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| Resource Library > Technology Transfer > Programs and Initiatives > Diffusion Samplers > When To Avoid |
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| Diffusion Samplers: When to Avoid |
Diffusion samplers are not appropriate for all compounds or in all applications. Situations where diffusion sampling should be avoided include the following:
- Groundwater contains target analytes not favorable to diffusion sampling - Compounds not currently appropriate for diffusion sampling include:
- Inorganic ions
- Metals
- Organic compounds with high aqueous solubility, such as methyl ethyl ketone (MEK), acetone, or methyl-tert-butyl ether (MTBE)
- Semivolatile organic compounds
- Polychlorinated biphenyls (PCBs)
- Pesticides
- Initial site investigations - Diffusion samplers should only be used for sampling compounds which have been shown to be favorable to diffusion sampling. As such, diffusion sampling should not be used for initial site investigations where the composition of the groundwater is not fully characterized. Initial site investigations should use conventional groundwater sampling methods to determine if the analytes of concern are specific VOCs favorable to diffusion sampling.
- Little or no groundwater movement - Diffusion samplers rely on the free movement of water through the well screen. In situations where groundwater flows horizontally through the well screen, the VOC concentrations in the open interval of the well are probably representative of the aquifer water in the adjacent formation.
- Where the well screen material is less permeable than the aquifer - If the well screen is less permeable than the aquifer or the sandpack than under nonpumping conditions, flow lines may be diverted around the screen. In this case, the VOC concentrations in the diffusion samplers may not represent concentrations in the formation water because of inadequate exchange across the well screen.
- Groundwater temperatures less than 10 degrees Celsius - The equilibrium time between VOC concentrations in the groundwater and in the sampler are temperature dependent. Although many VOCs will equilibrate within 48 to 72 hours at 10 degrees C and above, there is currently no data on equilibrium times below 10 degrees C. Equilibrium times will increase with decreasing groundwater temperatures. Field testing may be required to demonstrate that a representative sample can be obtained at the groundwater temperatures present at a particular site.
- Instantaneous or grab sampling - Diffusion samplers integrate concentrations over time. At a minimum, VOC concentrations within the diffusion sampler will equilibrate with groundwater concentrations within 48 to 72 hours. However, the recommended minimum equilibration period is 14 days. This may be a limitation if the goal of sampling is to collect a representative sample at a point of time in an aquifer where VOC concentrations substantially change more rapidly than the samplers equilibrate.
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