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California Department of Transportation August 17, 1999 Shop 10 Maintenance Station, Stockton, California Project No 103872-02 r4. GROUNDWATER MONITORING ACTIVITIES ' 4.1 Groundwater elevation and hydraulic gradient On June 24, 1999, a representative from Nmyo & Moore measured depth to static ground- water in the twelve on-site groundwater monitoring wells Static groundwater levels were measured using a Heron air/water interface probe The probe was decontaminated prior to each use A summary of the current and previous depth-to-groundwater measurements is presented in Table 1 The general depth to groundwater has increased in the on-site wells ' since the last monitoring event in April 1999 Based on groundwater data collected during this round of monitoring, the inferred direction of groundwater flow beneath the site is to the southeast with a hydraulic gradient of 0 0015 feet per foot 42 Groundwater Sampling and Observations Using a 2-inch submersible pump, a minimum of three well casing volumes of water were purged from each of the twelve on-site wells prior to the collection of groundwater samples The wells were allowed to recover to at least 80 percent of their original static groundwater ' levels prior to sampling During well purging, temperature, pH, and conductivity were measured using a Hydac instrument Additionally, characteristics of the water (color, turbid- ity, odor, sheen) were noted on the field data sheets and are included in Appendix A 1 Subsequent to purging, samples were collected using disposable PVC bottom-discharging bailers The samples were transferred from the bailer to the appropriate sample containers, labeled, and placed in a cooler containing ice at 4 degrees Celsius under chain-of custody protocol The samples were transferred to Associated Laboratories (AL) in Orange, Califor- nia via United Parcel Service (UPS) Purged and decontamination water generated during sampling activities were transferred into labeled, reconditioned 55-gallon drums The drums 1 were left on-site to be maintained by Caltrans 1 shoplD dm 3 Y