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PamUs of AdMand ub•-d SM2 <br /> Rea For Me ao=M <br /> 838 Mokelumne Street Woodbridge,CaIMOrrM <br /> Also among the understandings were: a) analysis of selected soil samples for <br /> "petrophysical"properties, including vertical permeability and porosity; and b)a discussion <br /> would be included within the results report regarding soil moisture and evapo-transpiration <br /> (ET) data for the site. Although not so stated, Aegis views these additional <br /> understandings as selective in arguing for site closure, and to be based on the results of <br /> the chemical analysis of the soil samples. In the event the analytical results indicated <br /> concentrations of TPH, as diesel, in soil at levels such that they remained a concern, then <br /> the physical data on the soils forming the "confining" layer and the potential for movement <br /> of the hydrocarbons within the subsurface under the influence of infiltrating water would <br /> have been critical to the argument. <br /> However, as reflected in the enclosed report, only five of the sixty-one soil boring samples <br /> collected and analyzed indicated concentrations of TPH, as diesel, above the Practical <br /> Quantitation Reporting Limit (PQL) of 1.0 part-per-million (ppm) for soil (page 18 of the <br /> 'Tri-Regional Board Staff Recommendations for Preliminary Evaluation and Investigation <br /> of Underground Tank Sites," dated August 10, 1990) as revealed by the above analytical <br /> methods. These concentrations ranged from 1.1 to 3.5 ppm in samples collected from <br /> depths ranging from 31 to 33 feet below surface. The borings indicate the base of the <br /> confining layer extends to at least 37.5 feet below surface. In addition, the samples <br /> indicating these concentrations were collected either immediately above or within the first <br /> few feet of the "confining" layer. <br /> It is important to note; in each of the respective borings, deeper samples collected from <br /> within the "confining" layer indicated concentrations of TPH, as diesel, below the PQL. <br /> In the case where the samples collected from within the first few feet of the "confining" <br /> layer indicated residual TPH, as diesel, the boring also revealed the layer extended for at <br /> least another 4.5 feet below the deepest sample that indicated the hydrocarbons. <br /> On the basis of these results, Aegis elected not to pursue either analysis of selected <br /> samples for their physical characteristics or collection of data regarding the soil moisture, <br /> ET, or infiltration. In Aegis' opinion the investigation has confirmed: <br /> 1. The existence of the "confining" layer, in terms of: a) its depth relative to the level <br /> of the potentiometric surface of the shallow water-bearing zone; b) discernable soil <br /> type(s) observed in the field; and c) its demonstrable effectiveness in preventing <br /> downward migration of remaining hydrocarbons; and, <br /> 2. What hydrocarbons do remain in soil beneath the site do so in only very minor <br /> amounts that may be subject to migration. <br /> 89-031BB.LTR -2- <br />