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First Flush

5 young coho in a jar
Juvenile coho are impacted by the high pollutant load in the first flush of stormwater after a dry spell.

The first rainstorm after a long summer of dry weather is usually welcomed. But the first rain also delivers an unwelcome summer’s worth of pollution to nearby streams and water bodies. After a dry spell, our roads, highways, sidewalks, driveways, and other hard surfaces accumulate sediment, heavy metals, oils, pesticides, fertilizers and pollutants from various sources on the landscape. All of this unseen or unnoticed material is transported by the rain into ditches, stormdrains, and nearby water bodies. This potential tidal wave of toxins is sometimes referred to as “first flush.”

First flush can be bad news for a variety of aquatic species, especially Coho salmon who are especially sensitive to contaminants. They are returning to their native streams to spawn just when the first flush often occurs. Exposure to polluted stormwater can cause harm to adult and juvenile salmon. They can experience behavioral and physiological changes including loss of smell, inability to avoid predation, changes to blood chemistry, all of which often end in early death. As salmon populations decline there’s an impact up the food web, as orcas and other marine mammals are deprived of a food source.

Stormdrain that says dump no pollutants.The good news is that businesses, municipalities, and homeowners can all help to reduce the potential harm of a first flush.
Many businesses and municipalities have stormwater management responsibilities specified under an Industrial or Municipal Stormwater General Permit issued by the Washington Department of Ecology. The Industrial permit requires that affected businesses take a sample of stormwater discharge from the first annual fall storm event. Legally, that means the first time on or after September 1st that precipitation heavy enough to result in stormwater discharge occurs. (Note: the date was moved up a month from October 1.) In addition to sampling water from the first storm, businesses must also take a sample of the stormwater that leaves their property every quarter, if measurable precipitation occurs. If pollutant levels exceed set standards, the businesses must take measures to mitigate the risk. This can be costly. Preventing pollution at the source, before it gets into stormwater runoff is much less expensive. It also benefits the environment to reduce the potential impact of first flush before the first fall storm arrives. That way, organizations can stay in compliance with their permits’ requirements.

When the weather forecast calls for the first imminent storm of the season, it’s time for businesses to get busy. Some of the actions they can take, which are listed in the permit, include:

  • Cleaning paved surfaces with a vacuum sweeper made specifically for this purpose.
  • Keeping dumpsters closed unless adding or removing material.
  • Inspecting the facility for leaks and spills – and cleaning up immediately.
  • Making sure all scrap or product metal is kept under cover.
  • Cleaning out all catch basins and having the storm drain lines inspected and cleaned.
  • Putting filters in catch basins.

It can be challenging to navigate through all the Stormwater permit requirements, which can differ by industry and location. The Washington Stormwater Center can help, offering resources and training for municipalities and businesses on permit compliance.

It’s important for homeowners to prepare for the first flush, too. Here are a few suggestions:

  • Sweep the driveway and sidewalk and be sure to clean up any spilled fertilizer.
  • Fix oil leaks and clean up after any spills.
  • Pick up after pets regularly.
  • If you harvest rain from your roof, divert water from the first rainstorm away from the rain barrel to avoid collecting all the accumulated dirt, bird feces, and other contaminants in your storage container.
  • Consider building a rain garden.  A rain garden acts like a miniature native forest by collecting, absorbing, and filtering stormwater runoff from roof tops, driveways, patios, and other areas that don’t allow water to soak in.

This fall, keep an eye on the weather forecast, and get ready to do your part in reducing the effects of first flush.