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SECTIONFOUR M(POSUrsASS®SSMOUR <br /> For low-volatility organics the model assumes there is an infinite source of organics in the surface <br />' soils and uses a volatilization rate based primarily on chemical properties The equation for low- <br /> volatility organics is described as <br /> VFss= (2wps 1 Uanrbair) Dse#H/nr(Bws+k"+HOhs)x103 <br /> where <br /> Dseff=Effective diffusivity in vadose zone soil (function of water content, porosity, and <br /> chemical Henry's constant) <br /> ms's= Volumetric water content in the vadose zone (cm3-H20/cm3-soil) <br />' ks = soil water sorption coefficient(g-H20/g-soil) <br /> H= Henry's law constant (cm3-H20)/(cm3air) <br />' Oas= Volumetric air content in vadose zone soil (cm3-atr/cm3-soul) <br /> The dilution of organics in ambient air above the source zone due to mixing with ambient air <br /> moving across the site is calculated using the box model The box model was used to estimate the <br /> steady-state average concentration of the chemicals at the site within a well-mixed cubic volume <br /> of air The model assumes that the volume of air is bound at the top of the mixing zone, and is <br /> ventilated by a steady flow of wind across the box Chemical mass enters the box through steady- <br /> state emissions from the soil, and leaves the box by advection Based on these assumptions, the <br /> steady-state average concentration in the box is computed as follows <br />' Cair = Q 103 /(mWH) <br />' where <br /> Carr = chemical concentration in air(mg/m) <br />' Q = average volatile chemical emission rate for the exposure period (g/s) <br /> in = mean annual wind speed (m/s) <br />' W = width of the box perpendicular to the predominant wind direction(m) <br /> H — height of the mixing zone (m) <br />' 103 — conversion factor(g to mg) <br /> The box model should only be used to estimate the average concentration within a volume of air <br /> and does not account for the decrease in concentration with distance or height from the source <br /> Box models have been found to be the most applicable for evaluating exposure to receptors <br /> working or living in the vicinity of the contaminated area, and typically result in very conservative <br /> Woodww1 <br /> a-M� T11 ae7w79008NA1s rocKmmr=PoRT%sF oREPlt+DocefaPwanoogNAw4A 4-10 <br />