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RISC Manual Version 4 0 <br /> To match the adult-only case of the ASTM standard or other software, assign <br /> adult parameters to the child case and make sure that the total exposure <br /> duration equals 30 years For example, assign an exposure duration of 24 <br /> years for the adult and 6 years for the"child" <br /> $ <br /> � •v �,^ �t'S.. A r�`lr'S � 3� i`i.�u-�`-y #� s =.,. ���R'W-,-� ^Fs� °`"t-1� .'�'t<�"°�`��� <br /> et-;T - <br /> FI2 GREEN=AMiT MODELS:-X <br /> In Tier 1, the vadose zone leaching algorithm assumes that the vadose zone source <br /> extends to the water table and assumes that there is no degradation or dispersion <br /> occurring in the vadose zone The Green-Ampt model can be used as a model to <br /> predict the effects of degradation in the vadose zone The Domenico model <br /> (described in the next section) is used to account for both dispersion and degradation <br /> in the saturated zone <br /> The Green-Ampt equation (1911) is used to calculate a minimal travel time for a <br /> wetting front to move through the vadose zone (from the source to the water table) <br /> The travel time is then used to estimate a vadose zone attenuation coefficient <br /> considering degradation processes This algonthm is used by both the U S States of <br /> South Carolina and Ohio to estimate risk-based screening levels under their <br /> Underground Storage Tank divisions <br /> The Green-Ampt model assumes that water infiltrates through the vadose zone soil as <br /> a sharp wetting front The volumetric water content above the wetting front is <br /> assumed to be completely saturated (equal to the total porosity) Since saturated <br /> conditions produce the highest permeability in the vadose zone (permeability <br /> decreases as soil moisture content decreases) this equation estimates the "fastest <br /> possible travel time" Therefore it is conservative when used to calculate degradation <br /> losses <br /> Once the pore water velocity is calculated, the retarded velocity for each chemical is <br /> calculated The chemical-specific travel time from the middle of the source to the <br /> groundwater is used to estimate degradation losses on the way to the groundwater <br /> The SSTL (site-specific target level) sheet of the Tier 1 spreadsheet contains the new <br /> risk-based screening levels incorporating the Green-Ampt algorithm Two different <br /> H-4 <br />