Ground Water Sampling Using Low-Flow Techniques

摘要

Obtaining representative ground water samples is important for site assessment andrnremedial performance monitoring objectives. The sampling device or method used to collect samplesrnfrom monitoring or compliance well can significantly impact data quality and reliability. Low-flow orrnminimal draw-down purging and sampling techniques is an alternative method for obtainingrnrepresentative samples from conventional monitoring or compliance wells or from low permeabilityrnformations and fractured rock. Use of low-flow techniques can minimize purge volumes and decreasernmonitoring costs as well as provide representative and reproducible ground water samples. The use ofrndedicated sampling devices in conjunction with low-flow purging and sampling can eliminate initialrndisturbance into the formation caused by pump insertion and further minimize purge time. Periodicrnmonitoring of the water level within the well should be conducted throughout purging and sampling inrnorder to measure draw-down. The flow rate of the pump should be adjusted to match the ground waterrnentrance velocity into the well screen. Water quality indicator parameters (pH, redox, specificrnconductance, DO, turbidity, temperature) should be monitored during purging to determine when adjacentrnformation water has entered the well screen and sampling can be initiated. Stabilization criteria for waterrnquality indicators is defined as three successive readings, at three minute intervals, within ±0.05 for pH,rn± 5 mV for redox, ± 3% for specific conductance, ± 10% for DO, and ± 10% for turbidity. However,rnsome parameters are more sensitive than others (e.g., redox, DO) and should be evaluated with time overrnseveral sampling events before establishing a sampling protocol.