Far from the shores of Puget Sound lurks an under-the-radar environmental legal case that holds the potential to significantly set back efforts to protect the Sound from its largest source of toxic pollution.

Closely watched by builders and environmentalists, the case focuses on what happens when rainwater drains off roofs, streets, parking lots and other hard surfaces: It streams into nearby waterways carrying pesticides, fertilizer, oil, transmission fluid, dog poop and innumerable other residues of modern urban life.

The polluted water’s result: Poisoned oyster beds. Dangerous flooding.And the biggest  source of toxic pollution for Puget Sound. Even a threat to drinking-water supplies in Bellingham. 

In the case just ruled on by the Washington Pollution Control Hearings Board, environmental activists are challenging how the state Ecology Department is allowing Clark County, population 432,000 in southwest Washington, to comply with recently beefed-up rules to protect waterways from stormwater.

One aspect of the enormously complicated case is vesting. Clark County and the Building Industry Association of Clark County contend stormwater-control rules adopted by the county in April of 2009 don’t apply to developments approved earlier. The county and the builders contend that stormwater-control regulations are subject to vesting — that is, that they lock in as soon as the builder drops off development plans at city hall.

But, ruling from its offices in Tumwater, the pollution board last week rejected that notion, saying Clark County first dragged its feet in adopting new stormwater rules and then “unlawfully exempted” developers whose building plans were filed during an eight-month period.

While the number of developments that secured building rights in one county in that eight-month period may not seem significant, the pollution board’s ruling is far-reaching, because it says Ecology made a mistake to allow vesting to apply to stormwater regulations in the first place, said Jan Hasselman, the Earthjustice attorney representing environmentalists in the case.

“It’s going to be very difficult in the (future) for Ecology to sweepingly exempt all vested projects,” Hasselman said.

However, Clark County or the developers — or Ecology — could appeal the pollution board’s decision to Superior Court.

James Howsley, the Vancouver attorney representing the building association, said, “We’re not giving up our firm belief that vesting does apply.” For the courts to hold otherwise, he said, “is not only impractical, but it contravenes the state Legislature and local jurisdictions who have tried to throw a lifeline to the building industry by extending vesting.”

For environmentalists trying to protect Puget Sound, the case holds the promise of controlling stormwater pollution in a matter of decades instead of half a century or more.

Getting stormwater under control, they say, means changing building patterns so that developers use “low-impact development” techniques designed to slurp up stormwater before it can carry pollutants into nearby waterways. So far, these building methods have found mixed acceptance among developers and local governments. The pollution control board, however, has said the biggest cities must start to require developers to use these techniques.

Developers, for their part, counter that the majority of the stormwater problem stems from areas built up over many decades — not the relative handful of buildings being constructed now.

“If you want to make it more difficult to develop, tell me that’s your goal,” said Scott Hildebrand, policy director of the Master Builders Association of King and Snohomish Counties. “And if that is your goal, don’t be upset when housing prices are through the stratosphere.”

Opposing that view is Hasselman, the environmentalists’ attorney.

 “Washington state is falling behind the rest of the country in its ability to protect water quality from development,” Hasselman said. “We continue to build projects today using standards that we’ve known for years are harmful for salmon and that degrade water quality.

“If we care about recovering Puget Sound, we have got to get this sorted out.”

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