Use of Passive-Diffusion Bag Samplers to Profile Low-Level Trichloroethene Concentrations in Ground-Water Extraction Wells

Abstract

Using passive-diffusion bag (PDB) samplers as a ground-water monitoring technique has become more popular in recent years as various laboratory and field studies have shown acceptable data-quality objectives (DQOs) can be achieved through their use. The PDB sampler is deployed in an aqueous environment (e.g., well or borehole), where passive diffusion through the semi-permeable membrane of the sampler enables the water quality in the sampler, which is typically filled with deionized water, to equilibrate with that in the surrounding aqueous environment (Harter and Talozi, 2004). The type of membrane determines which solutes are able to pass into the sampler medium. Ehlke and others (2004) and Harter and Talozi (2004) discuss sampler membrane types and factors that affect their use. To obtain representative samples with PDB samplers, ground water must be able to flow through the well screen in sufficient volume to permit solute equilibration in the sampler medium during the sampler deployment period. After initial equilibration is achieved, sampler medium and ambient ground water should maintain equilibrium. However, because this constant re-equilibration process is not instantaneous, the sampler medium represents a time-averaged solute concentration of ground water during the preceding days (depending on the rate of equilibration) (Ehlke et al., 2004). The rate at which the medium within the PDB sampler equilibrates with ambient ground water depends on multiple factors, including the type of compound being sampled, ground-water temperature, contaminant distribution, and ground-water flow dynamics (Vroblesky, 2001). Various compounds more easily diffuse into the sampler medium simply because the molecular sizes of the compounds are smaller than the pore size of the PDB-sampler membrane. Vroblesky (2001) presents a list of volatile organic compounds of environmental interest that have shown effective and ineffective equilibration between the PDB-sampler medium and ambient water under laboratory conditions. As presented in Vroblesky (2001), …