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. AR[ADISGERAGHTY&MILLER <br />' retardation factor This rate of migration must be further modified <br /> to account for the amount of attenuation of the compound due to <br />' dispersion and biodegradation This series of calculations must be <br /> performed for each constituent of interest which is identified and <br /> quantified in the soil analysis <br /> 1 At each step of the calculation process, there are inherent and <br /> significant sources of error which ultimately make this <br />' quantification process highly uncertain For example, the above- <br /> described partitioning calculations required the use of a partitioning <br /> coefficient The value of the partitioning coefficient can only be <br /> obtained experimentally and varies significantly with soil type In <br /> instances where experimentally determined partitioning coefficients <br /> are not available, they can be estimated using regression equations <br /> The partition coefficient has been recognized as a key parameter in <br /> predicting the environmental fate of organic compounds The <br /> uncertainty (error) associated with estimation of the partitioning <br /> coefficient, when one is not able to relate the site soil to one <br />' identified under carefully controlled laboratory conditions can be <br /> orders of magnitude (i a greater that a factor of 10) A second <br /> example of the uncertainty in predicting the potential impact on <br /> groundwater based on a soil sample is that the hydraulic <br /> conductivity of the medium must be know in order to predict the <br /> advective rate of transport Since the value of hydraulic <br />' conductivity vanes significantly with soil type and even spatially <br /> within a soil type, the error associated with a calculation dependent <br /> upon assuming a hydraulic conductivity value or spatially <br />' extrapolating a measured hydraulic conductivity value can also be <br /> orders of magnitude Theses errors ultimately are multiplicative <br /> and can result a highly misleading assessment of the potential for <br />' future impact to downgradient groundwater <br /> Because the task of hand-calculating predictions of future <br /> downgradient groundwater impact is so onerous, some <br />' investigators have utilized numerical computer models to ease the <br /> calculation work load However, it is important to emphasize that <br /> while the burden of the onerous work load has been removed, the <br />' uncertainty attendant to the process descnbed above has not been <br /> removed To make matters worse, the uncertainty is hidden by the <br /> convenience of the automated calculation, unless a formal <br />' evaluation of the uncertainty is also included as part of the <br /> assessment <br />' Some regulatory agencies have in the past used more simplified <br /> calculations than those described above to set soil concentration <br /> guidance levels which are designed to be protective of <br /> groundwater These approaches have declined in usage and <br /> application as the knowledge of contaminant fate and transport has <br /> 1 3 <br />