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Remediation Report - 152 East 11th Street, Tracy, California Page 5 structure Areas beneath the streets adjacent to the property have been extensively excavated and backfilled for installation of sewers and other utilities The depth to groundwater is in the range 7 to 9 ft Seasonal variations in the elevation of the groundwater table amount to approximately one foot or less (see Appendix A - Table A2) The general direction of the groundwater gradient is northwest across the 152 East 11 th Street property The slope of the groundwater gradient is typically about 0 003 ft /ft The groundwater contours shown on Figure 2 were constructed from measurements of the depth to groundwater in each of the NAV and ULT series monitoring wells that were made on November 15th 1995 (Ref 19) and are representative of the generally prevailing conditions Hydrostratigraphy As noted above, the soils beneath the 152 East 11th Street property and adjacent areas are composed of clays, silty and sandy clays and sands that are complexly interbeded i However, over most of the area, from the ground surface to a depth that varies locally from approximately 12 to 14 ft , the soils are dominantly silty clays with only a few discontinuous lenses of sand Beneath the clays there is a fairly extensive stratum of sands that is t}pically some 11 ft thick, but which includes lenses of clay and which may be locally discontinuous in areas where silts and clays dominate the matrix Beneath the sand stratum, at a depth that varies between some 21 and 25 ft below the ground surface, and close to the maximum depth of exploration, there is a second stratum of silty clays that appears to be arealy extensive When the sands described above were encountered in borings drilled to install groundwater-quality monitoring wells on and around the 152 East 11th Street property they were generally composed of fine to medium-course, sub-rounded particles with some silt They are locally very loose (less than 4 blows per foot) When the borings for wells NAV 5 6, 7 and 8 were drilled, the sand encountered in those holes at depths between 14 and 16 ft beneath the ground surface flowed into the hollow-stem augers In one case - well NAV 6 - when drilling reached the sands, the sampling tools became trapped in the boring (Ref 9) This was a significant finding because such loose sands beneath the water table would readily flow into any excavation that penetrated into them Figures 3 and 4 are hydrostratigraphic sections along the lines A'-A' and B'-B' shown on Figure 2 The hydostratigraphy shown in the cross sections is an interpretation of the stratigraphic data from the well boring logs that divides the sediments into two classes, the highly permeable sands and the comparatively less permeable silty and sandy clays This presentation made it possible to reduce the details of the stratigraphy to a tractable degree of complexity by distinguishing between the different soil types based on properties that were of importance to the design and implementation of the CAP (i a relative hydraulic conductivity and cohesion) The subsurface zone affected by gasoline is also shown on the sections