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r GROUNDWATER MONITORING AND SAMPLING REPORT BP Oil Company Service Station No. 11195 16500 South Interstate 5 Lathrop, California Project No. 10-151-06-002 �i July 23,1998 :F :- INTRODUCTION :zl This report presents the results and findings of the May 7. 1998 groundwater monitoring and nducted by Alisto Engineering Group at BP Oil Company Service Station sampling co No. 11195, 16500 South Interstate 5,Lathrop,California. A site vicinity map is shown on Figure 1. _j FIELD PROCEDURES Field activities were performed in accordance with the procedures and guidelines of San,Joaquin County Public Health Services and the California Regional Water Quality Control Board,Central Valley Region. Before purging and sampling,the groundwater level in each well was measured. from a permanent mark on top of the casing to the nearest 0.01 foot using an electronic sounder. The Y.L F- depth to groundwater and top of casing elevation data were used to calculate the groundwater °--J elevation in each well in reference to mean sea level. The survey data and groundwater elevation measurements collected to date are presented in Table 1. The potentiometric groundwater elevation as calculated from the results of this monitoring event are shown on s, Figure 2. Before sample collection;each well was purged of 3 casing volumes while recording field readings of pH, temperature,electrical conductivity, and dissolved oxygen. Groundwater samples were collected for laboratory analysis by lowering a bottom-fill, disposable bailer to ===i just below the water level in the well. The samples were transferred from the bailer into + • laboratory-supplied containers for chemical analysis. The water sampling field survey forms are presented in Appendix A. The results of laboratory analysis of the groundwater samples for this and previous sampling events are summarized in Tables 1 and 2. The results of groundwater analysis are shown on Figure 3. On May 7. 1998,recovery testing was performed on Monitoring Wells MW-1 through MW-6 to assess the spatial variation in hydraulic conductivity in the shallow water-bearing zone across the site. Recovery testing was performed by evacuating groundwater to the maximum extent practicable from the wells using a submersible pump,measuring groundwater recovery, and plot the measurements to calculate hydraLLlic conductivity. The results of recovery testing will be submitted under separate cover.