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Technical Justification for Soil Screening Levels for Direct Contact and Outdoor Air Exposure Pathways (Final 03-15-2012) 1 EXECUTIVE SUMMARY Soil screening levels have been proposed to be used in conjunction with vapor intrusion criteria and groundwater criteria for identifying sites posing a low-threat to human health. That is, these soil screening levels are just one of three sets of criteria that should be evaluated to determine if a site is low-threat. The soil screening levels discussed in this document have been developed for benzene, ethylbenzene, naphthalene, and polyaromatic hydrocarbons (PAHs). The exposure pathways considered in the conceptual site model are: incidental ingestion of soil, dermal contact with soil, and inhalation of dust and volatile emissions from soil. Note these exposure pathways are assumed to occur simultaneously, i.e., the screening levels are protective of exposure from all four exposure pathways for each chemical. Standard U.S. EPA risk assessment equations were used to derive the screening levels. The exposure parameter values, chemical toxicity values, and chemical fate and transport properties are based on default values used in California. Risks posed by direct exposure to multiple chemicals with similar health effects are considered to be additive or cumulative. For example, the total risk posed by the presence of carcinogenic chemicals is the sum of the theoretical risk posed by each individual chemical. The same is true for chemicals that cause noncarcinogenic health effects. Use of these screening levels for single chemicals is limited to the extent that the screening levels remain protective of human health should other chemicals with similar health effects are present. Assuming all four chemicals are present at the same location and at concentrations at their respective screening levels, the estimated total risk is 4x10-6. For reference, the USEPA National Contingency Plan (NCP) is commonly cited as the basis for acceptable risks in risk management decisions. According to the NCP, an acceptable site-specific lifetime incremental cancer risk falls with the range of 1 in a million (1x10-6) to 100 in a million (1xle). Cancer risks below or within the range of 10-6 to le are generally considered protective of human health by the USEPA. The estimated total risk for the four chemicals considered in this document (4x10-6 ) falls within this range. Two sets of screening levels were developed for two soil horizons: one from 0 to 5 feet below ground surface (bgs) and one from 5 to 10 feet bgsi and three exposure scenarios (residential, 1 There are several definitions of what constitutes surface soil ("near" surface soil is typically the top 6 inches of soil) to quantify potential exposures in health risk assessments. Surface soil can be defined as soil to a depth of 2 feet below ground surface 1