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NOA Drywell Closure and Investigation Page 5 of 26 <br /> May 30, 2014 <br /> 2.6.2 Site-Specific Hvdrogeology <br /> ' Groundwater was first encountered in the western drywell (DW-1) on March 28, 2014 at 65.5 <br /> feet bgs. However, groundwater quickly dropped. Groundwater was again encountered at 75 <br /> feet bgs. Groundwater was encountered in the eastern drywell (DW-2) on April 1, 2014 at 58.5 <br /> feet bgs. Groundwater was sufficient to collect samples; further drilling was not performed. <br /> There are several factors that may explain the variance in groundwater levels between the two <br /> I (2) adjacent drywells. <br /> A hollow stem auger was used to drill the boreholes for sample collection. An accurate <br /> groundwater reading cannot be obtained using this drilling method, because it takes time for <br /> 1 groundwater to seep from the bottom of the boring into the annular space of the auger and <br /> through the auger connections. The time it takes for groundwater to stabilize is highly <br /> dependent on the permeability of the soil. <br /> Groundwater encountered at 58.5 feet bgs in DW-2 may be a perched lens in coarse sand <br /> underlain by a low permeability clayey sand aquitard. The perched lens of groundwater at 58.5 <br /> feet bgs is likely thin and may therefore have been drilled through without observation in DW-1. <br /> Groundwater encountered at 75 feet bgs in DW-1 is likely part of the deeper regional aquifer, <br /> based on known groundwater depths in the area. Groundwater encountered at 65.5 feet bgs in <br /> DW-1 quickly permeated into underlying soil. Groundwater at this depth may also be part of the <br /> deeper regional aquifer, or is another perched lens of groundwater. <br /> In general, differences in groundwater levels recorded during sampling activities may be <br /> explained by the measurement inaccuracy that results from the hollow stem auger drilling <br /> method, as well as by possible measuring and sampling in the regional aquifer in DW-1, and a <br /> perched lens of groundwater in DW-2. <br /> �W2 SH»yl1 n.(y ,�r �erc�o� <br /> j <br /> ' 902 Industrial Way• Lodi, CA 95240• 209.367.3701 • Fax 209.333.8303 ®2014 Neil 0.Anderson @ Associates,Inc. <br />