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Vadose Zone Model <br /> 1 <br /> ' Degradation is considered in the dissolved phase for the region between the <br /> source and the water table <br /> The vadose zone model is automatically linked with the dissolved phase model <br /> in RISC so that concentrations in groundwater may be estimated <br /> The model accounts for the presence of NAPL by limiting the dissolved phase <br /> ' concentration in the source region to the chemicals'effective solubility limit <br /> This is important for two reasons <br /> 1 Otherwise the leachate and groundwater concentrations may be greatly <br /> ' over-estimated <br /> 2 Limiting the leachate concentrations to effective solubility limits may <br /> cause the source to deplete at a slower rate,thereby increasing the <br /> ' length of time for loading to groundwater Depending on the problem <br /> being simulated,this approach may be more conservative than ignoring <br /> solubility limitations because it increases the exposure duration and <br /> ' hence the estimate of long-terrn cancer risk <br /> ' The source is assumed to be located in the vadose zone The region in the <br /> vadose zone below the source and the groundwater aquifer itself is assumed to <br /> be "clean" at the start of the simulation This means that the vertical extent of <br /> the contamination in the vadose zone must be known If the groundwater is <br /> . already contaminated there are several ways to model the situation Here are <br /> two ideas <br /> ' 1 Assume that the start of the simulation coincides with the date that the <br /> spill occurred("back it up in time") Run the model through the <br /> present day using the current site information to calibrate the model <br /> (e g make the model predict the current distribution of contaminants in <br /> the soil and groundwater) <br /> 2 Skip the vadose zone portion and model the site with one of the <br /> ' groundwater models If there is still a large amount of contamination <br /> in the vadose zone but the system appears to be at steady-state(i a the <br /> groundwater concentrations are not increasing with time)then use the <br /> dissolved phase model with a constant source <br /> It is important to choose the source concentration and size of the source <br /> ' carefully The best approach is to estimate(roughly)the amount of mass <br /> thought to be in the vadose zone The average concentration multiplied by the <br /> source size should not exceed the estimated mass This seems obvious, <br /> however, it is very easy to use an unreasonably large source mass especially if <br /> maximum concentrations and the maximum extent of detection are used <br /> simultaneously <br /> ' A-3 <br />