Passive Diffusion Devices & Polyethylene Diffusion Bag (PDB) Samplers
Passive Diffusion Samplers are generating a lot of interest in the environmental community for four main reasons:
•Water samples are more representative of the area adjacent to the well screen or area of interest than those taken by other conventional means.
•Well purging is not required.
•They are simple to deploy.
•They are inexpensive when compared to other sampling techniques.
The principle behind passive diffusion is that compounds will migrate or diffuse through a semi-permeable membrane until a concentration equilibrium is established on either side of the membrane. The following three passive devices are currently commercially available or will be soon.
Polyethylene Diffusion Bag (PDB) Samplers
Columbia Analytical is a licensed manufacturer and supplier of the polyethylene diffusion samplers used to collect groundwater samples for long term monitoring of volatile organic compounds (VOCs). Most common halogenated and aromatic VOCs-of-concern readily diffuse through polyethylene film and equilibrate in as little as 24 hours. To insure equilibration, 14 days is the recommended minimum deployment period.
Columbia Analytical’s PDB Samplers are made of thin low-density polyethylene (LDPE) film seamed along the sides to prevent abrasion. They are available in diameters that accommodate most monitoring wells (¾” to 4”) and in lengths needed to supply the volume of sample (200-300ml) for analysis of VOCs and the attendant QA/QC. Lengths of 24 inches or less are routinely ordered for the common two inch diameter well to assure strata specific sampling within a well screen. Other lengths and widths are available. Hanging assemblies may be customized for each well based on the client’s placement specifications. Wells that exhibit stratification can be monitored easily and cost effectively by deploying PDB Samplers in tandem (see figure at right).
The most commonly supplied PDB Samplers come field-ready, i.e., filled in the laboratory with certified analyte-free water, sealed and ready for deployment. Alternately, Columbia Analytical can supply field-filled samplers that are similar in design to the field-ready but have a cap for filling and pouring.
Validation of PDB Sampler
Since the introduction of the samplers in 1998, they have undergone hundreds of side-by-side comparisons with traditional purge procedures. Through the multi-state/multi-federal agency and industrial organization the Interstate Technical Regulatory Council (ITRC) Diffusion Sampler Team, many position papers, technical documents, and studies have been collected and produced concerning applicability of PDB Samplers technology at a wide variety of sites. This organization has stated that samples taken through the use of PDB Samplers are as representative as those taken by other conventional sampling techniques. However, certain VOCs, semivolatiles and inorganic analytes cannot be sampled by PDB Samplers because of limited diffusion or non-diffusion across the polyethylene membrane. We recommend including your regulatory agency when reviewing the suitability of this sampling technology for your site.
It has been estimated that use of PDB Samplers can decrease your field deployment costs by 60 – 70% over traditional purge and bail or low-flow techniques. A recent cost comparison for McClellan AFB showed a savings of 67% of field and labor costs over conventional purge samples and an almost identical cost savings over micropurge technology. The ITRC website (www.itrcweb.org) has a cost comparison model that will allow you to estimate your potential cost savings.
Passive Vapor Diffusion (PVD) Samplers
Columbia Analytical provides passive diffusion samplers for the collection of air (vapor) samples in the pore water of sediment. These PVD Samplers consist of an empty, uncapped, glass vial enclosed in two layers of LDPE film. When samplers are placed in sediments, concentration equilibrium is reached between VOCs in the pore water and the air in the vial in one to three weeks, depending on the analyte. The concentration of the VOCs in the pore water can be determined from the analysis of the air sample. The relative concentration partitioning into the air and water varies among VOCs, thus the PVD Samplers are designed to be used primarily as screening tools.
Nylon-Screen PDB Sampler
The use of PDB and PVD Samplers is confined to collection of air and water samples for the analysis of VOCs, due to the poor diffusion rates of other contaminants across the polyethylene membrane. However, Columbia Analytical is participating in studies to develop samplers suitable for the collection of groundwater samples for analyses of inorganic, semivolatile and the poorly diffusing VOC analytes. The availability of passive sampling devices for these analytes is becoming critical as more monitoring of natural attenuation occurs at contaminated sites where the treatment of choice is the augmentation of the natural biochemical activity in the soil.
Diffusion samplers using nylon screens have been used to obtain solute concentrations in wells and in pore water for several years. Don Vroblesky, of the USGS, started research in 2002 for AFCEE and NFESC on simplification of nylon screen samplers to obtain larger sample volumes and to examine their applicability for some emergent contaminants. Columbia Analytical is participating in these studies and in January of 2003 began laboratory studies on a limited suite of inorganics, semivolatiles and VOCs (Benzene, TCE, Perchlorates, 1,4-Dioxane, Arsenic, Iron, Chloride and MTBE) to determine if these contaminants diffuse sufficiently across a nylon-screen and, if so, how long before equilibrium is established. Don Vroblesky reported the results as favorable to the ITRC Diffusion Team in March and August of this year. Additional studies are under way to determine the optimum design of a sampler suitable for field trials.