What is considered unacceptable drawdown with low-flow purging?
What is considered unacceptable drawdown with low-flow purging? And, what happens if you cannot purge slow enough to meet this requirement?
3/10 of a foot or 1/10 of a meter seems to have carried over from some early guidance documents, but it is really not a limit. Unfortunately, it does still exist in some cases. But, basically an unacceptable drawdown would be essentially dewatering the well.
If the water level is above the screen zone before you start purging, the goal is to try to get the water level to stabilize and, if at all possible, not dewater the screen zone. But if the well is a low producer, you can drop down a few feet into the screen zone provided that you maintain steady state pumping conditions. You will get a representative sample as you continue to purge until parameters are stabilized.
If you can't actually purge the well slow enough to get the drawdown to stop, you will want to look at an alternative method. You probably need to use a passive or no-purge approach.
3/10 of a foot or 1/10 of a meter seems to have carried over from some early guidance documents, but it is really not a limit. Unfortunately, it does still exist in some cases. But, basically an unacceptable drawdown would be essentially dewatering the well.
If the water level is above the screen zone before you start purging, the goal is to try to get the water level to stabilize and, if at all possible, not dewater the screen zone. But if the well is a low producer, you can drop down a few feet into the screen zone provided that you maintain steady state pumping conditions. You will get a representative sample as you continue to purge until parameters are stabilized.
If you can't actually purge the well slow enough to get the drawdown to stop, you will want to look at an alternative method. You probably need to use a passive or no-purge approach.