Field Evaluation of Well Purging Procedures

In order to avoid contamination of ground water samples by stagnant water in the well bore, it is generally recommended that the well be purged prior to sampling. There is however, a divergence of opinion both on the need for purging and the best methods of purging. This paper describes detailed field tests in which non-reactive tracers were used to examine, from a well hydraulics point of view, the need for purging and also the effectiveness of various purging procedures. Results show that in the permeable geologic materials of the test site, and for the non-reactive tracers, the water within the screened interval will be purged by the natural flow of water through the screen, while the water above will remain stagnant. The volume of water above the screen is referred to here as one bore volume. It,is suggested that with consideration of the required sample volume, the volume of water stored in the screen, the sampling rate, and the position of the sampler intake, dedicated samplers could be used to obtain representative ground water samples without prior purging of the well.
Of the purging procedures tested, pumping from just below the air-water interface in the well, or the method of "complete removal" of the water within the well bore were the only effective means for complete removal of the stagnant water. Using these procedures, it appeared that representative samples could be obtained with the removal of only two to three bore volumes of water.