Passive Sampling Suggestions


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

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Be aware of contaminant stratification

Depending on your Data Quality Objective (DQO), contaminant stratification in monitoring wells might be something that should be considered when using passive sampling devices. Field studies conducted on wells with ten foot screens at the North Island Naval Air Station (San Diego, CA) have shown concentration changes in the hundreds to thousands of parts per billion within a vertical distance of three feet. Other field studies included in Part II of the USGS User’s Guide have reported similar results. This emphasizes the need to properly evaluate the potential for contaminant stratification to be present when deploying passive devices in wells with greater than five feet of screen or open hole.

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Add sufficient weight

Because there is some amount of buoyancy associated with passive samplers, care should be taken to add sufficient weight to the samplers to ensure they remain suspended at the intended target zone within the well.

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Different results do not mean passive sampling devices are wrong

Differences between results from passive sampling devices and other methods do not necessarily mean that the diffusion sampler results are wrong. Differences may result from a variety of factors. The PDB samplers represent approximate point samples (over about 1.5 ft). Low-flow samples represent an approximate point sample in some wells and an integration of water over larger areas in other wells. The area integrated and mixed in low-flow sampling may vary from well to well, and may include the entire screened interval in some wells. Sampling by purging 3 or more casing volumes prior to sample collection mixes and integrates water over an even larger area, sometimes moving water to the well screen from areas not adjacent to the screen. The differences between sampling methods are most probable to be seen in wells containing hydrogeologic or chemical heterogeneity.

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Combining multiple passive samplers and bore-flowmeter

The combination of multiple passive samplers and borehole-flowmeter testing can be effective in determining contaminant inflow points and stratification in wells.

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