Passive Diffusion Bag Samplers
Description
Passive Diffussion Bag (PDB) Samplers take advantage of semi-permeable
membrane technology to gather contaminants from groundwater. 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. Other
types of groundwater sampling for volatile organic
compounds (VOCs)
have the potential to lose volatiles during collection.
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.
In low-permeability formations, 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 a more 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.
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.
PDB Samplers are used for characterization, but it is not
recommended that they be used for site close-out.
Temperature of the groundwater must be greater than 50
degrees Fahrenheit.
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.clu-in.org/char/technologies/passdiff.cfm
http://www.itrcweb.org/Documents/DSP-3.pdf
http://www.itrcweb.org/Documents/DSP_4.pdf
http://www.itrcweb.org/Documents/DSP-1a.pdf
http://costperformance.org/pdf/wrir014060.pdf
http://www.ert2.org/PDST/tool.aspx
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.