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' California Department of Transportation September 6 2 p S p OOI Stockton Maintenance Station, Stockton, California Project No 400296001 4. GROUNDWATER MONITORING ACTIVITIES ' 4.1. Groundwater Elevation and Hydraulic Gradient On June 4, 2001, a representative from Ninyo & Moore measured depth to static groundwa- ter in the twelve on-site groundwater monitoring wells and two vapor extraction wells Static groundwater levels were measured using a Solinst water level probe The probe was decon- taminated prior to each use A summary of the current and previous depth-to-groundwater measurements is presented in Table I The general groundwater elevation has decreased in ' the on-site wells since the last monitoring event in March 2001 Based on groundwater data collected during this round of monitoring, the inferred direction of groundwater flow be- neath the site is to the east with a hydraulic gradient of 0 13 feet per foot (ft/ft) t4.2. Groundwater Sampling and Observations Using a 2-inch submersible pump, which was decontaminated between wells, a mini- ,® mum 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 re- cover to at least 80 percent of their original static groundwater levels prior to sampling During well purging, temperature, acidity, and conductivity were measured using an Altrameter 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 trans- ferred into labeled, reconditioned 55-gallon drums The drums were left on-site to be maintained by Caltrans i � 0246-01 stockwn 2QO1 doc 3