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California Department of Transportation June 19, 2001 p p Shop 10 Maintenance Station, Stockton, California Project No 400296001 • benzene, toluene, ethylbenzene, and total xylenes (BTEX) and methyl tertiary butyl ether (MTBE) have been detected in the on-site monitoring wells 4. GROUNDWATER MONITORING ACTIVITIES 4.1. Groundwater Elevation and Hydraulic Gradient On December 12, 2000, a representative from Ninyo & Moore measured depth to static groundwater in ten of the twelve on-site groundwater monitoring wells Well MW-14 and VM-2 were 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 September 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 0 004 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 the ten accessible on-site wells prior to the collection of groundwater samples The wells were allowed to recover to at least SO per- cent of their original static groundwater levels prior to sampling During well purging, temperature, acidity, and conductivity were measured using a Hydac instrument Addition- ally, 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 Laboratories (AL) in Orange, Califor- 0296 O i r shop I Q FN L 4Q2000 do, 3