THE PENINSULA’S BIGGEST source of water pollution?

Storm water.

When rain falls on forests and meadows, trees and plants take up moisture. Roots slow the flow, allowing water to seep down and replenish streams and aquifers as lakes, ponds and marshes slowly fill.

Nature employs strategies from forest duff to soil microbes to store, use and reuse fresh water before it reaches the oceans.

Although Earth is a water planet, only 3 percent of its water is fresh; less than 1 percent is available to use.

Given wide areas, water slowly soaks into the ground — which works splendidly until lots of water falls on areas covered with impervious surfaces like highways and houses, driveways and parking lots.

Washing over asphalt, cement, packed dirt, fertilized fields and lawns, storm water picks up a nasty assortment of metals, pesticides, oil, grease, chemicals from plastics and vehicles and pet poop.

Half an inch of rain on a typical Walmart parking lot generates some 250,000 gallons of ick-laden storm water.

Across Jefferson and Clallam counties, rain follows countless paths, sometimes slowing and meandering to allow Nature to cleanse it before it flows into the ocean.

Pollution is so high that our beloved orcas, at the top of the food chain, become toxic waste when a carcass washes ashore.

In cities like Port Angeles, street drains funnel storm water into the city’s sewer system and ultimately to the water treatment plant.

That’s OK — until a big storm.

Then, storm water and sewage exceed the treatment plant’s capacity and everything overflows straight into the Strait of Juan de Fuca.

In Portland, Ore., its innovative “Green Streets” program transformed its storm sewer system into an eco-tourist attraction.

“When we started this 10 or 12 years ago, there was a lot of skepticism,” Dean Marriott, director of Portland’s Bureau of Environmental Services, told USA Today.

“Today, many cities are moving in this direction. People want to see how it’s done.”

Homeowners disconnected their gutter downspouts from the storm-water system — the cheapest way to improve storm-water quality — so water flows from rooftops to gardens and rain barrels.

Portland’s rooftop rain gardens cover 24 acres, capturing about 80 percent of its rainfall.

Savvy North Olympic Peninsula gardeners, like Kristina Lawrence, owner of Out On a Limb Landscape Services, already harvest water from their roofs.

Her Port Angeles home will be part of the June 26 Master Gardeners tour.

A quarter-inch of rain on a typical roof fills a 55-gallon barrel.

Want to disconnect your downspouts and use the free — and chemical-free — rain from your roof?

Portland offers a nifty online guide at http://tinyurl.com/cwmae7.

Yard drains and sump pumps may also be disconnected from sewer systems.

Homeowners can also reduce storm water by using permeable pavement for patios, driveways and walkways.

Permeable paving locks pollutants in the soil while allowing water seepage to recharge ground water. Such built-in stormwater management usually costs less than impervious pavement plus other storm ­water systems.

Managing storm water at its source, often called low impact development, uses a site’s natural features and rain gardens, swales and strategic plantings.

Many communities are examining LID guidelines for new development and retrofitting what’s already there, motivated by more than a simple desire to provide clean water.

The Environmental Protection Agency is starting to issue specific stormwater quality standards.

Clallam County’s new Stormwater Work Group is meeting on the second and fourth Thursdays in April and May.

Interested people are welcome to attend this week’s meeting, Thursday from 6 p.m. to 8 p.m. at Peninsula College, 1502 E. Lauridsen Blvd. in Port Angeles.

Bonus: This meeting includes a tour of the rain garden on the campus.

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Diana Somerville, an author and science writer, lives in Clallam County and can be contacted via www.DianaSomerville.com.

Her column on sustainability and the environment on the North Olympic Peninsula appears every other Tuesday.