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Stockton 99 Speedway Preliminary Endangerment Assessment Workplan April 12,2007 3.7.2.3 Leaching The potential for chemicals to leach from soil depends on their physical and chemical properties, their concentration, soil type, pH (for metals), and other Site-specific conditions. For example, chemicals with high water solubilities tend to leach more readily than chemicals with lower solubilities. In addition, a chemical's K,,c is important for assessing the degree of chemical sorption to soil particles; chemicals with a high sorption potential do not tend to leach as readily (i.e., metals). Site-specific conditions are also important for assessing whether leaching may occur, such as soil type (leaching occurs more readily in sandy soils than in clayey or silty soils), amount of rainfall, gradient, etc. In addition, other competing migration pathways can affect the tendency of a chemical to leach. Because the metals are expected to sorb strongly to soil particles, and because VOCs are expected to migrate upwards and volatilize, leaching is not expected to occur at the Site to any significant extent. In addition, groundwater is suspected to occur at a depth of 45 to 65 feet bgs; therefore, leaching to groundwater is unlikely to occur. 3.9.2.4 Stormwater Runoff Stormwater runoff from areas of contaminated soil has the potential to transport organochlorine pesticides, metals, VOCs, and petroleum hydrocarbons. The predominant mechanism for transport of organochlorine pesticides, metals, VOCs, and petroleum hydrocarbons by stormwater runoff is through suspension of silt or clay particles that have adsorbed organochlorine pesticides, metals, VOCs, and petroleum hydrocarbons. Dissolution of these compounds in surface runoff is not viewed as significant due to the relatively low solubilities of these compounds. Surface runoff will carry the organochlorine pesticides, metals, VOCs, and petroleum hydrocarbons adsorbed sediments until the velocity of the flow decreases to a point where the suspended particles will settle out. The greatest potential for impact caused by stormwater runoff is the retention basin located on the Site. Surface water runoff is expected to collect in this area; therefore, the Retention Basin Area has been identified as an AOC. 3.1.3 Potentially Exposed Receptors The third component necessary for an exposure pathway to be complete, potential receptors at the Site, is discussed in this section. Hypothetical human and ecological receptors identified for evaluation in this assessment were identified on the basis of proximity to the Site, proposed activities that could possibly result in direct or indirect contact with Site-related contaminants, and Site use. The Site is currently zoned mixed use (commercial/industrial) and may be rezoned for residential use. Potential ecological receptors may include terrestrial biota, which will be identified in an ecological survey as part of the PFA process. Ecological receptors that are identified during the ecological survey will be evaluated and incorporated in the CSM. The preliminary CSM identifies the following hypothetical human receptors: • Hypothetical Onsite Indoor Commercial/Industrial Worker Receptor; • Hypothetical Onsite Outdoor Construction Worker Receptor; and PFA Workplai.Final.doc 3-3 The Source Group,Inc.