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' California Department of Transportation July 2, 2001 Stockton Maintenance Station, Stockton, California Project No 400296001 • 1 4. GROUNDWATER MONITOIUNG ACICIVIITIES 4.1 Groundwater EIevation and Hydraulic Gradient On March 20, 2001, a representative from Ninyo & Moore measured depth to static groundwater in the twelve on-sate groundwater monitoring wells and two vapor extraction wells Static groundwater levels were measured using a Solinst water level 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 groundwater elevation has increased in the on-site wells since the last monitoring event in December 2000 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 0 003 feet per foot 4.2 Groundwater Sampling and Observations lie 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 f 0296-01 Stockton draft IQOI dM 3