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California Department of Transportation August 17, 1999 IStockton Maintenance Station, Stockton, California Project No 103872-02 44. GROUNDWATER MONITORING ACTIVI'T'IES 4.1 Groundwater elevation and Hydraulic gradient On June 24, 1999 two representatives from Ninyo & Moore measured depth to static groundwatei in the twelve on-site gioundvvatei nionitoiing wells and t,,\o vapor extiaction wells Static groundwater levels were measured using a Heron air/water interface probe The 1 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 south with a hydraulic gradient of 0 002 feet per foot 42 Groundwater Sampling and Observations Using a 2-inch submersible pump, a minimum of three well casing volumes of watei were purged from each of the fourteen on-site wells prior to the collection of groundwater sam- ples 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 conductiv- ity were measured using a Hydac instrument Additionally, characteristics of the water ' (color, turbidity, odor, sheen) were noted on the field data sheets and are included in Appen- dix A 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-cu8tody protocol The samples were secured in a cooler and transferred to Associated Laboratories 1 (AL) in Orange, California via United Parcel Service (UPS) Purged and decontamination water generated during sampling activities were transferred into labeled, reconditioned 55- gallon drums The drums were left on-site to be maintained by Caltrans Stockton dm 3