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Formation and one interval of the Riverbank Formation Figure A-5 shows four principal sediment . types in the interval sampled at the depths of 15 and 20 feet, which is the interval occupied by the light reddish brown planar sand bed in the upper Modesto Formation The map defines an east-west trending linear sand body that is bordered on the south by silt in GT-1 Farther south, silty sand is again prevalent (GT-9) The shape of the sand body suggests an overbank or unchannelized flood deposit or a very shallow stream channel, the coarser sand at the base in GT-7 may be a mid-channel bar (Figure 6) Figure A-6 shows three principal sediment types in the interval sampled at 35, 40, 45, 50, and 55 feet, which is the interval occupied by the Modesto channel The northeast-southwest trending belt of = coarse-gramed channel sand is bordered to the south by finer-gramed sand Figure A-7 shows four facies in the Riverbank Formation sand body in the interval sampled at 60, 65, and 70 feet In general, the Riverbank Formation becomes finer-grained southward across the site The hardpan is represented by clayey to silty fine-grained sand around GT-1 and GT-3 4.2 Subsurface Structure The new borings also make it possible to decipher the structure of the bedding beneath the site, which may be important in influencing the direction of groundwater flow Figure 8 is a contour map drawn on the top of the upper Modesto sand, which forms a distinctive near-surface marker bed The map shows that the bed dips northeastward beneath the site and ranges from 12 feet in depth in GT-2 to 19 feet i1 GT-6 Cross section G-H (Appendix A) trends in this direction and shows the inclination of this bed clearly The dip is approximately 2 degrees A northeastward dip is contrary to expectations In general, stratigraphic units in the eastern San Joaquin Valley dip westward off the flanks of the Sierra Nevada This northeastward dip is probably local in extent 4 3 Hydrogeology The depth to groundwater has fluctuated significantly in both time and space at this site When the first well was drilled in 1990, the depth was 85 feet below the surface When the next wells were drilled in 1994, the depth was 75 feet (Table 1) Since then the water table has risen steadily, reaching 57 feet in May 1996 When the fifth boring (GT-4) was dulled on August 16, 1996, groundwater was encountered between 47 and 48 feet This was the first time since investigation of the site began that groundwater was present in the Modesto Formation However, when GT-5 and GT-6 were drilled three days later, saturated conditions were not encountered until almost 65 feet, in the Riverbank Formation This indicates that the hydrogeology of the site is complex, and involves at least three recognizable hydrogeologic units (Figure 5) In order to better understand this complexity, several samples were submitted for Iaboratory analysis of their hydrogeologic characteristics These and other aspects of the hydrogeology of the site are considered in the following sections 7