4.1 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 ).

4.2 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.

1.1 This guide covers methods for purging wells used for groundwater quality investigations and monitoring programs. These methods could be used for other types of programs but are not addressed in this guide.

1.2 This guide applies only to wells sampled at the wellhead.

1.3 This standard describes seven methods (A-G) for the selection of purging methods. Method AFixed Volume Purging, Method BPurging Based on Stabilization of Indicator Parameters, Method CPurging Based on Stabilization of Target Analytes, Method DPurging Based on Fixed Volume Combined with Indicator Parameter Stabilization, Method ELow Flow/Low Volume (Minimal Drawdown) Purging, Method FWell Evacuation Purging, and Method GUse of Packers in Purging.

1.4 This guide offers an organized collection of information or a series of options and does not recommend a specific course of action. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this guide may be applicable in all circumstances. This ASTM standard is not intended to represent or replace the standard of care by which the adequacy of a given professional service must be judged, nor should this documente be applied without consideration of a project's many unique aspects. The word Standard in the title of this guide means only that the document has been approved through the ASTM consensus process.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and to determine the applicability of regulatory limitations prior to use.