Frequently Asked Questions (FAQ)


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

Q1:  When comparing passive sampling methods to traditional purge methods, how it is determined that the passive sampling groundwater data produces more reliable results than the traditional sampling groundwater data?

Each groundwater sampling technique characterizes contamination in the groundwater differently. It is important to understand the conceptual basis of any sampling technology since results from each method may differ. These differences should be considered when comparing sampling methods for interpreting sampling results. Differences may occur when comparing 3-volume purge, low-flow or passive sampling techniques. These differences do not necessarily indicate inaccuracies, but reflect the nature of the sampling methods.

When comparing passive groundwater data with data collected using other types of sampling devices (even other types of passive sampling devices), the team found that the more variables introduced into your sampling technique, the less reliable the data. When compared to passive sampling methods, traditional groundwater sampling methods introduce more variables such as where your groundwater sample is being collected (i.e., biased toward zone of highest hydraulic conductivity), increased turbidity concern since increase disturbance to the well, and possible unusable data due to cross contamination from well to well. Passive sampling methods require minimal handling procedures and equipment needs; therefore, reduce these errors.

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Q2: In the ITRC DSP-5 training and document, the Team states that passive samplers have "reproducible" results. What margin of error should one expect from the samplers to conclude that I am getting high reproducibility?

When performing ongoing sampling events, it is critical to place the passive sampler in the same location or depth for consistency and comparability of results over time.  Sampling at a consistent deployment depth can improve data reproducibility.  The Team expects high reliability or reproducibility of groundwater data if for each sampling event you are collecting groundwater with the same device at the same interval using the same sampling methodology. At this time, the Team has not designated a "margin of error" associated with reproducibility.  In general, the Team agrees that in a well that has a low temporal variability, a 1:1 correlation is expected if using the same sampling device with the same methodology.

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Q3: What are the costs associated with each device?

The Team has put together a Matrix Table that itemizes costs for each of the samplers that are discussed in the DSP-5 document.  This table is included in the Team's DSP-4 document "Overview of Passive Sampling Devices", as Chapter 14, page 83.  The document can be downloaded at http://www.itrcweb.org/guidancedocument.asp?TID=12 .

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Q4: How does groundwater pH (high and low values) affect the integrity of the diffusion-type sampling devices?

The Team has had no reports of any negative effect of high or low groundwater pHs on Polyethylene Diffusion Bags (PDBs), Rigid Porous Polyethylene (RPPs) samplers, or Regenerated Cellulose Dialysis Membrane (Dialysis) samplers (i.e., diffusion-type sampling devices).  Polyethylene materials in general are resistant to attack by both acidic and basic solutions.  PDBs, RPP samplers and Dialysis samplers have been used to successfully sample ground waters and acidified laboratory test solutions with pHs ranging from 2 to 11 after equilibration periods of up to 4 weeks without any compromise of sampler integrity. 

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