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3.3 Water Sampling 3.31 Groundwater Monitoring,August 1996 Groundwater monitoring took place on August 28 and October 7 On August 28, the depth to groundwater in the five existing wells (GT-1, GT-2, GT-3, GT-5, and GT-6) was measured with a Soh nst water level meter and the elevations of the top of the well casings of GT-5 and GT-6 were surveyed with a builder's level relative to GT-3 (Appendix B) These data were used to construct a groundwater gradient map (Figure 3) and select a location for a monitoring well down gradient from GT-3 Approximately 25 gallons of water were purged from GT-5 and GT-6 to reduce the turbidity and tighten the sand pack, but no samples were collected on August 28 3.3.2 Groundwater Monitoring, October 1996 On October 7, after GT-9 had been drilled, the depth to groundwater was measured in all six wells, GT-9 was developed and surveyed relative to the other wells, and samples were collected from GT-5, GT-6, GT-9 and the drinking water well at 3851 Newton Road Approximately three well volumes were purged from GT-5 and GT-6 and approximately 25 gallons were purged from GT-9 before sampling During purging, a Coning Checkmate Ph/conductnvnty meter was used to measure the temperature, Ph, and conductivity of the purge water to determine when these parameters stabilized (Appendix B) After purging, water samples were collected in two 40-m1 vials and one 1-hter amber bottle The purged water was placed in 55-gallon drums on the south side of the Gilhes property The samples were labeled and placed in a cooled ice chest and slupped under chain of custody to McCampbell Analytical Laboratory 4 0 SITE GEOLOGY 4.1 Subsurface Stratigraphy Figure 4 is a generalized stratngraphic column for the near-surface deposits of the eastern San Joaquin Valley In the eastern part of San Joaquin County, the two main stratigraphnc units that are of interest at most contaminated sites are the Modesto and Riverbank Formations, which are exposed at the surface and/or are present at shallow depths Both formations can be recognized at the Gilhes Trucking site Younger(modern) deposits are less common in the eastern part of the county, and older deposits are buned at greater depths and are not affected by near-surface contamination When combined with the previous four borings, the sic bonngs drilled in August and October make it possible to construct a detailed, site-specific stratngraphnc column to depict the stratngraphnc details and hydrogeologic units of the Modesto and Riverbank Formations (Figure 5) Logs of these bonngs (Appendix A) were used to construct two-dimensional cross sections m various directions (Appendix Figures A-1 through A-4), which were then combined into a three-dimensional fence diagram (Figure 6)to illustrate lateral variations in the stratigraphy 4