Passive Sampling


Information on this page is courtesy of the ITRC Passive|Diffusion Sampler Team. Analytical testing dots

Passive SamplingA passive groundwater sampler is one able to acquire a sample from a discrete position in a well without active media transport induced by pumping or purge techniques. “Passive sampling” is synonymous with “no-purge sampling.”

Groundwater passive sampling devices:

  • Passive diffusion bags (PDB) / Rigid, porous polyethylene sampler (diffusion/equilibrium-type samplers)
  • Snap Sampler™ (grab-type well water samplers)
  • Hydrasleeve™ (grab-type well water samplers)
  • Regenerated-cellulose dialysis membrane sampler
  • GORE™ Module (a diffusion and sorption–type sampler).

All groundwater samplers or sampling methodologies attempt to collect a sample that is
formation-quality water of the groundwater adjacent to the well. Studies have shown that most
wells receive groundwater flow through the screened interval of the well. This screened interval,
considered in equilibrium with the adjacent groundwater (formation water), can be sampled with
passive samplers with little or no well-water agitation, which can alter the contaminant
concentrations in the sampled water.

Passive samplers, which remain submerged during a deployment period, collect from a discrete
position within a well a sample of water in ambient equilibrium with adjacent groundwater.

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Advantages of passive samplers:

  • relatively easy to use
  • can be deployed in most wells
  • practical for use where access is difficult or where discretion is desirable
  • can sample discrete intervals in a well
  • can be deployed in series to provide a vertical contaminant profile
  • have no depth limit
  • reduce field sampling variability, resulting in highly reproducible data
  • allow rapid field sample collection
  • decrease field labor and project management costs for long-term monitoring
  • eliminate purge-water production and thus all or most disposal cost.

 

Disadvantages of passive samplers:

  • must be submerged in the screened interval during deployment
  • require the aquifer to be in hydraulic communication with the screened portion of the well;
  • require special consideration in wells having a layer of free product;
  • may have volume/analyte limitations;
  • require consideration of contaminant stratification.

 

Passive sampling retrievalNot all well water is thoroughly mixed within the screened interval. Passive samplers can be deployed at any location within the screened interval to evaluate the highest or lowest contaminant concentration in a stratified-flow screened interval. Deployed in a series within a screened interval, passive samplers can provide a contaminant concentration profile of the screened interval.

According to 17 states responding to a questionnaire, there are no specific regulatory barriers to using passive samplers to collect groundwater samples. There is, however, guidance that specifically requires purge-type sampling, thereby requiring passive samplers to obtain an exception when used. However, most states require some sort of comparative study if passive samplers are intended to replace an existing sampling program.

Analytical testing dots3 categories of passive sampler devices:

1. Devices that rely on diffusion and sorption to accumulate analytes in the sampler.
Samples are a time-integrated representation of conditions at the sampling point over the
entire deployment period. The accumulated mass and duration of deployment are used to
calculate analyte concentrations in the sampled medium.

  • GORE Module

2. Devices that recover a grab well water sample. Samples are an instantaneous
representation of conditions at the sampling point at the moment of sample collection.

  • HydraSleeve
  • Snap Sampler

3. Devices that rely on diffusion of analytes across the sampler membrane to reach and
maintain equilibrium with the sampled medium.
Samples are time-weighted toward
conditions at the sampling point during the latter portion of the deployment period. The
degree of weighting depends on analyte- and device-specific diffusion rates.

  • regenerated-cellulose dialysis membrane (dialysis) sampler
  • passive diffusion bags (PDBs)
  • rigid, porous polyethylene (RPP) sampler

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