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The Lower Duwamish Waterway

Seattle Soil Dioxin Study

Ecology recently completed a state-wide study where concentrations of certain chemicals were measured in soil collected from urban and rural areas. The overall goal of the study was to define the range of  concentrations of dioxins and furans (dioxins) and carcinogenic polycyclic aromatic hydrocarbons (cPAHs) found in the soil of urban and rural areas of Washington.

For the rural part of the study, Ecology collected soil samples from state parks throughout the state. Samples were collected to help Ecology understand what levels of these chemicals were present in areas not influenced by human activity. Samples were analyzed for dioxins, cPAHs and arsenic. Soil concentrations were similar to levels reported in previous studies conducted in Washington and other parts of the United States.  A link to the report is provided below.

For the urban part of the study, Ecology partnered with City of Seattle to collect soil samples in six Seattle neighborhoods. These neighborhoods were South Park, Georgetown, Ravenna, Capitol Hill, West Seattle and Ballard. In each neighborhood, Ecology and the City collected 20 samples from randomly selected planting strips. Samples were analyzed for dioxins and cPAHs.  A link to the report is provided below.

The range of dioxin and cPAH concentrations in the Seattle soil samples were similar to the range of concentrations measured in other cities.  In general, the average cPAH levels were somewhat lower than levels reported in other cities and average dioxin levels were somewhat higher than those found in other cities.

Why is this study important?

The Seattle neighborhood soil sampling data will help Ecology in several ways. Study results will be used to:

• Determine if more sampling is needed in one or more of the six Seattle neighborhoods and where.
• Design future studies in other Washington cities.  No two cities are completely alike. However, study results will help Ecology to design studies to provide context for decisions on soil cleanup actions in other areas.  
• Help identify ways to prevent sediment contamination in Seattle rivers and lakes.  For example, Ecology is currently exploring the relationships between contamination levels in soil, storm water and sediment.  These data will help Ecology better understand geographic patterns of contamination.

The rural sampling data will help Ecology define natural background levels for dioxins and cPAHs. Ecology evaluates natural background levels when making cleanup decisions under the MTCA rule. The rule specifies that soil cleanup levels shall not be established at levels below natural background levels. Under Ecology’s MTCA rule, “natural background” is defined as pollutant concentrations (e.g. dioxins) that have not been influenced by human activities

Project Documents

• Fact Sheet
• Español
• tiếng Việt
• 中文
• Urban Soil Dioxin Study Report  
• Rural Soil Dioxin Study Report
• Seattle Soil Dioxin Sampling and Analysis Plan
  • Figures 1-4
  • Figures 5-8
• South Park and Georgetown Neighborhood Sampling Fact sheet
• West and North Seattle Neighborhood Sampling Fact sheet