetland detention basin was designed by Robert Roseen of the UNH Stormwater Center. It is being used in places where a lot of sediment needs to be removed from stormwater, DOT's Jonathan Bradstreet said.

Bradstreet was the contract administrator for the Exit 2 park and ride/bus station in Salem.

The 442-space lot off Pelham Road was the first place one of these detention basins was built. It was completed last year.

Permits for the 20-mile I-93 widening project, between Salem and Manchester, require added environmental protection.

Stormwater from the interstate and off-ramp at Exit 1 in Salem is also cleansed by a gravel detention basin. The design works something like a fish-tank filter, I-93 project manager Pete Stamnas said.

Sand, other solids and pollutants drain into the basin and are directed across it and through crushed stone, cleansing the stormwater before it is directed back into the environment.

The gravel basins are expected to be used elsewhere along the 20-mile corridor, Stamnas said. They are also planned for other locations in the state including Portsmouth, Bill Arcieri said.

Arcieri, a stormwater specialist with Vanasse Hangen Brustlin Inc., reviewed stormwater treatment needs for the I-93 corridor.

Arcieri said Roseen's design is appropriate for areas where lots of sediment and other materials need to be removed from stormwater.

Roseen's design requires a larger area than a traditional detention pond, which merely collects runoff before it is released. The new design also requires more materials and construction is more involved, he said.

So, in places where less sediment needs to be removed, the more traditional detention pond is more cost effective.

Sediment typically builds up on impervious surfaces such as asphalt parking lots, then is washed off the surface during heavy rains.

Excess sediment and salt can wreak havoc on wetlands, streams and ponds, spurring the growth of invasive plant species and killing fish.

Bradstreet said the cleansing method has proven very effective at Exit 2, where water drains into Policy Brook.

"I've seen some really dirty water coming in and it has always been crystal clear coming out," he said.