How bad is it if you dewater the well with a low-flow system?

If you dewater the well and the sand pack, you radically change the conditions: you will see an upward shift in pH and have the potential for gas exchange. So, how bad is it?  Well, it’s unfortunate, and it’s just not the best sample you can take. If you dewater the well, sample, and then recover, it’s no worse than anything that you have done in the past, but you have to recognize that limitation. If you know that a well readily dewaters, I would highly recommend changing over to a method where you are sampling the water that’s standing in the water column using a minimum purge approach or a passive/no-purge approach. Now, something to think about here, if you are watching the water level drawdown during purging, I would recommend stopping at that point, don’t keep going until you’ve dewatered the well, but instead, now indicate that water level stabilization could not be obtained, then turn the flow rate down very low, collect samples from the water that’s present in the well, and indicate you did so. Those samples are going to be better than the samples you would get after that well recovers. If you completely dewater the well and let it recover, you are going to see far more of an impact on sample chemistry than you would if you partially purge the well or at least purge the pump tubing volume and collect those samples. It’s better to keep an eye on the water level during purging and make sure you cut your losses, stop the pump before you go too far into the process and sample using the minimum purge approach.