See figures: Click here for bar charts which show the distribution of arsenic and lead results (as percent of reported values occurring in defined concentration ranges). The highest concentrations are seen to occur relatively infrequently.
Relationship Between Arsenic and Lead
Arsenic and lead demonstrate a strong and statistically significant correlation. While the ratio of lead:arsenic varies from sample to sample, lead concentrations are about twice the corresponding arsenic level on average.
Spatial Pattern
The highest arsenic and lead concentrations occur on Maury Island and South Vashon Island, followed by the mainland (with a much lower sampling density in this study). North Vashon Island generally has lower concentrations, although all parts of the study area show some elevation above background levels.
The spatial pattern of soil contamination levels, as well as the strong relationship between observed arsenic and lead concentrations, are consistent with deposition from Ruston smelter emissions being the primary contributing source. This conclusion is also supported by other available information, including wind roses and precipitation chemistry studies performed by the University of Washington during the period of smelter operations.
Variability in Concentrations
The study design included collection and analysis of closely-spaced samples at 34 grid cell locations. (At 20 of those locations, samples were analyzed at both the 0-2 inch and 2-6 inch depths). The results for these "spatial scale" samples illustrate that arsenic and lead concentrations can vary several-fold (i.e., by several hundred percent) even over distances of as little as 50 feet. This result is observed even though all sampling in this study targeted the least-disturbed forested areas. Land use areas with greater degrees of soil disturbance may well show even greater variability. The observed variability in concentrations indicates that one soil sample, or a few soil samples, may not accurately represent contamination levels (either average or maximum concentrations). Arsenic and lead concentrations should be expected to vary significantly over even small distances. As a result, extrapolating from one sampling location to nearby unsampled areas is not recommended.
Depth Profiles
In this study, analyses were performed for two soil depths: 0 to 2 inches, and 2 to 6 inches. The analysis of depth profiles is therefore limited to near-surface soils. There is evidence from some earlier, more limited soil studies on Vashon-Maury Island that arsenic and lead contamination can extend below 6 inches; this becomes even more likely when there is a history of soil-disturbing activities. Absent significant soil disturbance, contamination is generally not expected to extend more than a foot or so below the ground surface.
The 2 to 6 inch soil samples were analyzed more frequently in areas of higher contamination (South Vashon Island, Maury Island, and the mainland).
Comparing the patterns of arsenic and lead concentrations over the two sampling depths, there is evidence that arsenic moves somewhat more easily than lead to lower soil depths. For both arsenic and lead, however, the 2 to 6 inch concentrations can exceed, and sometimes substantially exceed, the 0 to 2 inch concentrations. This occurs more frequently with arsenic than with lead. These depth profile results indicate that analyses of only 0 to 2 inch soils may not accurately reflect the degree of contamination that occurs at a property.
Consistency With Prior Studies
This study represents by far the most extensive soil testing to date for Vashon-Maury Island. More than a dozen previous, limited studies of soil contamination by arsenic (and in some cases other metals) on Vashon-Maury Island over the past 30 years are known. Differences in how samples were collected and analyzed exist between those previous studies and the current study. In some cases these differences in protocols may significantly affect the results. Nevertheless, the current study results are generally consistent with the cumulative information from previous soil studies and other studies of contamination patterns (e.g., precipitation chemistry studies). The current study supports the first comprehensive evaluation of soil contamination island-wide.
This study represents an initial "pilot" study of the mainland areas. Previous soil studies cumulatively provide only a very small number of results. Other types of investigations (e.g., precipitation chemistry studies) have provided some relevant information on possible soil contamination patterns. The results in this study confirm the existence of significantly elevated arsenic and lead concentrations at some mainland shoreline areas. The spatial extent of this contamination on the mainland has not yet been defined.
