Ten Mile Creek runs clear and cold near Clarksburg Town Center in northwest Montgomery County. The region’s “last best stream,” in the words of one conservationist, it flows into Little Seneca Lake, which serves as part of the Washington area’s emergency water supply.

It’s not mentioned anywhere in a new bill and zoning amendment sponsored by Council member Craig Rice (D-Upcounty) to encourage the use of permeable surfaces in new development. But TenMile figured prominently nonetheless in Tuesday’s hearing on the proposals. This summer the County Planning Board is weighing changes to the Clarksburg Master Plan that will determine how much development to allow in the Ten Mile Creek Watershed. Two major retail projects are in planning stages, and and Rice’s measures could have a direct bearing on what is ultimately built.

Stormwater management regulations currently require developers to minimize the use of impervious surfaces, which allow rainwater to pick up pollutants as it flows into streams and rivers. In environmentally sensitive areas, special water quality plans are required if impervious surfaces exceed 8 percent of a development.

The current zoning code does not distinguish between permeable and impervious surfaces. Rice’s bill would give developers a 25 percent credit for use of permeable surfaces when calculating whether limits on impervious surfaces are satisfied.

Rice said the county already encourages the use of permeable pavement — which is designed to absorb runoff — by homeowners and that it makes sense to apply the emerging technology to commercial development as well.

But the proposals got pushback Tuesday from the environmentalists and the county planning staff, which expressed opposition. Planners said there are adequate credits in stormwater management regulations for developers who use permeable surfaces. They said changes to the zoning code would only result in larger scale construction in sensitive areas such as Ten Mile Creek.

“The unintended consequence would be greater development footprints,” said Planning Board Chair Francoise Carrier in her appearance before the council Tuesday.

“The fact remains that permeable or impermeable, pavement is still pavement,” said Ginny Barnes, vice-chair of Conservation Montgomery. “Sensitive watersheds are not the place to offer credit for using it. Such use is counter to the intent of establishing impervious limits.”

Diane Cameron, representing the Audubon Naturalist Society, said Rice’s bills are aimed at “helping an industry, makers and installers of pervious pavement products.” Cameron acknowledged that her group supported the county in promoting green stormwater practices that include the use of permeable surfaces, but she said this was a different matter because it endangered fragile watersheds.

Rice offered no comments during the hearing. He said afterward that he has been working on the bill for two years and that it has nothing to do with trying to facilitate development in Clarksburg. He said it is difficult to for environmentalists to argue that permeable surfaces are acceptable in residential settings but not in commercial projects.

“You can’t cut it both ways,” said Rice. “It doesn’t make sense.”