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' Harding Lawson Associates <br /> ' A consistent decreasing or increasing trend in chemical concentrations over <br /> time is not observed in sampled wells. Fluctuations in concentrations may be <br /> associated with on-site remedial pumping in Wells 1, 15 and 17. <br /> ' Most of the wells with detectable chemical concentrations are screened only <br /> in silty clay units. Wells with saturated silty sand screened intervals (Wells 7 <br /> ' and 9) either showed no detection of chemicals (Well 7), or were not sampled <br /> (Well 9). <br /> C. Chemical Migration <br /> Although the potential for off-site chemical migration may exist, factors <br /> that would decrease this potential are 1) variable hydraulic gradient direction, <br /> ' 2) an aquifer zone with a low hydraulic conductivity, and 3) sufficient capture <br /> ' radii of extraction wells. Off-site investigation may not produce conclusive <br /> evidence of off-site migration due to the proximity of other potential sources. <br /> Y ' More appropriate is an investigation into the potential for off-site migration. <br /> As previously discussed, tidal fluctuations may affect hydraulic gradient <br /> ' directions on site. A hydraulic gradient reversal or a ground-water divide may <br /> ' restrict off-site migration and/or ground-water flow to surface waters. A tidal <br /> influence study should provide an estimation of changes in hydraulic gradients. <br /> ' Clay materials such as those comprising the aquifer zone on-site tend to <br /> have low hydraulic conductivities. Low hydraulic conductivities indicate a slow <br /> ground-water velocity, thereby decreasing potential for off-site migration. An <br /> aquifer test could be used to provide estimates of aquifer parameters such as <br /> transmissivity, hydraulic conductivity, and storativity. <br /> 1 <br /> November 24, 1987 8 of 9 <br /> B2208-R <br />