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1 <br /> contamination beneath the Langston property, but has apparently prevented the plume <br /> ' from migrating off-site (Figures 6 and 7). As a result, the plume of contamination is not <br /> Yet fully defined to either the east or to the south <br /> 5.3 Discussion <br /> Benzene concentrations found in the groundwater in MW-1 and MW-3 are well in excess <br /> of both State and Federal Action levels Benzene concentrations in MW-4 are dust slightly <br /> in excess of both State and Federal Action Levels The absence of toluene, ethylbenzene, <br /> total xylenes and non detectable levels of TPH in the gasoline range MW-4, suggests that <br /> plume migration perhaps had ,lust started to reach this location before the direction of <br /> ' groundwater flow changed The very low concentrations of TPH, essentially g/L 0 4 m in `��� <br /> MW-2 with no volatile aromatic compounds present, again suggests the possibiity that flow <br /> directions have varied with time As a result, Figure 7 is drawn to represent the <br /> configuration of the contaminant plume as it is believed to exist at the time of this <br /> investigation. <br /> Since there are numerous domestic water wells present within1/2-mile radius of the <br /> Langston property (Figure 5), further assessment and remediation is warranted—However <br /> the low hydraulic conductivity of the aquifer coupled with a very low, nearly level gradient <br /> across the site ( 001 feet per foot as calculated from surveyed water table elevations in the <br /> four monitoring wells) has prevented large lateral spreading of the contaminant plume <br /> away from the initial point of release, but has also contributed to the possibility that flow <br /> directions are highly variable, and may be hydraulically connected to deeper resources <br /> being periodically pumped by the City of Lathrop (Figure S) <br /> Since the primary aquifer material consists of silt and fine-grained sand with a hydraulic <br /> conductivity of no more than 01 to 10 feet per day, the average Darcian velocity <br /> (v=Kdh/ndl, where K is the hydraulic conductivity, dh/dl is the hydraulic gradient, and n <br /> is porosity) is within the range of 004 to 0 04 feet per day if we assume a porosity of 0 4 <br /> Although there is no immediate concern that nearby domestic water wells could be <br />' 16 <br /> 1 <br />