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1.0 INTRODUCTION The second phase of a drilling investigation that began in the fourth quarter of 2004 was conducted during March 2005 at 3931 Newton Road The first phase had confirmed that diesel fuel is locally present in three semi-confined aquifers of the late Pleistocene Riverbank Formation beneath the main zone of contamination in the overlying Modesto Formation The purpose of the second phase was to locate the limit of contamination in these aquifers by installing additional wells farther from the contaminant source Data from the first phase and from earlier investigations in 1994, 1996, and 1998 were used to determine appropriate locations of the new wells These locations were approved by the Environmental Health Department of San Joaquin County, which is the Local Oversight Agency in charge of the investigation 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 2 0 PROCEDURES • 2.1 Drilling and Soil Sampling g Drilling was performed on March 15-18 and 21 V&W Drilling, Inc mobilized a BK-81 hollow- stem auger drilling rig and advanced six borings (GT-12B, GT-16, GT-17, GT-18, GT-20, and GT- 21) into the Riverbank Formation under the supervision of Upgradietit Environmental Consultants Boring locations are shown in Figure 2 Boring GT-12B twinned the previously drilled GT-12A, which was terminated at a depth of 105 feet, therefore, soil samples were not collected from GT- 12B above a depth of 100 feet The other borings were sampled at 5-foot intervals from 10 feet to the bottom of the hole Boring logs were prepared and samples were screened for visual or olfactory evidence of diesel or gasoline contamination Only one sample emitted diesel odors, but a total of nine samples were preserved in a cooled ice chest for subsequent laboratory analysis When the borings reached total depth, 2-inch diameter monitoring wells were constructed The boring logs are contained in Appendix A During the first phase of drilling in November 2004, boring GT-12A encountered flowing sand at a depth of 95 feet and the boring had to be terminated at 105 feet This problem was not encountered during the second phase, and boring GT-12B reached a total depth of 125 feet The monitoring well was constructed within the 3`d Riverbank aquifer in the depth interval of 115-125 feet Borings GT-16 and GT-17 were drilled to approximately 90 feet, and wells were constructed in the ! I" and 2nd Riverbank aquifers between 69-70 and 80-90 feet GT-18 reached a depth of 125 feet, • but the percussion hammer cable broke during sample retrieval and the sampler and hammer could 2 4