What if your wells are low-yield and you cannot get the water level to stabilize even at a very low flow rate, or you have a 5-foot drawdown at a well with a 10-foot column of water, can low-flow sampling still be used?

In this instance you are going to have to use a passive or no-purge method. Or you can use a minimum purge sampling approach where you purge out just a volume of the pump tubing and collect samples from there. Under those conditions, you will be better off collecting water samples from the standing water column then you would be from evacuating that well. Now, if you have a 10-foot water column and 5 feet of drawdown, as long as you can get the water level to stabilize at that 5 feet of drawdown, you can continue pumping until indicated parameters are stabilized and still use the low-flow approach.

 

Many monitoring well screens are not submerged from the start, or in some cases the water level was initially above the well screen when it was installed, but because of long term drought or seasonal changes in water level now part of the screen is exposed. But, all you need to be concerned about is steady state flow conditions, so even if you have 20, 30, or even 40 percent of the screen being drawn down as long as you can maintain a constant pumping water level, use the low-flow sample approach.  However, if the water level won't stabilize, low-flow purging and sampling is not really a viable choice, and you will need to either use a passive or no-purge approach, or a minimum purge sampling method where you purge out one to two times the volume of the pump and tubing and then collect the samples.