PASSIVE DIFFUSION GROUND-WATER SAMPLERS: APPLICATION IN HETEROGENEOUS SYSTEMS

Passive diffusion (PD) ground-water samplers offer several potential technical and cost-related advantages; consequently, they are used increasingly in long-term ground-water monitoring programs at contaminated sites. However, there are inherent differences between PD sampling and traditional purge-and-sample methods, and relatively few field data collected with PD samplers have been presented and discussed in the literature. Additionally, several important application issues, such as sampler equilibration time, have not been thoroughly analyzed. In this paper we present the results of pilot study at a heterogeneous site where VOCs in groundwater were characterized with PD samplers, low-flow sampling, and purge-and sample methods. Unlike conventional sampling methods, PD samplers measure discrete intervals and were able to characterize the vertical concentration profile across the well-screen interval. At some locations, order-of-magnitude variations in concentration were observed within a monitoring well over a 0.6-m vertical interval. We show how PD samplers can be used in tandem with traditional purge-and-sample methods to characterize plume stratification and identify zones of high chemical flux. Additionally, we present a numerical solution that can be used to determine PD sampler equilibration time when well concentrations vary temporally. For most sampler configurations, equilibration time is relatively short (several days), and therefore, PD sampler measurements represent nearly discrete-time measurements.