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commercial/industrial, and a utility trench worker) were considered. This document describes the technical background for the development of the soil screening levels. 2 INTRODUCTION The equations used to develop the soil screening levels are identical to the equations used to derive the USEPA's Regional Screening Levels (RSLs; USEPA 2011). Exposure parameter values were assumed to equal the default values used in California Department of Toxic Substances (DTSC) Office of Human and Ecological Risk (HERO) "Human Health Risk Assessment (HHRA) Note Number 1" (DTSC 2011). The soil screening levels presented in this document are sufficiently protective because the assumptions used to calculate the values are based on conservative assumptions and exposures. The volatilization factor used in the RSLs was replaced with volatilization factors obtained from the American Society of Testing Material's (ASTM's) Standard Guide for Risk-Based Corrective Action Applied at Petroleum Release Sites (ASTM 1995). The ASTM volatilization factors used to calculate concentrations in outdoor air consider mass balance. The volatilization algorithm used in the RSLs can overestimate the amount of contaminant volatilizing into outdoor air (Cal/EPA, 2005). In the ASTM volatilization algorithm 2, if the calculated volatilization rate depletes the source before the end of the exposure duration, the volatilization rate is adjusted so that the total source mass is assumed to volatilize by the end of the exposure duration. By using this mass-balance check, it is ensured that the total amount volatilized does not exceed the total amount of contaminant in soil. For incidental ingestion of soil, dermal contact with soil, and inhalation of dust, the concentration in soil is assumed to be constant at the screening level for the entire exposure duration. This assumption is conservative for volatile chemicals or chemicals that are expected to biodegrade in soil, such as benzene and ethylbenzene. 2.1 Screening Levels vs. Risk The soil screening levels represent concentrations, below which, indicate the site is a low-threat risk for human health. Multiple conservative assumptions were made when developing these soil screening levels. Actual site risk is expected to be lower than the risk targets used to develop the screening levels. For example, a residential receptor is assumed to come into contact with soil at concentrations or as indicated in the supporting documentation for the CHHSLs and ESLs, a depth of approximately 10 feet is generally used to delineate between shallow soil, where a potential exists for regular direct contact, and deep soil where only periodic exposure is considered likely. 2 The ASTM VF differs from the approach used by USEPA in the development of the RSLs. The VF used to calculate the RSLs assumes infinite sources over a large source area for the dispersion term. The ASTM VF algorithm is considered more appropriate for leaking underground fuel tank (LUFT) sites. Further details on the differences are presented in the Draft California LUFT Guidance Manual, version 2.0, 2010). 2