Passive Diffusion Bag Samplers

Description

Groundwater sampling for volatile organic compounds (VOCs) has the potential to lose volatiles during collection. A technology called passive diffusion bag samplers (PDBs) helps to eliminate this problem. The bag is made of low-density polyethylene (LDPE), which acts as a semi-permeable membrane that allows most chlorinated VOCs to pass through it. The membrane is in the shape of a sealed, long cylindrical bag, filled with de-ionized water. Volatile organic compounds in the groundwater diffuse across the membrane into the de-ionized water in the bag until equilibrium is established between the diffused volatile organic compounds in the groundwater and in the de-ionized water. The PDBs take about 2 weeks to equilibrate in the aquifer. Once this equilibration has occurred, sample recovery takes place.

On the outside of the PDB sampler, a low-density polyethylene mesh is sometimes used to protect against abrasion in open boreholes and as a means of attachment at the prescribed depth. The PDB sampler can be positioned at the target horizon by attachment to a weighted line or by attachment to a fixed pipe.

PDB samplers are relatively inexpensive and sample recovery is rapid.

Limitations and Concerns

In order to prevent cross contamination, PDB samplers should not contact non-aqueous phase liquid (NAPL) during deployment or retrieval.

PDB samplers provide measurements of concentrations over a given time frame. If the goal of sampling is to collect a representative sample at a time when VOC-concentrations change rapidly, this goal may not be met.

Tests on methyl-tertiary-butyl ether (MTBE), acetone and most semi-volatile organic compounds have not yielded accurate results. Additionally, test data on semi-volatile compounds in contact with PDB samplers showed a higher concentration of phthalates (compounds found in plastics to make them flexible) inside the PDB sampler than outside the PDB sampler, suggesting that the polyethylene may contribute phthalates to the enclosed water. Thus, the samplers should not be used to sample for phthalates.

Guidelines for equilibration times and the applicability of PDB samplers in low-permeability formations have not yet been established. Therefore, in such situations, a side-by-side comparison of PDB samplers and conventional sampling methodology is advisable to ensure that the PDB samplers do not underestimate concentrations obtained by the conventional method.

The effects of long-term (greater than 1 month) PDB-sampler deployment on sampler and sample integrity have not yet been thoroughly tested for a broad range of compounds and concentrations. In some environments, development of a bio-film on the polyethylene may result from long-term deployment.

The effectiveness of the use of a single PDB sampler in a well is dependent on the assumption that there is horizontal flow through the well screen and that the quality of the water is representative of the groundwater in the aquifer directly adjacent to the screen. Using multiple PDB samplers within a well may be more appropriate for sampling if any of the following conditions exist: there is vertical flow in the well; there are multiple intervals of the formation contributing to flow; or concentrations of VOCs vary vertically within the well.

Applicability

The PDB is used to monitor groundwater contaminated with VOCs. Typical applications are for long term monitoring and delineating contaminant stratification.

Technology Development Status

The technology is commercially available.

Web Links

http://www.itrcweb.org/DSP-1BothPartsA.pdf

http://www.itrcweb.org/DSP-1BothPartsB.pdf

http://www.caslab.com/products/dif/

Other Resources and Demonstrations

A current list of vendors and PDB-sampler construction details can be obtained from the U.S. Geological Survey Technology Transfer Enterprise Office, Mail Stop 211, National Center, 12201 Sunrise Valley Drive, Reston, Virginia 20192 (telephone 703-648-4344; fax 703-648-4408).

Also see http://diffusionsampler.itrcweb.org/common/default.asp for a database on PDB sampler use nationwide.