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k The new wells force extensive revision of the isopach map for the 3`d Riverbank aquifer In the November 2004 Fourth Quarter Report we postulated a west-trending channel from GT-6 through the former UST cavity toward GT-13, where the thickness of this aquifer reaches 15 feet The map predicted that the aquifer thins northward toward GT-15 and CPT-6 and would die out before reaching GT-12 and GT-18 Instead, both wells penetrated 3rd Riverbank sand at a depth of 120 feet and were terminated in flus sand at 125 feet (Appendix A) This suggests that this channel trends more southwesterly from the truck repair garage toward GT-13 and GT-2 (Figure 9) Both Figure 3 and Figure 5 are transverse to the postulated channel axis, which is shown passing between CPT-2 and GT-15 in Figure 3 and beneath GT-7 in Figure 5 Boring CPT-1 (depth 130 feet) is the only boring that completely penetrated the 3rd Riverbank Channel and reached the underlying aquitard, but lacking samples it is uncertain whether the channel overlies a paleosol (as shown in Figure 4) of unweathered sediment (as shown in Figure 5) However, a few inches of mottled reddish-blown silt were penetrated at the bottom of GT-13 at a depth of 127 feet, supporting the suggestion of a paleosol below the channel (see boring log of GT-13, Appendix A,Fourth Quarter 2004 Report) 7 k 3.2 Paleosols Weathered deposits interpreted as paleosols were recognized as early as 1990 at this site, when Richesin and Associates drilled GT-3 (termed MW-1 in their report) These ancient soils have been consistently recognized in all borings and used as excellent stratigraphic markers in correlating from boring to boring throughout Upgradient Environmental's investigation Brief descriptions have been given in various reports, and here we provide a more thorough discussion of the characteristics of these marker units and interpretation of their significance Several features of these units make them readily distinguishable from unweathered deposits of the Riverbank and Modesto Formations Color is the most obvious criterion for recognizing paleosols Where unweathered, the deposits are gray to dark gray(sand) or light gray to pale tan (silt and clay) In weathered zones, the sediment is oxidized to reddish-brown or dark brown or is mottled in color with veins and streaks of white, patches of yellow or red, or blotches of black or bluish black The white veins are some type of pedogemc precipitate (perhaps calcium carbonate or gypsum) and the black blotches are either weathered organic material or manganese oxide minerals Examination of most boring logs will reveal several zones in which the parent material has been discolored by weathering and soil formation Another criterion that can be used is the degree of lithification, exemplified by blow counts or the difficulty of extruding the sediment from the sample tubes Unweathered sand is normally loose and unconsolidated, and silt is generally moderately consolidated to soft Blow counts in unweathered sand range from 5 to 15 and in silt from 10 to 30 Where the sediment has been affected by soil- forming piocesses, the sand may be sufficiently compacted to be difficult to extrude from the sampler, and silt can be extracted only with difficulty Blow counts increase to more than 20 in sand beds and more than 50 in silt and clay beds A third criterion is the development of soil structures (soul peds) Blocky peds are the most easily recognized in the core samples, but irregular or columnar peds have also been observed Fhe ped surfaces are characterized by re-sedimented clay (clay skins) that has leached downward from 5