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California Department of Transportation November 24, 1999 Stockton Maintenance Station, Stockton, California Project No 103872-02 i ' 4. GROUNDWATER MONITORING ACTIVITIES ' 4.1. Groundwater elevation and hydraulic gradient On September 22, 1999 two representatives from Ninyo & Moore measured depth to static 1 groundwater in the twelve on-site groundwater monitonng wells and two vapor extraction wells Static groundwater levels were measured using a Solinst water level probe The probe 1 was decontaminated prior to each use A summary of the current and previous depth-to- groundwater measurements is presented in Table 1 The general groundwater elevation has ' decreased in the on-site wells since the last monitonng event in June 1999 Based on groundwater data collected dunng this round of monitoring, the inferred direction of ' groundwater flow beneath the site is to the southeast with a hydraulic gradient of 0 002 feet per foot 4.2. Groundwater Sampling and Observations Using a 2-inch submersible pump, which was decontaminated between wells, a minimum of three well casing volumes of water were purged from each of the fourteen on-site wells prior ' to the collection of groundwater samples The wells were allowed to recover to at least 80 percent of their onginal static groundwater levels prior to sampling During well purging, rtemperature, pH, and conductivity 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 Appendix 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-custody protocol The samples were secured in a cooler and transferred to Associated Laboratories ' (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 doe 3