To Purge, or not to Purge

I was discussing a recent study in Bozeman with a student of groundwater hydrology where the local water quality district tested values of nitrates from wells that are purged before sampling versus nitrate levels from wells that were not purged. The results indicated that the difference in values between purged wells and water that had been sitting stagnant in the casing was subtle, but apparent.

I have not been able to find the study, as I don't think the results have been published, but this is an interesting topic since most of the groundwater in privately-owned wells is tested by untrained well owners who do not purge their wells. Thus, while the difference is not enough to be significant for owners who are just finding a baseline value for their drinking water, it is significant in terms of the data collected from owners throughout the state which is used to examine trends in groundwater by, say, the university.

When I worked for the USGS in California, we always sampled from purged wells to ensure that we sampled a representative portion of the groundwater, and not just stagnant water that hung around in the casing and did not represent the composition of the groundwater. We thus ensured an unbiased sample and that the results of our lab tests accurately reflected the actual groundwater below the well.