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Location of soil contamination

We now know that arsenic and lead from the smelter were carried by the wind throughout all of the areas in King County that were studied, including Federal Way north to the Snohomish border and east to Maple Valley, Issaquah, and Duvall. In general, there is less contamination the farther away one goes from the smelter.

The amount of Asarco arsenic and lead is in the soil in any area of King County depends on several factors, including distance from the smelter, topography, and history of the property in question. Here are some rules of thumb to figure out the chances that your property has contamination:

  • Distance: The most contaminated soils tend to be in coastal King County from Seattle to Federal Way and on Vashon-Maury Island.

  • Topography: Soil on hillsides that face southwest tended to get more of the contamination than east and north facing hillsides because of the way the wind traveled and carried the contamination.

  • History of the property: Arsenic and lead tends to stay in the top six inches of soil. If the soil on a property was dug up, moved, or otherwise "disturbed" over the last 100 years, there may be less contamination there than on a property that was undisturbed for the entire time the smelter operated (from the late 1800s to 1986).

For example, an old growth forest or long-standing park would likely be more contaminated than a newer development where the soil was leveled and/or filled in. An older neighborhood that has not significantly changed over the years may have more chances of contamination than a newer subdivision.

Residential soil testing

Tacoma Smelter contamination is unpredictable from property to property. There is tremendous variation from one property to another, and even across parts of a property. The only way to know for sure if your property is contaminated is to test the soil.

For more information, contact us at 206-477-DIRT (3478); email us at dirtalert@kingcounty.gov; or use our online form.

Maps

View soil studies, reports and maps.

Gardening

Since arsenic and lead in soils can be a health hazard, gardeners and others who work or play in contaminated soils should take precautions to limit and reduce the amount of soil they swallow or breathe.

The goal of these suggestions is to reduce the amount of contaminated soil that you unintentionally swallow or breathe in while gardening or working around your home. Follow these guidelines to reduce your exposure:

  • Avoid gardening in soils with arsenic in excess of 20 ppm (parts per million). Bring in clean soils and build a raised bed instead.
  • Add clean soils or soil supplements such as compost or mulch to your existing garden. Clean soils are ones that are known to be contaminant-free. If you are unclear whether your new soils are clean you may consider testing. For more information on composting and mulch visit the King County Soils and Composting page.
  • Wear gloves while gardening.
  • Wash all home grown vegetables carefully and peel vegetables where possible. Be sure to remove particles of soil on the food item. Wash inside crevices (e.g. broccoli and cauliflower).
  • Though there is evidence that vegetables and fruits may take up small amounts of arsenic into their roots or leaves, a more serious risk could come from eating fruits and vegetables that have bits of contaminated soils stuck to them.
  • Dampen soils with water before you garden to limit the amount of dust you inhale.
  • Consider wearing a mask if you spend time in dusty soils.
  • Review related posters and brochures.