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the concentrations measured in samples from the three monitoring <br /> wells. For benzene and toluene, these averages are 0.26 mg/L and <br /> 0.025 mg/L, respectively. Multiplying by 0.0214, receptor con- <br /> centrations are found to be 0.006 mg/L benzene and 0.0005 mg/L <br /> for toluene. The benzene concentration exceeds the Primary <br /> Drinking Water Standard (PDWS) , while the toluene level is far <br /> below the standard. <br /> Attenuation mechanisms---biodegradation, sorption, and volatiliza- <br /> tion---will further reduce the contamination levels predicted in <br /> the receptor well . However, these effects were not included in <br /> the model, for lack of adequate data. <br /> 3. 3 EXPOSURE ASSESSMENT <br /> 3.3.1 Soils Ex osures <br /> Humans become exposed to contaminants in soils through a number <br /> of potential pathways, including incidental soil ingestion, inha- <br /> lation of soil particles, and derma contact. These direct ex- <br /> posure routes are associated with surficial soils (approximately <br /> the top 5 to 12 inches) . Unless the soil structure becomes dis- <br /> turbed, there are no direct exposure routes to deeper ^oils. <br /> Generally, humans become exposed to deeper soil contamination <br /> indirectly, through transport in the vapor phase and in aqueous <br /> solution. <br /> i <br /> At the Stockton site most of the contaminated soils were exca- <br /> vated and piled onto the soil surface. This probably increased <br /> the exposure of surrounding communities to soil particulates and <br /> to benzene, toluene, and xylene for a short period. Wind would <br /> have eroded some particles which would then have been inhalable. <br /> The current and future exposures to benzene, toluene, and xylene <br /> are not expected to be significant, because there are no detect- <br /> able concentrations left in the soils (Table 3; Bane and Taggart <br /> 1987) . <br /> 21 <br />