If a low-flow pump retrieves a representative sample from the entire screen zone, how can you collect a sample from a discrete interval?

That is a great question. Getting a discrete sample from a well screen zone under pumping conditions is virtually impossible, but this was one of the things we always believed we could do in the past. Research has shown that once you turn a pump on in the well, even at a very low flow rate, you start pulling water from all parts of the screen zone where water can flow in. So, the only way to get a discrete sample from a specific zone within the well screen is to use a passive or no-purge sampling device. Now, having said that, there is another consideration and that is the potential for vertical ambient flow within the borehole. If you have vertical ambient flow, you have some mixing occurring and potential redistribution of the contaminant mass. So the only way to really collect a discrete sample within a screen zone is to do it under non-pumped conditions, using devices like passive and no-purge samplers. But you will want to do some research in the borehole to determine if you have ambient vertical flow and mixing and whether you are truly getting a discrete sample.