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j u SIERRA of the area New wells have been drilled in the eastern part of the city and groundwater has been supplemented with water from the Calaveras River through the Stockton-East Water District Aqueduct (page 44) The site is located in the Great Valley geomorphic province which is a northwest trending asymmetric structural trough The basin is crossed by two major faults, the Stockton fault,which separates the Sacramento and the San Joaquin sub-basins and the White Wolf fault in the southern part of the basin The site is underlain by Cenozoic non-marine alluvial deposits which consist of interbedded sand, silt and clay' The groundwater level at the site was recorded at approximately 45 feet below grade in February 1987 5 The depth to ground water in August 1991 is approximately 55 to 56 feet below grade The ground water gradient beneath the site is nearly flat (approximately 0 001 ft/ft) and is easterly (Figure 2, Appendix A) Fiv mile Creek approximately The closest surface water is e pp y one-half mile to the northwest 2. SUBSURFACE INVESTIGATION On July 30, 1992, Soils Exploration Services (C-57 license #582696) of Benicia, California installed one on-site monitoring well (MW-6) using a hollow-stem auger drill rig The monitoring well location is shown on Figure 2 (Appendix A) 2.1 Soil Boring Soil samples were collected during drilling in accordance with SES Standard Operating Procedure -Soil Sampling (Appendix C) The boring was logged in accordance with SES Standard Operating Procedure - Logging Method (Appendix C) Soil samples were screened for volatile organics during drilling in accordance with SES Standard Operating Procedure - OVM Readings (Appendix C) Geology of Northern California 1966, Califorrmia Division of Mines and Geology Bulletin 190, Geologic Map of California, Plate 1 5 Groundwater Technology 1987, Consultants report prepared for Chevron February 27, 1987, 8 pp and 3 appendices Page 4