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| Resource Library > Technology Transfer > Programs and Initiatives > Diffusion Samplers > Designs |
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| Diffusion Samplers: Design |
A variety of diffusion sampler configurations have been utilized in well applications. The earliest versions were simply re-sealable polyethylene sandwich bags (Vroblesky and Hyde, 1997). Currently, a typical diffusion sampler consists of a 12-18 inch section of 4-mil low-density polyethylene (LDPE) lay-flat tubing, heat-sealed at both ends, and filled with laboratory-grade deionized water (see figure). The typical diameter of a diffusion sampler is about 2 inches, however, other dimensions may be used to match the well diameter. Often, the samplers are placed in "flex-guard" low-density polyethylene mesh tubing for structural support and protection against abrasion and attached to a weighted rope with nylon cable ties.
Diffusion Sampler Bags
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| Source: Vroblesky, D.V., 2001 |
- Sampler with protective mesh
- Sampler without mesh
- Sampler with attachment loop
The U.S. Geological Survey (USGS) and General Electric Company (GE) have co-patented the diffusion sampler design described above as U.S. Patent # 5,804,743 - Passive Diffusion Bag Sampler. For all non-Federal uses, the user must purchase commercially produced samplers from a licensed manufacturer or purchase a non-exclusive license for sampler construction from the USGS Technology Enterprise Office. Licensing is not required for construction of samplers to be deployed at Federal installations. Authorized distributors as of February 2001 were Columbia Analytical Services (206 824-8933) and Eon Products (800 474-2490).
Applicable Contaminants
Diffusion samplers have been investigated in laboratory and/or field studies (Vroblesky and Hyde, 1997; Vroblesky, D.A., and Campbell, T.R., in press) and found to be applicable for a number of water-soluble, low-molecular weight volatile organic compounds (VOCs), including TCE, DCE, DCA, and others . Numerous other VOCs are being tested and may prove to be appropriate for sampling with diffusion samplers. The diffusion sampler method is not currently appropriate for sampling inorganic ions, organic compounds with high aqueous solubility, such as methyl ethyl ketone (MEK), acetone, or methyl-tert-butyl ether (MTBE), semi-volatile organic compounds (SVOCs), pesticides, or polychlorinated biphenyls (PCBs).
Deployment and Equilibration Times
A variety of approaches can be used to deploy the diffusion samplers in wells. A typical approach, is to attach the samplers to a weighted line. Weights may be attached directly to the sampler if the attachment point is sufficiently strong. The weights used are stainless steel and must be thoroughly decontaminated before use or reuse.
The sampler is lowered into the well and secured at the proper depth via a rope or wire. Materials used include polyester rope, stainless-steel wire-braid, or Teflon-coated stainless-steel wire. Coated stainless-steel wire is preferred and can be reused, if decontaminated. To prevent cross contamination, rope and uncoated stainless-steel should be single-use only.
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| Source: Vroblesky, D.V., 2001 |
Samplers in Monitoring Well
In many cases, one diffusion sampler placed at the midpoint of the well screen may be sufficient to collect a sample that is representative of water in the entire well or screened interval. Diffusion samplers may also be placed at discrete depth intervals to develop a vertical profile of the VOC contamination along length of the well screen and identify specific geologic units or strata, if any, that may be contributing the highest concentrations. This methodology eliminates the potential for collecting samples that represent an integration of different water types (i.e., contaminant mass averaging). In addition, once the vertical profile of the well contamination is defined, a sampling program can be developed that monitors only the geologic units that contribute the highest VOC concentrations. Previous field investigations of the methodology showed a close correlation between VOC concentrations in water obtained using the samplers without prior purging and concentrations in water samples obtained from the respective wells using traditional purging and sampling approaches (Vroblesky and Hyde, 1997; Vroblesky et al., 1999).
The VOC concentrations within diffusion samplers equilibrate with the surrounding volatile organic compound concentrations under laboratory conditions within approximately 48 hours for several VOCs, and 96 and 168 hours for vinyl chloride and some chloroethanes. However, current field practice is to allow at least 2 weeks of equilibration in most wells screened in aquifers consisting of fine- to medium-grained sands, fine silty sands, and weathered fractured rocks. In less permeable geologic units, longer equilibration times may be required. Following the initial equilibration period, the samplers can be left in place until recovery (3 months or more).
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