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
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.