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. approximately 250,000 years old The Modesto Formation is thought to be less than 125 000 years <br /> old Due to its greater age and the effects of sod formation, sediment of the Riverbank Formation is <br /> more consolidated than Modesto sediment Whereas only 10-30 blows of the drill harnmer are <br /> typically required to collect a soil sample from the lower portion of the Modesto Formation, 30-80 <br /> blows are commonly needed to sample the upper part of the Riverbank Formation, where a slightly <br /> cemented soil horizon ("hardpan") occurs locally This farther aids in identifying the Modesto- <br /> Riverbank contact <br /> In its Apnl 10, 2003 letter to Gillies Trucking EHD suggested drilling one boring in the vicinity of <br /> the former UST cavity to a depth of 110-120 feet Boring CPT-I was advanced to 120 feet in this <br /> area, but the zone from 85-120 feet appeared to consist of consolidated (stiff) fine-grained sand <br /> interbedded with silt and clay, and exhibited poor aquifer quality on the CPT log (Appendix A) <br /> The boring was then extended to 130 feet,but aquifer characteristics did not improve and the boring <br /> was therefore terminated Water samples were collected at 69-74 feet and 80-85 feet and (with <br /> difficulty) at 114-119 feet Borings CPT-3 and CPT-4 were advanced to 120 feet, but the sediment <br /> below 100 feet again exhibited relatively poor aquifer characteristics and only CPT-3 was sampled <br /> below 100 feet Borings CPT-2, CPT-5, and CPT-6 were terminated at 100 feet, and samples were <br /> collected at approximately 65-70 feet and 80-85 feet in these borings <br /> Thirteen samples were analyzed for TPH-diesel, and ten were analyzed for TPH-g and BTEX The <br /> sample collected at 80 feet in CPT-2 was analyzed for TPH-g and BTEX but not for TPH-d, <br /> because the diesel bottle was broken during transit to the laboratory The three samples from CPT-1 <br /> • were also analyzed for gasoline oxygenates and 1,2-DCA The laboratory report is in Appendix B <br /> 2.2 Groundwater Monitoring <br /> Groundwater monitoring took place on December 3 The wells were opened and allowed to <br /> equalize with atmospheric pressure, and then the depth to groundwater was measured with a water <br /> level meter The measurements were recorded on the Well Monitor Report(Appendix C) before the <br /> wells were purged <br /> Samples were collected in disposable plastic bailers and transferred to i-liter amber bottles The <br /> samples were labeled and placed into a cooled ice chest for later shipment to the laboratory All five <br /> samples were analyzed for TPH-diesel The laboratory report is contained in Appendix D <br /> 3.0 RESULTS <br /> 3.1 Lithology and Stratigraphy <br /> Four cross sections were constructed through the CPT and other,borings and used to identify and <br /> correlate stratigraphic units across the site (Figures 3-6) Correlation is relatively straightforward, <br /> and three principal sand bodies can be identified <br /> The uppermost sand body occurs in the lower part of the Modesto Formation, and has been referred <br /> to in previous reports as the Modesto Channel This sand body is at least 25 feet thick in the vicinity <br /> rof the former UST facility, but thins to the north and is no more than 5 feet thick in CPT-6 (cross <br /> 3 <br />