Introduction

Diffusion samplers (DSs) have been used since at least the 1990s to sample volatile organic compounds (VOCs) in ground-water [1,2]. DSs can be advantageous for sampling VOCs in ground-water primarily because they have the potential to reduce costs substantially compared with pumping approaches to well sampling. The depth-specific characteristic of the samples can also be advantageous in certain investigations. In general, the types of DSs used to examine VOCs in ground-water can be divided into sorption devices and equilibrium DSs. Sorptive devices are discussed briefly in the following paragraph; however, this chapter concentrates on equilibrium DSs.

Sorption devices for measuring VOC concentrations in ground-water typically consist of a semi-permeable membrane enclosing a sorptive medium, such as hydrophobic carbonaceous resins or polymeric resins. In this type of sampler, dissolved VOCs partition into a vapor phase in order to cross the hydrophobic membrane (examples include low-density polyethylene (LDPE) and Gore-Tex®1) to the internal sorption material in an air space. A simple example consists of an air-filled glass vial containing a wire coated with activated carbon. The vial is enclosed in a plastic zip-lock bag and buried in the bottom sediment in a pond. This type of inexpensive DS has been used successfully to map the zone of VOC-contaminated ground-water beneath a tidal pond where the contaminated ground-water was discharging to surface-water [1]. Because sorptive-type samplers continue to sorb analytes until the sorptive xThe use of trade names does not imply endorsement by the U.S. Geological Survey.

Comprehensive Analytical Chemistry 48

R. Greenwood, G. Mills and B. Vrana (Editors)

Volume 48 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)48013-2

© 2007 Elsevier B.V. All rights reserved. 295

medium is saturated, the samplers are highly sensitive, and some anal-ytes can be measured to the parts per trillion level.

Equilibrium-type DSs typically consist of a closed receptacle composed of a semi-permeable or permeable membrane containing vapor or water free of the target analytes. When these types of DSs are deployed in VOC-contaminated water, equilibrium begins to develop between VOC concentrations in the ambient water and in the water or air of the DS. Once the VOC concentrations attain equilibrium, the VOC concentrations within the DS maintain equilibrium with the concentrations in the ambient water and can be used to track changes in the ambient water [3]. The equilibrated DS can be recovered, and the sample can either be sealed in the DS or transferred to sealable sample vials, depending on specific sampler requirements.

An equilibration time of 1-7 days is typical for equilibrium-based membranes [4-7]. To allow the deployment disturbance to dissipate, however, most equilibrium-based samplers are usually left in place for at least 2 weeks prior to recovery. In poorly permeable sediments, longer times may be required [8].

Semi-permeable or permeable membranes used for this application include LDPE [9,10], regenerated cellulose [11], polysulfone [12], silicone polycarbonate [13], porous polyethylene [14,15], and others. LDPE membranes have been tested extensively for monitoring VOCs in ground-water at wells and at the ground-water/surface-water interface [9,16-20]. The pore size of LDPE membranes is about 1nm. The samplers constructed from these membranes for use in ground-water applications typically are either a water-filled LDPE lay-flat tube sealed at both ends, or a vapor-filled vial with the membrane covering one end. The water-filled samplers are called polyethylene or passive diffusion bag (PDB) samplers and typically hold about 220-350 mL (Fig. 13.1). The vapor-filled vials are called polyethylene or passive vapor diffusion (PVD) samplers, and typically are constructed of an open 20 mL vial containing air and sealed in two LDPE bags (Fig. 13.2). The samplers are deployed at the target horizon in a well or in stream-bottom sediment and allowed to equilibrate. During the equilibration period (typically about 2 weeks), aquifer VOCs diffuse through the LDPE membrane and into the sampler in accordance with the principles of Fickian diffusion. Fick's first law states that the flux of solute movement is directly proportional to the concentration gradient and inversely proportional to the resistances to flow. At the end of the deployment period, water-filled samplers contain the same dissolved-phase VOC concentrations as the ambient water, and

Fig. 13.1. Typical water-filled polyethylene diffusion bag (PDB) samplers used in wells, including (A) a diffusion bag with polyethylene protective mesh, (B) a diffusion bag without mesh, and (C) a bag and mesh attached to a bailer bottom.

Fig. 13.2. Passive vapor diffusion (PVD) sampler showing (A) an empty vial, (B) a completed sampler consisting of the open vial enclosed in two polyethylene bags and attached to a survey flag, and (C) a crimp cap.

vapor-filled samplers contain headspace concentrations in equilibrium with ambient water.

The polyethylene samplers are not permeable to water, but are permeable to many VOCs that constitute some of the most common priority contaminants. These VOCs include chlorinated ethenes and ethanes, benzene, toluene, ethylbenzene, and others [10]. The samplers are also suitable for monitoring helium, neon, hydrogen, oxygen, and nitrogen [6]. Some very soluble VOCs, such as methyl-tert-butyl ether (MTBE) do not move through the LDPE rapidly enough to be effective target anal-ytes. Polar molecules, such as most inorganic solutes, do not diffuse through the membrane. In some cases when sampling for VOCs, this may be an advantage. For example, PDB samplers may be particularly practical in carbonate environments where alkalinity can be high enough to cause effervescence when an acid preservative is added. The effervescence results in loss of VOCs by volatilization. Collection of VOCs without addition of acid preservative prevents volatilization loss, but significantly shortens the sample holding time. An investigation by Vroblesky and Pravecek [21] in an alkaline aquifer in Hawaii showed that the PDB sampler membranes transmitted VOCs, but not alkalinity. Therefore, they collect VOCs in a non-alkaline matrix and allow for the addition of an acid preservative even when sampling from a highly alkaline aquifer. Technical and regulatory guidance documents are available for monitoring VOCs with the PDB [10,21,22] and the PVD [18] samplers.

Regenerated cellulose dialysis samplers are equilibrium-based DSs that have been used in a variety of investigations for VOCs and inorganic constituents in ground-water [2,7,11,23]. The nominal pore size of cellulose-based membranes used for DSs in environmental studies for VOCs ranges from about 1.8nm [7,11,23] to about 4.0nm [24,25]. Cellulose-based membranes tend to biodegrade once deployed [23,26] so the sample deployment time needs to be minimized. Harter and Talozi [27] found that regenerated cellulose DSs could be deployed in warm, bioactive ground-water monitoring wells with sufficient ambient advective exchange for up to 4 days without being compromised by biodegradation.

Rigid porous polyethylene samplers (RPPSs) are equilibrium-based devices that have been used to sample VOCs in wells and also are capable of sampling inorganic constituents. RPPSs are porous polyethylene tubes filled with water. Pore size is approximately 6-15 mm. A field study in 2004 of wells showed that the RPPS produced VOC and 1,4-dioxane concentrations similar to low-flow sampling results [15]. A recent laboratory study of RPPSs showed that they produced accurate concentrations of most VOCs [14].

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