Wells used in ground-water quality investigations or monitoring programs are generally purged prior to sampling (Note 1). Purging is done to minimize the bias associated with stagnant water in the well, which generally does not accurately reflect ambient ground-water chemistry (Note 2).

Note 1—Some sampling methods, such as passive sampling, do not require the practice of purging prior to sample collection (1,2).³

Note 2—This guide does not address the practice of post-sample purging (purging again after sampling is completed), which is intended for purposes other than the minimization of bias associated with stagnant water in the well.

There are various methods for purging. Each purging method may have a different volume of influence within the aquifer or screened interval. Therefore, a sample collected after purging by any one method is not necessarily equivalent to samples collected after purging by the other methods. The selection of the appropriate method will be dependent on a number of factors, which should be defined during the development of the sampling and analysis plan. This guide describes the methods available and defines the circumstances under which each method may be appropriate.

Wells used in groundwater quality investigations or monitoring programs are generally purged prior to sampling (Note 1). Purging is done to minimize the bias associated with stagnant water in the well, which generally does not accurately reflect ambient groundwater chemistry (Note 2).

Note 1—Some sampling methods, such as passive sampling, do not require the practice of purging prior to sample collection (1,2).

Note 2—This guide does not address the practice of post-sample purging (purging again after sampling is completed), which is intended for purposes other than the minimization of bias associated with stagnant water in the well.

There are various methods for purging. Each purging method may have a different volume of influence within the aquifer or screened interval. Therefore, a sample collected after purging by any one method is not necessarily equivalent to samples collected after purging by the other methods. The selection of the appropriate method will be dependent on a number of factors, which should be defined during the development of the sampling and analysis plan. This guide describes the methods available and defines the circumstances under which each method may be appropriate.