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2.3. Monitoring Well Development and Sampling • Well developmer►t and sampling-procedures were conducted'in accordance with the ' guidelines of the SJCEHD and RWQCB. Field procedures for groundwater monitoring well development'arid sampling-are presented in'Appendix D. Wells were developed on Maich 30, 1994, before installing the bentonite and neat cement seal, by removing at least 10 casing volumes or until groundwater was relatively free of sediment. Monitoring WelLs MW-1 through MW-7 were sampled on April_20 and 2"5, 1994. To obtain ' groundwater samples that were representative of the shallow water-bearing'zone, the wells were purged of at least 3 well casing volumes before sample collection and,while monitoring pH,'specific conductivity, and temperature. The samples were then transported in an iced ' cooler to a state-certified laboratory following proper chain of custody procedures. Field observations during well development and sampling are presented in the sampling forms in ' Appendix E. ' 2,4 Groundwater Level Monitoring and Well Surveying 1 The wells were surveyed'to the tap of casing in reference-to a box on top of the curb at the east end of the westerly driveway at the BP Oil`station, with an elevation of 4.55 feet above mean sea level. On April 25, 1994, the depth to groundwater in the wells was measured 1 from the top of the well casing to the nearest 0.01 foot, using an electronic sounder. The survey data and relative groundwater elevation measurements are presented in Table 1. The well elevation survey map prepared by a license surveyor is included in Appendix D and a graphical interpretation of the groundwater gradient beneath the site is shown in Figure 3. 10 3.0 SITE GEOLOGY AND HYDROGEOLOGY Soils encountered while drillingduring this and previous investigations enerall consisted g Pgenerally of approximately 5 feet of silty clay underlain by interbedded sand and silt to the total depth of the borings. Hydrogeologic cross sections prepared using boring logs generated during this and previous investigations are shown in Figure 4. •Lines of.hydrogeologic cross sections are shown in Figure 2. , _ Near-saturated soil conditions encountered during drilling and sampling were noted at approximately 18 feet below'grade, with,the depth to groundwater measured in the monitoring wells at between 17 and 18 feet below grade. Groundwater elevations in all the ' wells as measured on April 25, 1994, were'used to develop the groundwater potentiometric surface map shown in Figure 3. The groundwater gradient as interpreted from these measurements is approximately 0.002 foot per foot in a general_noitheasterly direction across the site: 1 ' E:\0\10.136\SSLRM0622% 3 i Rem