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INITIAL RELEASE DDJC Tracy pesticides DDE, DDT, chlordane, and dieldrin have been much less frequently detected near comparison values. Though these contaminant concentrations exceed the very conservative comparison values used for screening, these concentrations are typical of background concentrations in soil for the vicinity. ATSDR determined that infrequent contact with these levels of contamination in this surface soil is not likely to result in an increased risk of adverse health effects for on-site workers or visitors. No puhlic health hazards are associated with exposure to contaminants in on-site soil for either children playing in the Day Care Center play yard or workers and visitors who may infrequently contact exposed surface soil. Inhalation of Soil Gas Exposure to soil gas becomes a concern when high levels of VOCs migrate via soil gas into basements through cracks in the foundation walls of buildings. Soil gas accumulates in the small spaces between soil particles when chemicals volatilize from contaminated soil or groundwater. Generally, VOCs in the upper layers of soils gradually diffuse to the surface through soil gas. Under certain circumstances, however, such as when a low-permeability layer in the unsaturated subsurface zone inhibits the upward diffusion of gas, horizontal spreading of soil gas may occur (EPA, 1992a). When this happens, contaminants may migrate with soil gas into a building's basement through foundation walls. As part of its RI activities, DDJC Tracy sampled soil gas to characterize the presence of VOCs in soil and groundwater. Depot structures (including those overlying the plume and most likely to accumulate vapors) and residential structures located downgradient of the site do not have basements. As a conservative measure, however, ATSDR reviewed data to determine whether soil gas could potentially contribute to the indoor air environment of on-site buildings. Of the VOCs measured in soil gas, PCE (0.004 to 950 ppb) and TCE (0.007 and 220 ppb) were the most frequently detected (Montgomery Watson, 1996b). Although the amount of TCE or PCE that may enter a building varies as a function of several properties (e.g., soil characteristics), EPA predicts that indoor air contaminant concentrations typically do not exceed 5% of the surrounding soil gas concentrations (EPA, 1992a). Applying this estimate to PCE and TCE concentrations measured in on-site soil gas, ATSDR predicts indoor air concentrations between 0.0002 and 47.5 ppb for PCE and 0.00035 and 11 ppb for TCE. These estimated ranges include values that are within or near ATSDR's comparison values for airborne PCE and TCE (40 ppb and 100 ppb, respectively). Therefore, ATSDR estimates that even if soil gas migrates into indoor environments, VOCs are not likely to accumulate to concentrations that may pose health hazards. 15