5. Nature and Extent of Area-Wide Soil Contamination

The Task Force considered what is known and not known about the location and magnitude of elevated levels of arsenic and lead in soil from historical smelter emissions, use of pesticides containing arsenic and lead, and combustion of leaded gasoline. Much of the Task Force's deliberations focused on how to communicate this information in a way that would present information accurately without causing undue alarm. As discussed below, the Task Force decided that a tiered series of maps, along with accompanying information and tools, should be used to communicate information on area-wide soil contamination in a balanced and useful way. The Task Force also recommends updating the maps regularly to improve their precision and developing local maps of area-wide soil contamination where such maps do not exist (primarily for areas affected by lead arsenate pesticides). Recommendations for additional research on contamination from combustion of leaded gasoline are discussed in Section 11.

The Task Force's findings and recommendations in this section are organized according to three questions the Task Force considered:

• What is currently known about the nature and extent of arsenic and lead soil contamination in Washington State?
  
• How should information on the nature and extent of area-wide soil contamination be communicated?
  
• What steps should be taken to improve our understanding of the nature and extent of arsenic and lead soil contamination?

What is Known About the Nature and Extent of Area-Wide Arsenic and Lead Soil Contamination

	What is Area-Wide Soil Contamination? Area-wide soil contamination is low- to moderate-level contamination that is dispersed over a large geographic area, ranging in size from several hundred acres to many square miles.

Elevated levels of arsenic and lead are present in soil in some areas of Washington State from three historical sources: air emissions from metal smelters, lead arsenate pesticides, and combustion of leaded gasoline. In areas affected by off-site deposition of smelter emissions and areas where lead arsenate pesticides were applied to crops, concentrations of arsenic and lead in soil generally are higher than concentrations that occur naturally in Washington soils and higher than State soil cleanup levels established under MTCA. However, concentrations generally are lower than those found at smelter operation sites and at sites where lead arsenate pesticides were mixed in preparation for application. Low-to-moderate arsenic and lead soil contamination associated with areas affected by off-site deposition of smelter emissions, lead arsenate pesticide application, and combustion of leaded gasoline is referred to as "area-wide soil contamination" to distinguish it from the higher concentrations and smaller geographic extent of contamination at more traditional cleanup sites.

What Are Other Sources of Arsenic and Lead Contamination? Other sources of arsenic contamination include wood treated with chromated copper arsenate (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., stained 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.

The precise boundaries of land affected by area-wide soil contamination are not known; however, certain places have a higher likelihood of arsenic and lead soil contamination based on the locations of metal smelters or the probable use of lead arsenate pesticides from approximately 1905 to 1947. To support Task Force deliberations, the contractor project team conducted a detailed study of available data on the nature and extent of area-wide soil contamination. Based on this study, 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, Everett, Northport, and Trail, BC, and on Harbor Island (in Seattle). 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. Table 1, later in this section, describes the number of acres potentially affected by area-wide arsenic and lead soil contamination based on information currently available.

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According to the study prepared to support Task Force deliberations, the range of concentrations of arsenic and lead in soil associated with area-wide soil contamination is quite broad. Total arsenic concentrations range from natural background levels (7-9 mg/kg statewide) to over 3,000 mg/kg in smelter areas. Average concentrations of total arsenic in soil at developed properties with area-wide soil contamination generally are less than 100 mg/kg. Total lead concentrations range from natural background levels (11-24 mg/kg statewide) to over 4,000 mg/kg in orchard top soils (higher concentrations are likely areas where pesticides were mixed prior to application). Average concentrations of total lead in soil at developed properties with area-wide soil contamination generally are less than 700 mg/kg. By comparison, the MTCA soil cleanup levels for unrestricted land use for total arsenic and total lead are 20 mg/kg and 250 mg/kg, respectively. 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 soil³. While some downward movement of arsenic occurs in most soils, substantial downward movement has been detected on occasion and appears to be restricted to heavily leached sandy- to medium-textured soils with very uniform soil profile characteristics⁴. Currently there does not appear to be evidence of ground water contamination associated with area-wide soil contamination. The long-term consequences of the very slow downward movement of arsenic in soil require further evaluation.

Concentrations of arsenic and lead at properties affected by area-wide soil contamination are highly variable and depend on the historical use and development of the property. For example, during development of a property, surface soils are often mixed with underlying soils and redistributed; this disturbance tends to dilute the concentrations of arsenic and lead in soil and distribute them in unpredictable patterns. Contaminant concentrations on one property cannot reliably be used to predict concentrations on neighboring properties.

Information on the nature and extent of arsenic and lead soil contamination provided the basis for Task Force deliberations on what actions should be taken to respond to area-wide soil contamination in important ways. For example, the knowledge that most added arsenic and almost all added lead remains in surface and near-surface soils, coupled with lack of evidence for ground water contamination, suggests that ground water contamination is not likely an issue for properties with area-wide soil contamination. Similarly, the understanding that arsenic and lead contamination tends to be highest in undisturbed soils, with other considerations, led to the Task Force's recommendations on additional steps that should be taken when converting open land into developed properties.

³ Landau Associates, Preliminary Estimates Report, Area-Wide Soil Contamination Strategy, Washington State, prepared for the Washington State Department of Ecology, Olympia, WA, 2003 (pending).

⁴ Peryea, F.J., and T.L. Creger, "Vertical Distribution of Lead and Arsenic in Lead Arsenate-Contaminated Soils." Water, Air and Soil Pollution 78 (1994): 297-306.

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