Formatted Title:

Assessing Background Arsenic in Portland Harbor Sediment and Riverbank Soil

Background/Objectives:

Many inorganic materials for which cleanup levels are set to protect human health and the environment occur naturally in soil and sediments in varying concentrations. It is therefore critical to determine a reliable representation of the background concentrations of these materials in sites that are subject to remediation. Portland Harbor has been a Superfund site since 2000 and is subject to remedial design across multiple project areas along a 10-mile stretch of the Willamette River. Arsenic is one of the contaminants of concern in sediment and riverbank soil, as well as other media. A large proportion of sediment and soil data in the project areas exceed the clean-up level of 3 mg/kg set for the Site. This situation impacts remedial design, source control evaluations, and assessments of recontamination, and will make it difficult to find backfill that will meet this criterion. The goal of this study is to use multiple methods to establish a representative background arsenic concentration for Portland Harbor sediment and soil.

Approach/Activities:

EPA typically defines background constituents as those that are not influenced by the releases from a site and can be categorized as either naturally occurring or anthropogenic. Concentrations of naturally occurring arsenic in sediment and soil are influenced by the weathering of the volcanic bedrock in the region, and some of the pyroclastic and volcaniclastic deposits in the Willamette River watershed can be particularly enriched in arsenic. Upstream mining and the historical use of arsenic-bearing pesticides/herbicides and chromated copper arsenate-treated lumber can contribute to anthropogenic background arsenic. We assess available soil and sediment data from the region representative of natural and anthropogenic arsenic concentrations and compare these values to samples from one of the Portland Harbor project areas. We use several lines of evidence to assess an arsenic dataset representative of background conditions, including comparison to previous estimates of background arsenic, rank order curves and element-element scatter plots.

Results/Lessons Learned:

All the methods that we used demonstrated that background arsenic in soil and sediment in the region exceed the 3 mg/kg clean-up level. The multiple lines of evidence evaluated do not convene to a single value of background arsenic but consistently support a narrow range. In regions where inorganic materials are present due to naturally occurring and anthropogenic sources, a robust evaluation of existing conditions should be considered prior to remedy design to ensure the goals of cleanup are achievable.

Debbie Silva

Affiliation

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(Group 1, Poster Board #55) Assessing Background Arsenic in Portland Harbor Sediment and Riverbank Soil