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. limit benzene migration include hydrocarbon retardation by adsorption and related <br /> processes, biodegredation by in-situ microbes, and diffusion <br /> The predicted benzene concentrations through time and its distance of migration could be <br /> overestimated or underestimated, depending on how accurately the mean of the most <br /> recent benzene concentrations in MW-1, MW-2, SW-1, and SW-2 represent the actual <br /> mean concentration within the contaminant plume For example, the concentration in <br /> SW-1 in 1998 was higher than its most recent value, but the 1998 concentrations are ��d f CL <br /> questionable because groundwater pumping (purging) at the time of sampling may have <br /> concentrated benzene within the well bore For that reason., the higher concentrations may <br /> not be representative of true conditions at that location However, even if the higher <br /> concentrations are valid, test runs of the simulation using the higher values indicated that <br /> both the migration distance and the concentration of benzene remain low This implies <br /> that benzene concentrations that have been detected in the wells are not high enough to <br /> have a mayor impact on the simulation Moreover, key input parameters like hydraulic <br /> conductivity and hydraulic gradient were intentionally overestimated to provide an extra <br /> measure of safety and the model makes other conservative assumptions, so the estimated <br /> concentrations and migration distance are more likely to be overestimated than <br /> underestimated Hence, the modeling results support our previous conclusion that the site <br /> poses a very low risk to people or the environment Therefore,we recommend that the <br /> case be closed with no further action <br /> i <br /> I <br /> I <br /> I i I <br /> I I I <br /> I II y I II I I I ' <br /> I , <br /> fl II I <br /> I <br /> ` � �f•`II I I� � 4 I ESI ' I � � I � i� 14� i �f � Ifs l`� � <br /> IIIk f,I I <br /> II ��� 1�h��'I�IGI f flli'I'IV I II I °II f i`f �aE��'III�I'` '�II'�IE III I,f II'i1111 I l II Ij 'II,Ih���III IV� (,tl�I CI It �l;�IIII` �I�kl"�I I�111�11 II Itll I �i IClI}���IIfl IIII��i�I�IIiI I {k1� <br /> ��Ili <br /> 1,{� I I I�llll`I II II <br /> d 'I' <br />