Is there any value of evacuating a well after a low-flow sample has been collected
Is there any value of evacuating a well after a low-flow sample has been collected?
There used to be a guideline actually in the State of California that eventually got removed that was referred to as pre and post sample purging which was kind of an interesting one. I never could quite understand the basis for it I could understand the legal reason for it and that was that if you took a low flow sample from the well or any sample I should say, any purge sample from the well that the next time you go to purging sample of the well you want to make sure you are not sampling the same water. So you would evacuate the well thereby eliminating the possibility that any water that had been sampled previously would be sampled again. It was strictly a legal issue. It was the result of a legal challenge by one waste disposal company to the state’s guidelines on purging and sampling wells. But no, I don’t see any value to evacuating the well after a low flow sample has been taken. Because again if we accept the idea and the argument that passive and no-purge samples would represent the chemistry of the surrounding formation within the well screen zone through either groundwater flow or through diffusion, there should really be no need to remove that water afterwards.
During the purging of high yield wells in confined bedrock aquifer water levels may drop several feet prior to stabilization at rates as low as 300 to 500 ml per minute. Is it of any benefit with regard to the representativeness of the sample to further reduce the rate to a 100 milliliters per minute assuming that the drawdown is less but still several feet.
As long as you don’t have a stated drawdown with it that you have to meet, my inclination is to try and maintain the highest practical flow rate that I can. So if I am starting out at 300 to 500 milliliters per minute and I am getting several feet of drawdown the well provided that my water level then stabilizes very quickly after that I would prefer to stay at that upper limit. My reasoning being that when it comes to filling the sample bottles afterwards if you stick yourself with a 50 or 100 or 150 milliliter per minute flow rate and you have multiple large sample bottles to fill that could be a problem. I was working in a site in Ohio once where they were purging all wells at 200 milliliters a minute. Unfortunately they were filling eight 1 liter amber and four (Inaudible) So it took 20 minutes to purge the wells and an hour and a half to fill the sample bottles. But they weren’t considering any higher flow rate, they just simply used the stated flow rate of 200 because it was in their sampling and analysis plan. When we kick the flow rates up to about 500 mils a minute we had a little bit more drawdown in the wells but lo and behold you could fill at 500 mils a minute now, you could fill those 1 liter amber in just over two minutes. So I would say that provided you don’t have a stated limit for drawdown, I would lean toward the higher flow rate and the greater drawdown as long as I get my pumping water level to stabilize. If you have continual drawdown that doesn’t seem to be slowing down then you have no choice but to back off on the flow rate at some point because you don’t want to dewater the well entirely.
There used to be a guideline actually in the State of California that eventually got removed that was referred to as pre and post sample purging which was kind of an interesting one. I never could quite understand the basis for it I could understand the legal reason for it and that was that if you took a low flow sample from the well or any sample I should say, any purge sample from the well that the next time you go to purging sample of the well you want to make sure you are not sampling the same water. So you would evacuate the well thereby eliminating the possibility that any water that had been sampled previously would be sampled again. It was strictly a legal issue. It was the result of a legal challenge by one waste disposal company to the state’s guidelines on purging and sampling wells. But no, I don’t see any value to evacuating the well after a low flow sample has been taken. Because again if we accept the idea and the argument that passive and no-purge samples would represent the chemistry of the surrounding formation within the well screen zone through either groundwater flow or through diffusion, there should really be no need to remove that water afterwards.
During the purging of high yield wells in confined bedrock aquifer water levels may drop several feet prior to stabilization at rates as low as 300 to 500 ml per minute. Is it of any benefit with regard to the representativeness of the sample to further reduce the rate to a 100 milliliters per minute assuming that the drawdown is less but still several feet.
As long as you don’t have a stated drawdown with it that you have to meet, my inclination is to try and maintain the highest practical flow rate that I can. So if I am starting out at 300 to 500 milliliters per minute and I am getting several feet of drawdown the well provided that my water level then stabilizes very quickly after that I would prefer to stay at that upper limit. My reasoning being that when it comes to filling the sample bottles afterwards if you stick yourself with a 50 or 100 or 150 milliliter per minute flow rate and you have multiple large sample bottles to fill that could be a problem. I was working in a site in Ohio once where they were purging all wells at 200 milliliters a minute. Unfortunately they were filling eight 1 liter amber and four (Inaudible) So it took 20 minutes to purge the wells and an hour and a half to fill the sample bottles. But they weren’t considering any higher flow rate, they just simply used the stated flow rate of 200 because it was in their sampling and analysis plan. When we kick the flow rates up to about 500 mils a minute we had a little bit more drawdown in the wells but lo and behold you could fill at 500 mils a minute now, you could fill those 1 liter amber in just over two minutes. So I would say that provided you don’t have a stated limit for drawdown, I would lean toward the higher flow rate and the greater drawdown as long as I get my pumping water level to stabilize. If you have continual drawdown that doesn’t seem to be slowing down then you have no choice but to back off on the flow rate at some point because you don’t want to dewater the well entirely.