SHADOW ZONE CAPTURE DURING PURGING AND ITS IMPACT ON CONTAMINANT CONCENTRATION AVERAGING IN MONITORING WELLS

Although it is general practice to purge monitoring wells prior to sampling, the need for purging has been under debate for several years. The debate has been fueled by empirical studies that show the concentrations of volatiles collected prior to purging are about equal to those obtained following purging. These results are contrary to that of other field and modeling studies that show concentration averaging during purging and sampling could result in orders of magnitude differences between samples collected before and after purging. This study was performed to further investigate whether purging is necessary to sample a typical monitoring well. Unlike previous studies, here purging and sampling tests were conducted in one well over a period of 14 months to eliminate variance associated with different hydrogeologic conditions. It also involved examining a non-volatile, conservative constituent (chloride) in addition to a VOC (MTBE) to help eliminate constituent bias. Furthermore, a cluster of multilevel samplers upgradient allowed for an assessment of sampling results in consideration of flow to the well under ambient and pumping conditions, vertical concentration gradients, and concentration averaging in the well bore. The results of sampling without purging and with purging by a bailer and by low flow pumping were compared. No statistical differences were found. Also, the concentrations observed were close to calculated concentration averages. The lack of differences is likely due to drawing back down gradient water during purging that had passed through and had been averaged within the well bore between sampling rounds ("shadow zone"). Based on modeling, drawing the shadow zone back into the well during purging and sampling can dramatically moderate concentration averaging biases that result from vertical variations in concentration and hydraulic conductivity in the formation outside the well, water levels achieved in the well during purging and, perhaps, vertical borehole flow.