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Toxics Cleanup Program

Areawide Soil Contamination Project

Toolbox Chapter 1

Area-wide soil contamination is low-to-moderate level soil contamination that is dispersed over large geographic areas, ranging in size from several hundred acres to many square miles.  These areas generally have arsenic and lead levels that are higher than both naturally occurring concentrations and state soil cleanup levels established under the Model Toxics Control Act (MTCA). Area-wide soil contamination is different from most cleanup sites, which are typically smaller and have higher levels of contamination.  Area-wide soil contamination was caused by a number of historical activities, including past air emissions from metal smelting operations, the use of lead-arsenate pesticides in the early to mid-1900s, and combustion of leaded gasoline.

What is “Low-to-Moderate” Level?

In general, arsenic concentrations of up to 100 parts per million (ppm) and lead concentrations of up to 500 - 700 ppm are “low-to-moderate” levels of contamination for schools, childcare centers, and residential land uses. For properties where exposure of children is less likely or less frequent, such as commercial properties, parks, and camps, arsenic concentrations of up to 200 ppm and lead concentrations of up to 700 - 1,000 ppm are within the low-to-moderate range.  For comparison, the unrestricted site use cleanup levels for arsenic and lead under MTCA are 20 ppm and 250 ppm, respectively.  Arsenic occurs naturally in Washington State soils at approximately 5 - 9 ppm and lead at 11 - 24 ppm.  Ecology plans to ask the Science Advisory Board to review these values and their use in implementing the Task Force recommendations. 

 What Concentrations Have Been Observed?

Concentrations of arsenic and lead within areas affected by area-wide soil contamination are highly variable and the range of concentrations is quite broad.  Arsenic concentrations range from natural background levels to over 3,000 ppm in smelter areas. Average concentrations of arsenic in soil at developed properties are generally less than 100 ppm.  Lead concentrations range from natural background levels, to over 4,000 ppm in orchard top soils.  Average concentrations of lead in soil at developed properties are generally less than 700 ppm.  The higher concentrations were observed in smelter areas and in areas where lead arsenate pesticides likely were mixed in preparation for application.  Soil concentrations tend to be greater around the Tacoma smelter than in the other smelter areas, because the Tacoma smelter operated for a longer period and specialized in the processing of high-arsenic ore. 

Where found, arsenic and lead soil contamination tends to be relatively shallow.  In undisturbed soils, most of the arsenic and essentially all of the lead from historical smelter emissions and historical use of lead-arsenate pesticides typically are concentrated in the upper 6 to 18 inches of soil.  While some downward movement of arsenic occurs in most soils, substantial downward movement has been observed on occasion and appears to be restricted to heavily leached sandy soils and medium-textured soils with very uniform soil profile characteristics.  There are a few anecdotal reports of elevated arsenic concentrations in shallow drainage water derived from heavily irrigated land containing lead arsenate pesticide residues; however, currently there does not appear to be evidence of ground water contamination. 

Where is Area-Wide Soil Contamination Likely to be Found?

Areas affected by smelter emissions in King, Pierce, Snohomish, and Stevens counties have a higher likelihood of arsenic and lead soil contamination than other areas of the State due to historical emissions from metal smelters located in Tacoma, Harbor Island, Everett, Northport, and Trail, BC.  Areas where apples and pears were historically grown have a higher likelihood of arsenic and lead soil contamination than other areas of the State because of past use of lead arsenate pesticides.  Chelan, Spokane, Yakima, and Okanogan counties have a higher likelihood than other counties for elevated levels of lead and arsenic in soil based on the greater numbers of apple and pear trees in production there between 1905 and 1947.  Combustion of leaded gasoline produces lead-enriched particulates and aerosols that are emitted from exhaust pipes and deposited onto nearby soils.  The full extent of area-wide soil contamination from past use of leaded gasoline in Washington is not known; however, in general, land adjacent to any road constructed prior to 1995 and land in the center of highly populated urban areas has some likelihood of elevated levels of lead in soil from leaded gasoline. 

For more information about whether area-wide soil contamination is likely to be present on a specific locality, see the maps and accompanying information in section 2 and the tools and guidance for individual property assessments in section 3 of this toolbox.

How Much Land is Potentially Affected?

Preliminary Estimates of Area-Wide Soil Contamination in Washington

Area-Wide Soil Contamination Source

Estimated Land Area Affected

Smelters (Tacoma, Everett, Harbor Island, Northport, and Trail, B.C.)

489,000 acres

Orchard Land

188,000 acres


Unknown at present

Total Area-Wide Sources

677,000 acres

Notes:  The extent of affected area has not been fully characterized.

         The total area of land in Washington is about 42.6 million acres.

What Are Other Sources of Arsenic and Lead Soil Contamination?

Other sources of arsenic contamination include wood treated with chromated copper arsenic (often called “pressure-treated” wood), emissions from coal-fired power plants and incinerators, and other industrial processes.  Other sources of lead contamination include lead-based paint, lead-soldered water pipes, home remedies or health-care products that contain lead, hobbies that use lead (e.g., staining glass or sculpturing), foods and beverages, combustion of coal or oil, waste incinerators, and mining and industrial processes (such as battery and ammunitions manufacturing).  Both arsenic and lead also occur naturally in the environment at varying concentrations.