REPRODUCIBLE WELL-PURGING PROCEDURES AND VOC STABILIZATION CRITERIA FOR GROUNDWATER SAMPLING

Recent research has confirmed that low flow rate purging (i.e., approximately 11/min) is a valid technique for 2'' (5 cm) diameter monitoring wells with short screened intervals. The use of low flow, dedicated pumping devices for purging and sampling minimizes both the disturbance of stagnant water in the well casing and the potential for mobilization of particulate or colloidal matter which could lead to sampling artifacts. In addition, these techniques allow the use of purging indicator parameters (e.g., dissolved oxygen and specific conductance) to determine when to collect a sample for volatile organic compound (VOC) determinations. Detailed results from a monitoring network in a sand and gravel aquifer contaminated by organochlorine solvents support these findings. Recent publications on monitoring well hydraulic behavior and considerations of mass averaging effects due to pumping also support the use of low flow rate, minimal drawdown purging procedures to achieve reproducible sampling results. The suggested procedure includes documenting purging indicator parameters while purging with dedicated devices at low flow rates with minimal drawdown. This sampling method is less time-consuming and reduces the need to handle large volumes of purge water since VOC concentrations, O2, and specific conductance values stabilized consistently in less than one bore volume.