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California Department of Transportation June 29, 2001 Shop 10 Maintenance Station, Stockton, California Project No 400296001 4. GROUNDWATER MONITORING ACTIVITIES 4.1. Groundwater Elevation and hydraulic Gradient On March 20, 2001, a representative from Ninyo & Moore measured depth to static groundwater in ten of the twelve on-site groundwater monitoring wells Wells MW-14 and VM-2 have been lowered and paved over due to on site construction in the area and were not sampled Static groundwater levels were measured using a Solinst water level probe The probe was decontaminated prior to each use with an Alconox wash and a distilled water rinse A summary of the current and previous depth-to-groundwater measurements is pre- sented in Table 1 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 east with a hydraulic gradient of 0 006 feet per foot 4.2 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 the ten accessible 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 Dur- ing well purging, temperature, acidity, and conductivity were measured using a Hydac instrument Additionally, characteristics of the water (color, turbidity odor, and 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 transferred to Associated Laborato- ries (AL) in Orange, California via United Parcel Service (UPS) Purged and decontamination water generated during sampling activities were transferred into la- I 07%-ai.shoRio o.n 1Q01 a« 3