The parameters are stable but is below the recommended purge rate. Will a sample be representative?
The U.S. EPA recommends a purge rate between 100 to 300 milliliter per minute for stabilization. However, there is a site where the water level keeps dropping after 45 minutes down to 15 feet. But, if the rate is slowed to 50 milliliter per minute, the water level finally starts to level up.
Yes, the sample will be representative. There really is no lower limit for purging, it is really just whatever you can tolerate. We have actually purged wells as low as 5 to 15 milliliter per minute just to see what results we would get. Now, the purging time was really extended – an hour and a half to two hours of purging time and this was done specifically for research. However, on a practical basis, a good example would be the Texas landfill site that we worked on many years ago where the purging rates in most of the wells were between 50 to 80 milliliters per minute. We had to purge anywhere from 45 to 90 minutes per well in order to get stabilization. But we were able to get representative samples without the problems that we had previously associated with turbidity in these wells. These wells were properly constructed, adequately developed to a degree that they could be and they were set in a very fine grained clay material that had very low K. They wouldn’t produce a lot of water and it was virtually impossible to get all the solids out of the sand in the formation around it. So the answer to your question is even purging at 50 mL a minute, if you can get your water leveled off and monitor your parameters for stabilization, you have a representative sample.
Yes, the sample will be representative. There really is no lower limit for purging, it is really just whatever you can tolerate. We have actually purged wells as low as 5 to 15 milliliter per minute just to see what results we would get. Now, the purging time was really extended – an hour and a half to two hours of purging time and this was done specifically for research. However, on a practical basis, a good example would be the Texas landfill site that we worked on many years ago where the purging rates in most of the wells were between 50 to 80 milliliters per minute. We had to purge anywhere from 45 to 90 minutes per well in order to get stabilization. But we were able to get representative samples without the problems that we had previously associated with turbidity in these wells. These wells were properly constructed, adequately developed to a degree that they could be and they were set in a very fine grained clay material that had very low K. They wouldn’t produce a lot of water and it was virtually impossible to get all the solids out of the sand in the formation around it. So the answer to your question is even purging at 50 mL a minute, if you can get your water leveled off and monitor your parameters for stabilization, you have a representative sample.