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1 <br /> SECTIONF®UR Upesum bsessmeug <br /> groundwater data only, (4) groundwater screened within well mtervals, and (5) groundwater within <br /> well screens(<25 feet) All scenario results are included in Appendix A- <br /> 4 <br /> 4 4.3.2 Air Emissions from Soil Volatilization (Outdoor) <br /> Cherrucals may be emitted from soil into the air due to volatilization and wind erosion of soil <br />' particles These emissions are transported to receptors by wind advection and turbulence-driven <br /> dispersion Exposure of human receptors may then occur by inhalation of airborne contaminants <br />' The air dispersion models described in this section are from the ASTM RBCA model and simulate <br /> the airborne transport of vapor emissions and particulates from the source to the receptor <br />' Volatilization is the release mechanism by which a compound evaporates in the vapor phase to the <br /> atmosphere from the soil The ASTM RBCA approach models volatilization as the steady-state <br /> ratio of the concentration of an organic constituent in the ambient air breathing zone to the source <br />' concentration in the surface soil The surface volatilization factor incorporates two cross-media <br /> transfer elements 1) organic vapor flux from the surface soil mass to ground surface and 2) <br />' mixing of soil vapors in the ambient air breathing zone directly over the affected surface soil <br /> There are two calculation methods for estimating volatilization in the ASTM RBCA model The <br /> software defaults to the lesser of the two calculation methods as explained in ASTM RBCA <br /> (1995) For volatile organics, the ASTM RBCA model uses a mass balance approach with a <br />' constant emission rate over the duration of the exposure period (e g , 25-30 years) The surface <br /> volatilization factor for volatile organics is computed as follows <br />' Vfss[(mg/m3 —air)/(mg/kg—soil) - (Wpsd/,u5,r)xl03 <br /> where <br /> Vfss= surface volatilization factor [(mg chemical/m3-air)/(mg chemical/kg-soil)] <br /> W= width of source area parallel to wind or to groundwater flow direction(cm) <br /> Ps= soil bulk density (g-soil/cm3 soil) <br /> d= lower depth of surface soil zone (cm) <br /> U = wind speed above ground surface in ambient mixing zone (cm/s) <br /> 8 = ambient air mixing zone height (cm) <br /> z = average time for vapor flux(s) <br /> WoodwP'4W ® T%19971973009NAWrOCKTONREPORTISHOREPWI DOC -FEB-BB1973000NAMA 4-9 <br />