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not be retrieved The boring was then deepened to 135 feet, and the hammer dropped to that depth and was cemented in place Monitoring wells were constructed in the P and 2"d aquifers (113-123 feet and 75-85 feet) The shallower aquifer well in each boring was designated the A zone, and the lower aquifer was designated the B zone GT-20 and GT-21 were terminated at 70 feet and wells were installed in the I"Riverbank aquifer at a depth of 60-70 feet The soil samples were transported to Fxcelchem Analytical Laboratory The laboratory report is in Appendix B and the data are summarized in Table 1 2.2 Groundwater Monitoring Groundwater monitoring took place on March 30 The wells were opened and allowed to equalize with atmospheric pressure and then the depth to groundwater was measured with a water level meter In nested wells, the depth was measured in both casings The measurements were recorded on the Well Monitor Report (Appendix C) before the wells were purged Depth data are given in Table 2 A Grundfos purge pump was used to pump between 20 and 45 gallons of water from each of the newly constructed wells to remove sediment and tighten the sand pack The older monitor wells were hand-bailed of approximately 2-4 gallons of water The water was disposed of on site Samples were collected in disposable plastic bailers and preserved in 1-liter amber bottles The samples were labeled and placed into a cooled ice chest for later shipment to the laboratory The laboratory report is contained in Appendix D and the data are summarized in Table 3 After the wells were sampled, they were surveyed with a builder's level to determine the elevation of the top of the casing GT-6 was used as the relative base level benchmark Casing elevations are shown in Table 2 9 3.0 RESULTS 3.1 Lithology and Stratigraphy A thorough description and discussion of the subsurface geology was presented in the Fourth Quarter 2004 Report, using data collected during that phase of the investigation as well as data collected previously The data collected during this phase of drilling force several revisions in the interpretation of the 3-dimensional geometry of individual aquifers, but otherwise lead to only minor modification of previous desenptions These modifications are given in the following paragraphs Pre-drilling estimates for the thickness of the Modesto Channel in the new borings ranged from less than 8 feet in GT-I 8 to 20 feet or more in GT-20 and GT-21 Those estimates assumed a relatively linear trend to the channel in a northeast-southwest direction The measured thickness of 20 feet in GT-21 matches predictions well, but the thickness is greater in GT-18 and less in GT-16 and GT-20 than predicted (Figure 6) The greater thickness in GT-18 indicates a more sinuous or braided channel, and the main channel is shown branching to the west between GT-6 and GT-16 Both channels are illustrated in Figure 3, on either side of GT-15 In this well and in GT-8 (Figure 4) and • 3