5.1 Method Considerations-The objective of most groundwater sampling programs is to obtain samples that are similar in composition to that of the formation water near the well screen. The low-flow purging and sampling method uses the stabilization of indicator parameters to determine when the pump discharge is considered to represent a flow-weighted average of the formation water. Measurements of operational parameters are used to determine potential sampling bias (for example, artifactual turbidity and increased temperature) that may have been introduced by pumping operations and to ensure that the sample is representative of formation water. The low-flow purge rate minimizes lowering of the ambient groundwater level and thereby minimizes potential entrainment of blank-casing (and potentially stagnant) water above or below the screen into the screened-zone of the well. This sampling method assumes that the well has been properly designed and constructed as described in Practices D5092 and D6725, adequately developed as described in Guide D5521, and has received proper well maintenance and rehabilitation as described in Guide D5978 (see Note 1).

Note 1: This Standard is not intended to replace or supersede any regulatory requirements, standard operating procedure (SOP), quality assurance project plan (QAPP), ground water sampling and analysis plan (GWSAP) or site-specific regulatory permit requirements. The procedures described in this Standard may be used in conjunction with regulatory requirements, SOPs, QAPPs, GWSAPs or permits where allowed by the authority with jurisdiction.

5.2 Applicability-Low-flow purging and sampling may be used in a monitoring well that can be pumped at a constant low-flow rate without continuously increasing drawdown in the well (2). If a well cannot be purged without continuously increasing drawdown even at very low pumping rates (for example, 50 - 100 mL/min), the well should not be sampled using this sampling method as described in this standard; a passive sampling method, as described in Guide D7929, may be considered as an alternative.

5.3 Target Analytes-Low-flow purging and sampling can be used to collect samples for all categories of aqueous-phase contaminants and naturally-occurring analytes. It is particularly well suited for use where it is desirable to sample aqueous-phase constituents that may sorb or partition to particulate matter, because the method minimizes the potential for artifactual turbidity compared with high flow/high volume purging using a pump, bailer, or inertial-lift device (9-12).