Can a pumping rate be too low for purging
Are there any studies or data that suggest the pumping rates should not be to low (ex. <0.1 Lpm). I typically use a peristaltic pump and work in areas of very fine silts and clays. I have found that I can rarely achieve a 0.1 Lpm or higher rate without getting turbidity readings that are too high. On average my rates are about .05 Lpm.
No, there really is no pumping rate that is too low for purging. When you have wells set with very low hydraulic conductivity materials as you do, pumping at rates below 0.1 Lpm is often required to both control turbidity and to maintain a stabilized pumping water level. The double-whammy is that, because of the very low K materials around the well, you can't really accomplish much in the way of well development to reduce fines in the sand pack and adjacent formation, since you can't create much movement of water either through pumping or surging. But, if you're willing to pump slowly until your DO and SpC are stable, this is the best approach.
Use 0.1 Lpm as a 'practical' lower limit for pumping rate when doing low-flow purging. While you could pump much slower than this, the time required to purge can be protracted, and bottle filling can become a nuisance at very low rates if large volumes are needed. Below this rate, use an alternative sampling approach, either "minimum purge" sampling or what is commonly called "passive" or "no-purge" sampling. In any of these methods, the intent is to sample the water in the well screen rather than trying to replace this water with formation water. This is based on the idea that the water within the well screen, while affected by gas exchange and contact with well construction materials, etc., is a better representation of the formation water chemistry near the well, since evacuating the well would result in a significant change in redox conditions, pH, etc. The limitation is that, in many wells, there can be water column mixing that occurs due to density and head differences within the borehole, so sampling the water within the well may not accurately reflect formation water chemistry. It ends up being the "lesser of two evils" approach, since we generally lack any better way to sample existing wells for routine monitoring purposes.
Attached is a short white paper on sampling low-yield wells that might offer some additional details.
No, there really is no pumping rate that is too low for purging. When you have wells set with very low hydraulic conductivity materials as you do, pumping at rates below 0.1 Lpm is often required to both control turbidity and to maintain a stabilized pumping water level. The double-whammy is that, because of the very low K materials around the well, you can't really accomplish much in the way of well development to reduce fines in the sand pack and adjacent formation, since you can't create much movement of water either through pumping or surging. But, if you're willing to pump slowly until your DO and SpC are stable, this is the best approach.
Use 0.1 Lpm as a 'practical' lower limit for pumping rate when doing low-flow purging. While you could pump much slower than this, the time required to purge can be protracted, and bottle filling can become a nuisance at very low rates if large volumes are needed. Below this rate, use an alternative sampling approach, either "minimum purge" sampling or what is commonly called "passive" or "no-purge" sampling. In any of these methods, the intent is to sample the water in the well screen rather than trying to replace this water with formation water. This is based on the idea that the water within the well screen, while affected by gas exchange and contact with well construction materials, etc., is a better representation of the formation water chemistry near the well, since evacuating the well would result in a significant change in redox conditions, pH, etc. The limitation is that, in many wells, there can be water column mixing that occurs due to density and head differences within the borehole, so sampling the water within the well may not accurately reflect formation water chemistry. It ends up being the "lesser of two evils" approach, since we generally lack any better way to sample existing wells for routine monitoring purposes.
Attached is a short white paper on sampling low-yield wells that might offer some additional details.