How do we recognize similar conditions and stratified sedimentary settings that could affect the implementation of low flow sampling?

If you are using low-flow purging and sampling, in essence you are changing the head across into the screen, forcing water to flow in anywhere it can flow while being controlled by the geology, so vertical ambient flow becomes less of an issue as we purge the well.  Now, if the vertical ambient flow effect is very significant, the re-distribution of the contaminant mass within the well screen can be significant as well.  There can be either upward or downward movement of the contaminant mass caused by the well screen, when in fact the actual position of that contaminant mass in the formation is above or below the point that we are measuring in the well screen.  Vertical ambient flow is less of a factor in low-flow purging and sampling than it is for using methodologies that don’t change the head within the screen by purging.  So when using no purge or passive devices, it becomes critically important to know something about whether or not there is vertical ambient flow, but that is less important when we are using any methodology where we are changing the head within screen. There is a lot of work being published by Elci/Fred Molz about the effective, ambient vertical flow on sampling results and a number of other things that are related to contaminant investigations.