EVALUATION OF PURGING AND SAMPLING BIAS IN CONDUCTING REMEDIATION COMPLIANCE MONITORING

Technical objections to the use of bailers for purging and sample collection for remediation compliance monitoring have led to regulatory adoption of low flow purging protocols. Several empirical studies have concluded that samples obtained without purging provide comparable data to that obtained with purging. As such, it has been argued that purging is not needed. To examine this issue, a field study is being performed at the University of Connecticut Motor Pool, which has a history of gasoline contamination and remediation. The main objectives of this study are: 1) to determine if there are significant differences between samples that are collected without purging, after purging with a bailer, and using low flow methods; 2) to determine which of the three methods provides data that is most representative of the water in the formation; 3) to determine if the pre-purged, vertical concentration distribution in the well bore mimics that in the formation (as determined by multi-level samplers); 4) to determine whether or not the water in the well is thoroughly mixed following purging; and 5) to determine if a sample obtained by low flow methods is representative of an average of the well bore concentration distribution, the vertical average in the formation, or a discrete vertical interval in the formation. At the study site sampling is being performed in two monitoring wells and adjacent clusters of multilevel samplers. After several rounds of sampling, it was found that concentrations obtained for constituents that did not vary with depth were similar irrespective of the sampling protocol. The focus of subsequent sampling rounds was placed on constituents and parameters whose concentration varied with depth (e.g., T, EC, pH, ORP, DI, DO, turbidity, MTBE, Ca, Na and Cl). Amongst the different sampling methods we have found that there are concentration differences for these parameters. Despite concentration differences resulting from different sampling procedures, our data to date suggests similarities in temporal trends. Unlike past studies that are strictly empirical, we are systematically examining the underlying causative mechanisms for these differences, in consideration of the vertical concentration distributions of the above parameters in the well bore and formation, variations in hydraulic head and conductivity with depth, and flow to and from the well.