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The diffusion
sampling technique relies on natural molecular diffusion to cause the molecules from
volatile organic compounds to pass from the groundwater through a semi-permeable 42mm
diameter sampler which is pre-filled with deionized water. After sufficient time has
elapsed the concentrations in the sampler approximate those in the well. The technician
simply removes one sampler and replaces it with another for the next quarter. No bailing
or purging, minimizing on site time. About Diffusion Technology
For years, diffusion technology has been utilized for sampling of VOCs in soil vapor and
surface waters. Recent studies have demonstrated that ground-water samples obtained by
diffusion sampling can also produce results representative of in-situ VOC concentrations
in groundwater. Diffusion samplers utilize a membrane that is filled with water and
suspended in the screened interval of a well until chemical equilibrium occurs. The
sampler is then retrieved and the contents analyzed. Only a few short weeks are required
between deployment and retrieval. It's that simple!
The diffusion sampling method produces significant cost savings over traditional sampling
methods by eliminating field time for well purging, equipment decontamination/maintenance,
and purge water disposal. A paper published in Ground Water Monitoring & Remediation
(Vroblesky and Hyde, Summer 1997) documents the success of the diffusion technology for
groundwater quality analysis in monitoring wells. Call for a FREE copy!
Results of Pilot Testing
Bunne II-Lammons Engineering, Inc. (BLE) has conducted field trials that validate the
results obtained from the EON Diffusion Sampler. BLE's tests showed that where purgeable
halocarbons and aromatics were present in ground water, diffusion sampling provided highly
comparable results with traditional sampling technology. The chart opposite shows a
comparison of VOCs in groundwater samples.
Sampler Placement
Diffusion samplers are placed within the screened interval of a well where groundwater
flow is known to occur. Wells which have long screened intervals or bedrock wells that
intersect known fractures can utilize samplers connected in series to monitor more than
one interval or fracture set. Stainless steel weights are used to position the samplers at
the required depths. Locking caps with a bottom fastener are used to secure the suspension
cord at the well head.
How it works
Simply lower the water filled Diffusion Sampler into the monitoring well. Leave it in
place two or more weeks until molecular diffusion causes chemical equilibrium to occur
between the water in the sampler and the water in the well. The Diffusion Sampler is then
retrieved, and the water sample analyzed for VOCs using traditional laboratory methods. |
