Site investigations that anticipate soil screening within a risk-based corrective action (RBCA) program often require an understanding of naturally occurring or ambient soil conditions. Because most RBCA programs offer very limited, if any, pre-published values for naturally occurring metals (e.g., arsenic and lead), it is imperative that the risk assessment process be informed by defensible screening-level background threshold values (BTVs). In the absence of representative BTVs, conservative screening levels from the RBCA process may incorrectly “screen in” ambient conditions as a release (false positive or Type I error) when none has occurred. False positives add unnecessary cost and confusion to subsequent investigation or remediation decisions. This article demonstrates an effective approach to developing and evaluating soil data sets for BTV development during any stage of the risk-assessment process. Whereas this article focuses on the most common metals associated with Type I errors (e.g., lead and arsenic), the process to estimate BTVs works for any organic or inorganic contaminants when sufficient data is available. The approach outlined in this article is intentionally conservative in nature to both increase regulatory acceptance and simplify the statistical steps needed for BTV estimation at earlier stages of the risk-assessment process.

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