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1.0 INTRODUCTION A drilling investigation utilizing cone penetrometer testing (CPT) equipment was conducted during the fourth quarter of 2003 at 3931 Newton Road The primary purpose of the work was to collect groundwater samples from one or more aquifers within the Pleistocene Riverbank Formation to assess the magnitude and extent of groundwater contamination in this formation Quarterly sampling of the existing monitoring wells was also performed The results aie presented in this report The project site is located in an industrial area on the northeast outskirts of Stockton near U S Highway 99 (Figure 1) Originally agricultural property, the site was developed as a commercial truck terminal in 1973 A diesel and gasoline fueling facility, consisting of two underground storage tanks (UST's), was formerly present on the south side of the storage yard (Figure 2) This facility was removed in 1989, and evidence of soil contamination was discovered Several thousand cubic yards of contaminated soil were removed from the UST excavation in 1990 and treated using above-ground enhanced bioremediation methods, and several phases of, drilling have been completed since then to investigate the extent of groundwater contamination and residual soil contamination Based on the results of those earlier phases of work, the San Joaquin County Environmental Health Department (EHD) concluded that further investigation is required to determine the full extent of contamination In November 2002, EHD requested Gillies Trucking to perform several additional tasks, including the present investigation . 2 0 PROCEDURES 2.1 Drilling and Groundwater Sampling Drilling was performed on December 1-3, 2003 Gregg In-Situ,Inc mobilized a 20-ton CPT rig and advanced six borings into the Riverbank Formation under the supervision of Upgradient Environmental Consultants Boring locations were selected in consultation with Mi Nuel Henderson of EHD and are shown in Figure 2 Two borings were drilled at each locality The first boring was advanced to collect lithologic data, and cone-bearing pressure, sleeve friction, friction ratio, and pore water pressure data were automatically collected during drilling Graphical logs were retrieved (Appendix A) and were used to identify water-bearing zones and correlate stratigraphic units from one boring to another The second boring was drilled to collect water samples from the water-bearing zones that were identified on the graphical logs After the samples were retrieved, each bonng was filled with neat cement Data collected during previous phases of dnlling were used to estimate the required drilling depth The Riverbank Formation was penetrated in ten previous borings (TB-2, GT-1, GT-2, GT-3, GT-5, GT-6, GT-7, GT-8, GT-9, and GT-11) Logs of those borings were included in the Problem Assessment Report (October 1996) and/or the First Quarter 1999 Report 'I he top of the formation was identified in those borings at a depth of approximately 60 feet, where a reddish brown post- Riverbank paleosol is preserved beneath the light-gray clay, sand, and gravel of the overlying Modesto Formation This paleosol formed during an interval of landscape stability that followed the deposition of the Riverbank Formation, and has been estimated by various researchers to be 2