ASTM D6771-21

Standard Practice for Low-Flow Purging and Sampling Used for Groundwater Monitoring

SIGNIFICANCE AND USE
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-riser pipe (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/D5092M and D6725/D6725M, adequately developed as described in Guide D5521/D5521M, and has received proper well maintenance and rehabilitation as described in Guide D5978/D5978M (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 th...
SCOPE
1.1 This practice describes the method of low-flow purging and sampling used to collect groundwater samples from wells to assess groundwater quality.  
1.2 The purpose of this procedure is to collect groundwater samples that represent a flow-weighted average of solute and colloid concentrations transported through the formation near the well screen under ambient conditions. Samples collected using this method can be analyzed for groundwater contaminants and/or naturally occurring analytes.  
1.3 This practice is generally not suitable for use in wells with very low-yields and cannot be conducted using grab sampling or inertial lift devices. This practice is not suitable for use in wells with non-aqueous phase liquids.  
1.4 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are approximate mathematical conversions to inch-pound units that are provided for information only and are not considered standard.  
1.5 This practice offers a set of instructions for performing one or more specific operations. This document cannot replace education or experience and should be used in conjunction with professional judgment. Not all aspects of this practice 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 document be applied without consideration of a project's many unique aspects. The word “standard” in the title means only that the document has been approved through the ASTM consensus process.  
1.6 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, health, and environmental practices and determine the applicability of regulatory limitations prior to use.  
1.7 This international standar...

General Information

• Status: Published
• Publication Date: 31-Oct-2021
• ICS: 13.060.10 - Water of natural resources
• Technical Committee: D18 - Soil and Rock
• Drafting Committee: D18.21 - Groundwater and Vadose Zone Investigations

Relations

• Refers: ASTM D5978/D5978M-16(2024) - Standard Guide for Maintenance and Rehabilitation of Groundwater Monitoring Wells
• Effective Date: 15-Mar-2024
• Refers: ASTM D7069-24 - Standard Guide for Field Quality Assurance in a Groundwater Sampling Event
• Effective Date: 15-Mar-2024
• Refers: ASTM D6564/D6564M-17(2024) - Standard Guide for Field Filtration of Groundwater Samples
• Effective Date: 01-Feb-2024
• Refers: ASTM D5978/D5978M-16(2024) - Standard Guide for Maintenance and Rehabilitation of Groundwater Monitoring Wells
• Effective Date: 01-Jan-2024
• Refers: ASTM D6089-19(2023) - Standard Guide for Documenting a Groundwater Sampling Event
• Effective Date: 01-Nov-2023
• Refers: ASTM D6452-18(2023) - Standard Guide for Purging Methods for Wells Used for Ground Water Quality Investigations
• Effective Date: 01-Nov-2023
• Refers: ASTM D6517-18(2023) - Standard Guide for Field Preservation of Ground Water Samples
• Effective Date: 01-Nov-2023
• Refers: ASTM D5088-20 - Standard Practice for Decontamination of Field Equipment Used at Waste Sites
• Effective Date: 01-May-2020
• Refers: ASTM D7929-20 - Standard Guide for Selection of Passive Techniques for Sampling Groundwater Monitoring Wells
• Effective Date: 15-Feb-2020
• Refers: ASTM D6089-19 - Standard Guide for Documenting a Groundwater Sampling Event
• Effective Date: 01-Aug-2019
• Refers: ASTM D6517-18 - Standard Guide for Field Preservation of Ground Water Samples
• Effective Date: 01-Dec-2018
• Refers: ASTM D6452-18 - Standard Guide for Purging Methods for Wells Used for Ground Water Quality Investigations
• Effective Date: 01-Dec-2018
• Refers: ASTM D5521/D5521M-18 - Standard Guide for Development of Groundwater Monitoring Wells in Granular Aquifers
• Effective Date: 01-Jun-2018
• Refers: ASTM D6564/D6564M-17 - Standard Guide for Field Filtration of Groundwater Samples
• Effective Date: 15-Dec-2017
• Refers: ASTM D5092/D5092M-16 - Standard Practice for Design and Installation of Groundwater Monitoring Wells
• Effective Date: 15-Nov-2016

Overview

ASTM D6771-21, titled Standard Practice for Low-Flow Purging and Sampling Used for Groundwater Monitoring, establishes procedures for collecting representative groundwater samples from monitoring wells using low-flow methods. Developed by ASTM International, this standard is critical for groundwater quality investigations aiming to obtain samples that closely reflect the actual conditions of aquifers near the well screen. By focusing on minimal well disturbance and stabilization of key parameters, ASTM D6771-21 helps minimize bias introduced during field sampling and supports more accurate groundwater contamination assessments and monitoring.

Key Topics

• Purpose: The primary goal is to ensure that groundwater samples represent a flow-weighted average of solute and colloid concentrations under natural, undisturbed conditions. This method is suitable for analyzing both contaminants and naturally occurring constituents in groundwater.
• Methodology:
  • Samples are collected following a low-flow purging process, where water is pumped slowly to limit changes in ambient groundwater levels and to avoid mixing with stagnant or non-representative water.
  • The method relies on stabilization of "indicator parameters" (such as dissolved oxygen, oxidation-reduction potential, pH, and specific conductance) and "operational parameters" (such as turbidity, water level, and temperature) before sample collection.
• Applicability:
  • Suitable for wells with sufficient yield where drawdown can be stabilized at low pumping rates, generally not less than 50–100 mL/min.
  • Not applicable for wells with very low yields, wells containing non-aqueous phase liquids (NAPLs), or where only grab sampling/inertial lift devices can be used.
• Equipment Suggestions:
  • Use of adjustable-rate submersible pumps (e.g., bladder or peristaltic types) and equipment to monitor flow rates, turbidity, and water quality.
  • Dedicated equipment is recommended to reduce cross-contamination risks and minimize water column disturbance.

Applications

• Groundwater Contamination Assessment: Low-flow purging and sampling are essential for identifying and quantifying the presence of contaminants with greater accuracy, particularly in environmental site assessments, remediation projects, and regulatory compliance monitoring.
• Water Quality Monitoring: The standard supports routine monitoring of natural groundwater quality, facilitating early detection of changes due to anthropogenic or natural influences.
• Regulatory Environmental Programs: Government agencies, consultants, and industries reference ASTM D6771-21 as an accepted method for sample collection in support of quality assurance project plans (QAPPs), groundwater sampling analysis plans (GWSAPs), and standard operating procedures (SOPs).
• Site-Specific Investigations: The practice can be integrated with tailored site-specific protocols where local regulatory permits or project requirements allow, ensuring samples meet defensible and recognized standards.

Related Standards

ASTM D6771-21 is supported and complemented by a suite of related ASTM standards, which provide guidance on the full process of groundwater monitoring:

• ASTM D5092 / D5092M: Design and installation of groundwater monitoring wells.
• ASTM D5521 / D5521M: Development of groundwater monitoring wells in granular aquifers.
• ASTM D5978 / D5978M: Maintenance and rehabilitation of monitoring wells.
• ASTM D6634 / D6634M: Selection of purging and sampling devices for monitoring wells.
• ASTM D6089: Documentation of groundwater sampling events.
• ASTM D6564 / D6564M: Field filtration of groundwater samples.
• ASTM D7929: Selection of passive techniques for sampling groundwater wells.

These standards, when used together, help ensure a reliable, repeatable, and scientifically defensible groundwater sampling process.

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Keywords: ASTM D6771-21, low-flow purging, groundwater sampling, groundwater monitoring, flow-weighted average, indicator parameters, water quality, operational parameters, environmental sampling, ASTM standards.

Standards Content (Sample)

This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the
Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
Designation: D6771 − 21
Standard Practice for
Low-Flow Purging and Sampling Used for Groundwater
Monitoring
This standard is issued under the fixed designation D6771; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope* 1.7 This international standard was developed in accor-
dance with internationally recognized principles on standard-
1.1 This practice describes the method of low-flow purging
ization established in the Decision on Principles for the
and sampling used to collect groundwater samples from wells
Development of International Standards, Guides and Recom-
to assess groundwater quality.
mendations issued by the World Trade Organization Technical
1.2 The purpose of this procedure is to collect groundwater
Barriers to Trade (TBT) Committee.
samples that represent a flow-weighted average of solute and
colloid concentrations transported through the formation near
2. Referenced Documents
the well screen under ambient conditions. Samples collected
2.1 ASTM Standards:
using this method can be analyzed for groundwater contami-
D653 Terminology Relating to Soil, Rock, and Contained
nants and/or naturally occurring analytes.
Fluids
1.3 This practice is generally not suitable for use in wells
D5088 Practice for Decontamination of Field Equipment
with very low-yields and cannot be conducted using grab
Used at Waste Sites
samplingorinertialliftdevices.Thispracticeisnotsuitablefor
D5092/D5092M Practice for Design and Installation of
use in wells with non-aqueous phase liquids.
Groundwater Monitoring Wells
D5521/D5521M Guide for Development of Groundwater
1.4 Units—The values stated in SI units are to be regarded
Monitoring Wells in Granular Aquifers
as standard. The values given in parentheses are approximate
D5608 Practices for Decontamination of Sampling and Non
mathematical conversions to inch-pound units that are pro-
Sample Contacting Equipment Used at Low Level Radio-
vided for information only and are not considered standard.
active Waste Sites
1.5 This practice offers a set of instructions for performing
D5903 Guide for Planning and Preparing for a Groundwater
one or more specific operations. This document cannot replace
Sampling Event
education or experience and should be used in conjunction
D5978/D5978M Guide for Maintenance and Rehabilitation
with professional judgment. Not all aspects of this practice may
of Groundwater Monitoring Wells
be applicable in all circumstances. This ASTM standard is not
D6089 Guide for Documenting a Groundwater Sampling
intended to represent or replace the standard of care by which
Event
the adequacy of a given professional service must be judged,
D6452 Guide for Purging Methods for Wells Used for
nor should this document be applied without consideration of
Ground Water Quality Investigations
a project’s many unique aspects. The word “standard” in the
D6517 Guide for Field Preservation of Ground Water
title means only that the document has been approved through
Samples
the ASTM consensus process.
D6564/D6564M Guide for Field Filtration of Groundwater
1.6 This standard does not purport to address all of the
Samples
safety concerns, if any, associated with its use. It is the
D6634/D6634M Guide for Selection of Purging and Sam-
responsibility of the user of this standard to establish appro-
pling Devices for Groundwater Monitoring Wells
priate safety, health, and environmental practices and deter-
D6725/D6725M Practice for Direct Push Installation of
mine the applicability of regulatory limitations prior to use.
Prepacked Screen Monitoring Wells in Unconsolidated
Aquifers
These test methods are under the jurisdiction ofASTM Committee D18 on Soil
andRockandarethedirectresponsibilityofSubcommitteeD18.21onGroundwater
and Vadose Zone Investigations. For referenced ASTM standards, visit the ASTM website, www.astm.org, or
Current edition approved Nov. 1, 2021. Published November 2021. Originally contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
approved in 2002. Last previous edition approved in 2018 as D6771 –18. DOI: Standards volume information, refer to the standard’s Document Summary page on
10.1520/D6771-21. the ASTM website.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6771 − 21
D6911 Guide for Packaging and Shipping Environmental 4. Summary of Practice
Samples for Laboratory Analysis
4.1 General Objective—Under ambient conditions, the
D7069 Guide for Field QualityAssurance in a Groundwater
amount of groundwater flow through a monitoring well screen
Sampling Event
is dependent on the local hydrogeological conditions and well
D7929 Guide for Selection of Passive Techniques for Sam-
design (for example, well diameter, screen length, sand pack).
pling Groundwater Monitoring Wells
If a well is constructed, developed, and maintained properly,
hydraulic communication normally exists between the forma-
3. Terminology
tion and well under ambient conditions (1). With adequate
hydraulic communication and ambient aquifer flow, the com-
3.1 Definitions:
position of the formation water and pre-pumping well water
3.1.1 For common definitions of terms about soil and rock
may be very similar (Guide D7929). However, purging meth-
andthefluidscontainedinthem,refertoTerminologyinD653.
ods are commonly applied to assure the collection of
3.2 Definitions of Terms Specific to This Standard:
formation-quality water. Indicator parameters (for example,
3.2.1 artifactual turbidity—particulate matter that is not
dissolved oxygen and specific conductance) can be monitored
naturally mobile in the groundwater system and can be
to assess changes in the composition of the discharge water as
introduced to the subsurface during drilling or well
formation water is drawn into the well, mixes with existing
construction, sheared from the target monitoring zone during
well water, and displaces the pre-existing water in the screened
purgingofthewell,orproducedbyexposureofgroundwaterto
interval during purging. If the well is purged at a rate that
atmospheric conditions (abbreviated definition from D653).
results in substantial changes (that is, stress) to the ambient
3.2.2 blank-riser pipe water—waterintheriserpipeinterval
flow conditions, as can be shown by increases in operational
of a monitoring well above or below the well screen that is
parameters (drawdown and turbidity), the quality of formation
assumed to not represent formation quality water because it is
water entering the well screen can be altered. The low-flow
less susceptible to ambient well flushing and is potentially
purging and sampling method was developed to collect repro-
stagnant.
ducible samples that are considered to represent a flow-
weighted average of the formation water while minimizing
3.2.3 drawdown [L]—vertical distance the ambient (non-
changes to the ambient flow conditions (2).
pumping) water level is lowered due to continuous removal of
water from the well.
4.2 Minimizing Hydraulic Stress—Pumping that induces
excessive drawdown and/or groundwater inflow velocities
3.2.4 flow-through cell—vessel through which purge water
is transported in order to contact sensors for continuous throughthewellscreencanresultinsamplingbiasesassociated
measurement of indicator and operational parameters. with screen dewatering, water column aeration, artifactual
turbidity, and/or mixing of blank-riser pipe water into the
3.2.5 flow-weighted average concentration—single analyte
screened interval. The magnitude of these effects at a given
value that reflects a mixture proportional to the flow rate and
pumping rate are dependent on the well design and near-well
respective concentrations of groundwater entering the screen
hydrogeological conditions (for example, gradient and hydrau-
interval.
lic conductivity). Since the amount of hydraulic stress and
3.2.6 indicator parameters—chemical properties (oxygen,
related sampling biases that can occur at a given pumping rate
oxidation-reduction potential, specific conductance, and pH)
varies for each well, the overall goal of low-flow purging and
measured to determine when the discharge water is considered
sampling is to minimize hydraulic stress by reducing the
to represent a flow-weighted average concentration of the
pumping rate to the extent practical. Typically pumping rates
formation water.
on the order of 0.1 to 1.0 L/min can be used to minimize
3.2.7 operational parameters—physical properties (water changes to ambient flow conditions while preserving the
level, turbidity, and temperature) measured to determine
quality of formation water entering the well (2), although
whether pumping operations have introduced potential sam- higher rates can be used if appropriate.
pling biases.
4.3 Sample Composition—Groundwater samples collected
3.2.8 optimum pumping rate [L /T]—well-specific pump
by this method are considered to represent a flow-weighted
rate used to minimize the purge time required before sampling
average of the formation water entering the screened interval
while also minimizing changes to the ambient groundwater
based on the stabilization of indicator parameters (3-7). The
flow conditions and operational parameters.
vertical distribution of the inflow rate through the well screen
varies according to the vertical distribution of permeable
3.2.9 pumping water level [L]—free or unconfined water
materials in the surrounding formation and the presence of
elevation during purging and sampling.
verticalheadgradients(ifany).Somedegreeofverticalmixing
3.2.10 screen volume [L ]—quantity of water contained in
often occurs within the well under ambient flow conditions (4,
the screened interval of a monitoring well.
8). During pumping, the mixed pre-pumping well water is
3.2.11 stabilization—condition that occurs when changes in
incorporated with groundwater that enters the well screen and
indicator and operational parameter values are maintained
within a specified range over a selected number of consecutive
readings and it appears the readings will continue to remain
The boldface numbers in parentheses refer to a list of references at the end of
within that specified range during subsequent readings. this standard.
D6771 − 21
advances toward the pump intake. The purge time needed to 6. Benefits and Limitations of Low-Flow Purging and
achieve stabilization of indicator parameters is dependent on Sampling
the well design, the degree of in-well mixing, vertical hetero-
6.1 Benefits:
geneity of surrounding formation materials, and stratification
6.1.1 Purging and sampling at a low-flow rate provides
of the formation water quality (if any) entering the well screen.
more accurate and reproducible samples of the formation-
These factors control the volume of water to be purged. Where
quality water than high flow/high volume purging and sam-
the composition of formation water entering the well screen
plingmethodsbyminimizinghydraulicstressesontheambient
interval is relatively homogenous and/or is similar to the
flow conditions that may introduce one or more of the
pre-pumping well water (as signaled by the stabilization of
following biases to the sample (12, 13):
indicator parameters), a sample collected by low-flow purging
6.1.1.1 Artifactual Turbidity—Artificially elevated turbidity
and sampling reflects an acceptable mixture of the formation
levels induced by pumping rates that entrain colloidal sized
and pre-pumping well water (3, 4).
particles that are immobile under ambient flow conditions can
5. Significance and Use
result in increased concentrations of contaminants that are
sorbed or partitioned on those colloids (for example, metals
5.1 Method Considerations—The objective of most ground-
and some organics);
water sampling programs is to obtain samples that are similar
6.1.1.2 Artificial aeration, or oxygenation, of the water
in composition to that of the formation water near the well
column from percolation and/or cascading of water down the
screen. The low-flow purging and sampling method uses the
sand pack and well screen, respectively, when the well is
stabilization of indicator parameters to determine when the
rapidly dewatered. Water column aeration can also result from
pump discharge is considered to represent a flow-weighted
agitation by the sampling device. These processes can result in
average of the formation water. Measurements of operational
the loss of volatile organic compounds and dissolved gases, as
parameters are used to determine potential sampling bias (for
well as chemical changes associated with oxygenation; and
example, artifactual turbidity and increased temperature) that
6.1.1.3 Entrainment of blank-riser pipe (and potentially
may have been introduced by pumping operations and to
stagnant) water from drawdown or excessive agitation of the
ensurethatthesampleisrepresentativeofformationwater.The
water column.
low-flow purge rate minimizes lowering of the ambient
groundwaterlevelandtherebyminimizespotentialentrainment 6.1.2 Purging and sampling at a low-flow rate can provide
more cost and well-maintenance benefits than other purge and
of blank-riser pipe (and potentially stagnant) water above or
below the screen into the screened-zone of the well. This sampling methods by:
sampling method assumes that the well has been properly 6.1.2.1 Reducing purge-water volume, resulting in reduced
designed and constructed as described in Practices D5092/ exposure of field personnel to potentially contaminated purge
D5092M and D6725/D6725M, adequately developed as de- water;
scribed in Guide D5521/D5521M, and has received proper
6.1.2.2 Reducing well maintenance (for example, redevel-
well maintenance and rehabilitation as described in Guide
opment) through reduced pumping stress on the well and
D5978/D5978M (see Note 1).
formation, resulting in greatly reduced movement of fine
sediment into the filter pack and well screen;
NOTE 1—This Standard is not intended to replace or supersede any
6.1.2.3 Reducing purge-water volume, resulting in savings
regulatory requirements, standard operating procedure (SOP), quality
assurance project plan (QAPP), ground water sampling and analysis plan
of costs related to purge water handling and disposal or
(GWSAP) or site-specific regulatory permit requirements.The procedures
treatment; and
described in this Standard may be used in conjunction with regulatory
6.1.2.4 Potentially reducing purge time, particularly when
requirements, SOPs, QAPPs, GWSAPs or permits where allowed by the
using dedicated pumps (14), resulting in labor cost savings.
authority with jurisdiction.
5.2 Applicability—Low-flow purging and sampling may be
6.2 Limitations:
used in a monitoring well that can be pumped at a constant
6.2.1 Low-flow purging and sampling is generally not
low-flowratewithoutcontinuouslyincreasingdrawdowninthe
suitable for use in very low-yield wells (those that will not
well (2). If a well cannot be purged without continuously
yield sufficient water without continuously increasing draw-
increasing drawdown even at very low pumping rates (for
down while pumping at very low rates (for example, 50 – 100
example, 50 – 100 mL/min), the well should not be sampled
mL/min) over time).
using this sampling method as described in this standard; a
6.2.2 As with any sampling method, low-flow purging and
passive sampling method, as described in Guide D7929, may
samplingisnotsuitableforsamplinginwellsknowntocontain
be considered as an alternative.
light or dense non-aqueous-phase liquids (NAPL), because it
may misrepresent the risk to human health and may complicate
5.3 Target Analytes—Low-flow purging and sampling can
data interpretation.
be used to collect samples for all categories of aqueous-phase
contaminants and naturally-occurring analytes. It is particu- 6.2.3 Low-flow purging and sampling cannot be performed
larly well suited for use where it is desirable to sample using grab sampling devices (for example, bailers) or inertial-
aqueous-phase constituents that may sorb or partition to lift devices, because these devices can severely agitate the
particulate matter, because the method minimizes the potential water column in the well, and this typically results in aeration,
for artifactual turbidity compared with high flow/high volume excessive mixing of the water column, and artifactual turbidity
purging using a pump, bailer, or inertial-lift device (9-12). (see Guide D6634/D6634M).
D6771 − 21
6.2.4 Low flow purging and sampling could result in longer turbidity and temperature should be measured using a multi-
purge times than other purge methods described in Guide parameter instrument coupled with an in-line flow-through
D6452, such as fixed volume purging and well evacuation cell.Transparentflow-throughcellsarepreferredtoobservefor
purging. potential sediment accumulation and/or gas bubbles. Where
initial turbidity levels are high, it may be preferable to measure
7. Equipment Requirements for Low-Flow Purging and
turbidity ahead of the flow-through cell to prevent the accu-
Sampling
mulation of solids in the cell from affecting turbidity readings.
A flow diverter located upstream of the cell can be used to
7.1 A variety of adjustable-rate submersible pumps (for
collect a sample for measurement using a portable turbidity
example, bladder, centrifugal, peristaltic) capable of pumping
meter. The multi-parameter instrument and turbidity meter
at low flow rates can be used for low-flow purging and
should be properly calibrated and maintained in accordance
sampling. Because the purging and sampling processes are
with manufacturer instructions.
joined together into one continuous operation, the pump
selected(seeGuideD6634/D6634M)shouldbeappropriatefor
7.7 Other equipment and supplies used in low-flow purging
use both in purging and sampling the analytes of interest.
and sampling typically include decontamination supplies,
Equipment accompanying the pump selected should include a
sample bottles, calibration standards for field water quality
power source (for example, generator, compressor, compressed
instruments, and field documentation materials.
gas, or battery) and a controller capable of varying the pump
dischargewithintherangeofdesiredflowratesforpurgingand 8. Description of the Procedure
sampling (generally 0.1 to 1.0 L/min).
8.1 Preparation for Low-Flow Purging and Sampling—
7.2 Dedicated tubing or dedicated pumps with tubing (those Prior to conducting a sampling event, the sampling team
that are permanently installed in the well) are strongly recom-
should prepare themselves and any equipment and materials to
mended over portable pumps because they eliminate distur-
be used during the event in accordance with Practice D5903.It
bance to the water column in the well caused by deployment of is particularly important to review and bring to the site a copy
the pump and/or tubing prior to each purging event, and result
of the site-specific sampling and analysis plan, well construc-
in lower turbidity values, shorter purge times, and lower purge tion data for the wells to be sampled, and the field data from
volumes to achieve stabilized indicator parameter measure-
previoussamplingevents.Anyequipmentusedinthesampling
ments. However, portable pumps and disposable tubing can be program that could contact the water in the well, the water
used if care is taken to minimize disturbance to the water
collected during field parameter measurement, or the water
columnduringpumpandtubinginstallation(seeGuideD6634/ collected as a sample should be properly cleaned before (and
D6634M). In instances where the pump is at the surface, the
after) each use (see Practices D5088 and D5608). The clean
tubing should be dedicated to the well and preferably stored in equipmentshouldnotbeallowedtocontactthegroundorother
the well. Using dedicated pumps and tubing reduces the
surfaces that could impart contaminants. An effort should be
possibility of cross contamination from the previous well made to closely match the length of the tubing used for the
sampled (due to desorption of sorbed analytes), the need to
pumpwiththesamplingdepthsothatthereisnoexcesstubing.
decontaminate the equipment between sampling events, and Using excess tubing and/or leaving the tubing and flow-
thelossofanalytesduetosorptionespeciallybynewmaterials.
through cell in direct sunlight on hot days can elevate water
temperatu
...

This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the previous version. Because
it may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current version
of the standard as published by ASTM is to be considered the official document.
Designation: D6771 − 1821
Standard Practice for
Low-Flow Purging and Sampling for Wells and Devices
Used for Ground-Water Quality InvestigationsGroundwater
Monitoring
This standard is issued under the fixed designation D6771; the number immediately following the designation indicates the year of
original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A
superscript epsilon (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 This practice describes the method of low-flow purging and sampling used to collect groundwater samples from wells to assess
groundwater quality.
1.2 The purpose of this procedure is to collect groundwater samples that represent a flow-weighted average of solute and colloid
concentrations transported through the formation near the well screen under ambient conditions. Samples collected using this
method can be analyzed for groundwater contaminants and/or naturally occurring analytes.
1.3 This practice is generally not suitable for use in wells with very low-yields and cannot be conducted using grab sampling or
inertial lift devices. This practice is not suitable for use in wells with non-aqueous phase liquids.
1.4 Units—The values stated in SI units are to be regarded as standard. The values given in parentheses are approximate
mathematical conversions to inch-pound units that are provided for information only and are not considered standard.
1.5 This practice offers a set of instructions for performing one or more specific operations. This document cannot replace
education or experience and should be used in conjunction with professional judgment. Not all aspects of this practice 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 document be applied without consideration of a project’s
many unique aspects. The word “standard” in the title means only that the document has been approved through the ASTM
consensus process.
1.6 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, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.7 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
This practice is These test methods are under the jurisdiction of ASTM Committee D18 on Soil and Rock and isare the direct responsibility of Subcommittee D18.21
on Groundwater and Vadose Zone Investigations.
Current edition approved Sept. 1, 2018Nov. 1, 2021. Published September 2018November 2021. Originally approved in 2002. Last previous edition approved in 2018 as
D6771 –18. DOI: 10.1520/D6771-18.10.1520/D6771-21.
*A Summary of Changes section appears at the end of this standard
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
D6771 − 21
2. Referenced Documents
2.1 ASTM Standards:
D653 Terminology Relating to Soil, Rock, and Contained Fluids
D5088 Practice for Decontamination of Field Equipment Used at Waste Sites
D5092D5092/D5092M Practice for Design and Installation of Groundwater Monitoring Wells
D5521D5521/D5521M Guide for Development of Groundwater Monitoring Wells in Granular Aquifers
D5608 Practices for Decontamination of Sampling and Non Sample Contacting Equipment Used at Low Level Radioactive
Waste Sites
D5903 Guide for Planning and Preparing for a Groundwater Sampling Event
D5978D5978/D5978M Guide for Maintenance and Rehabilitation of Groundwater Monitoring Wells
D6089 Guide for Documenting a Groundwater Sampling Event
D6452 Guide for Purging Methods for Wells Used for Ground Water Quality Investigations
D6517 Guide for Field Preservation of Ground Water Samples
D6564D6564/D6564M Guide for Field Filtration of Groundwater Samples
D6634D6634/D6634M Guide for Selection of Purging and Sampling Devices for Groundwater Monitoring Wells
D6725D6725/D6725M Practice for Direct Push Installation of Prepacked Screen Monitoring Wells in Unconsolidated Aquifers
D6911 Guide for Packaging and Shipping Environmental Samples for Laboratory Analysis
D7069 Guide for Field Quality Assurance in a Groundwater Sampling Event
D7929 Guide for Selection of Passive Techniques for Sampling Groundwater Monitoring Wells
3. Terminology
3.1 Definitions:
3.1.1 For common definitions of terms about soil and rock and the fluids contained in them, refer to Terminology in D653.
3.2 Definitions of Terms Specific to This Standard:
3.2.1 artifactual turbidity—particulate matter that is not naturally mobile in the groundwater system and can be introduced to the
subsurface during drilling or well construction, sheared from the target monitoring zone during purging of the well, or produced
by exposure of groundwater to atmospheric conditions (abbreviated definition from D653).
3.2.2 blank-casing blank-riser pipe water—water in the casing riser pipe interval of a monitoring well above or below the well
screen that is assumed to not represent formation quality water because it is less susceptible to ambient well flushing and is
potentially stagnant.
3.2.3 drawdown [L]—vertical distance the ambient (non-pumping) water level is lowered due to continuous removal of water from
the well.
3.2.4 flow-through cell—vessel through which purge water is transported in order to contact sensors for continuous measurement
of indicator and operational parameters.
3.2.5 flow-weighted average concentration—single analyte value that reflects a mixture proportional to the flow rate and respective
concentrations of groundwater entering the screen interval.
3.2.6 indicator parameters—chemical properties (oxygen, oxidation-reduction potential, specific conductance, and pH) measured
to determine when the discharge water is considered to represent a flow-weighted average concentration of the formation water.
3.2.7 operational parameters—physical properties (water level, turbidity, and temperature) measured to determine whether
pumping operations have introduced potential sampling biases.
3.2.8 optimum pumping rate [L /T]—well-specific pump rate used to minimize the purge time required before sampling while also
minimizing changes to the ambient groundwater flow conditions and operational parameters.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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3.2.9 pumping water level [L]—free or unconfined water elevation during purging and sampling.
3.2.10 screen volume [L ]—quantity of water contained in the screened interval of a monitoring well.
3.2.11 stabilization—condition that occurs when changes in indicator and operational parameter values are maintained within a
specified range over a selected number of consecutive readings and it appears the readings will continue to remain within that
specified range during subsequent readings.
4. Summary of Practice
4.1 General Objective—Under ambient conditions, the amount of groundwater flow through a monitoring well screen is dependent
on the local hydrogeological conditions and well design (for example, well diameter, screen length, sand pack). If a well is
constructed, developed, and maintained properly, hydraulic communication normally exists between the formation and well under
ambient conditions (1). With adequate hydraulic communication and ambient aquifer flow, the composition of the formation water
and pre-pumping well water may be very similar (Guide D7929). However, purging methods are commonly applied to assure the
collection of formation-quality water. Indicator parameters (for example, dissolved oxygen and specific conductance) can be
monitored to assess changes in the composition of the discharge water as formation water is drawn into the well, mixes with
existing well water, and displaces the pre-existing water in the screened interval during purging. If the well is purged at a rate that
results in substantial changes (that is, stress) to the ambient flow conditions, as can be shown by increases in operational parameters
(drawdown and turbidity), the quality of formation water entering the well screen can be altered. The low-flow purging and
sampling method was developed to collect reproducible samples that are considered to represent a flow-weighted average of the
formation water while minimizing changes to the ambient flow conditions (2).
4.2 Minimizing Hydraulic Stress—Pumping that induces excessive drawdown and/or groundwater inflow velocities through the
well screen can result in sampling biases associated with screen dewatering, water column aeration, artifactual turbidity, and/or
mixing of blank-casing blank-riser pipe water into the screened interval. The magnitude of these effects at a given pumping rate
are dependent on the well design and near-well hydrogeological conditions (for example, gradient and hydraulic conductivity).
Since the amount of hydraulic stress and related sampling biases that can occur at a given pumping rate varies for each well, the
overall goal of low-flow purging and sampling is to minimize hydraulic stress by reducing the pumping rate to the extent practical.
Typically pumping rates on the order of 0.1 to 1.0 L/min can be used to minimize changes to ambient flow conditions while
preserving the quality of formation water entering the well (2), although higher rates can be used if appropriate.
4.3 Sample Composition—Groundwater samples collected by this method are considered to represent a flow-weighted average of
the formation water entering the screened interval based on the stabilization of indicator parameters (3-7). The vertical distribution
of the inflow rate through the well screen varies according to the vertical distribution of permeable materials in the surrounding
formation and the presence of vertical head gradients (if any). Some degree of vertical mixing often occurs within the well under
ambient flow conditions (4, 8). During pumping, the mixed pre-pumping well water is incorporated with groundwater that enters
the well screen and advances toward the pump intake. The purge time needed to achieve stabilization of indicator parameters is
dependent on the well design, the degree of in-well mixing, vertical heterogeneity of surrounding formation materials, and
stratification of the formation water quality (if any) entering the well screen. These factors control the volume of water to be
purged. Where the composition of formation water entering the well screen interval is relatively homogenous and/or is similar to
the pre-pumping well water (as signaled by the stabilization of indicator parameters), a sample collected by low-flow purging and
sampling reflects an acceptable mixture of the formation and pre-pumping well water (3, 4).
5. Significance and Use
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 blank-riser pipe (and potentially stagnant) water above or below the screen into the screened-zone of
The boldface numbers in parentheses refer to a list of references at the end of this standard.
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the well. This sampling method assumes that the well has been properly designed and constructed as described in Practices
D5092D5092/D5092M and D6725D6725/D6725M, adequately developed as described in Guide D5521D5521/D5521M, and has
received proper well maintenance and rehabilitation as described in Guide D5978D5978/D5978M (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).
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6. Benefits and Limitations of Low-Flow Purging and Sampling
6.1 Benefits:
6.1.1 Purging and sampling at a low-flow rate provides more accurate and reproducible samples of the formation-quality water
than high flow/high volume purging and sampling methods by minimizing hydraulic stresses on the ambient flow conditions that
may introduce one or more of the following biases to the sample (12, 13):
6.1.1.1 Artifactual Turbidity—Artificially elevated turbidity levels induced by pumping rates that entrain colloidal sized particles
that are immobile under ambient flow conditions can result in increased concentrations of contaminants that are sorbed or
partitioned on those colloids (for example, metals and some organics);
6.1.1.2 Artificial aeration, or oxygenation, of the water column from percolation and/or cascading of water down the sand pack
and well screen, respectively, when the well is rapidly dewatered. Water column aeration can also result from agitation by the
sampling device. These processes can result in the loss of volatile organic compounds and dissolved gases, as well as chemical
changes associated with oxygenation; and
6.1.1.3 Entrainment of blank-casing blank-riser pipe (and potentially stagnant) water from drawdown or excessive agitation of the
water column.
6.1.2 Purging and sampling at a low-flow rate can provide more cost and well-maintenance benefits than other purge and sampling
methods by:
6.1.2.1 Reducing purge-water volume, resulting in reduced exposure of field personnel to potentially contaminated purge water;
6.1.2.2 Reducing well maintenance (for example, redevelopment) through reduced pumping stress on the well and formation,
resulting in greatly reduced movement of fine sediment into the filter pack and well screen;
6.1.2.3 Reducing purge-water volume, resulting in savings of costs related to purge water handling and disposal or treatment; and
6.1.2.4 Potentially reducing purge time, particularly when using dedicated pumps (14), resulting in labor cost savings.
6.2 Limitations:
6.2.1 Low-flow purging and sampling is generally not suitable for use in very low-yield wells (those that will not yield sufficient
water without continuously increasing drawdown while pumping at very low rates (for example, 50 – 100 mL/min) over time).
6.2.2 As with any sampling method, low-flow purging and sampling is not suitable for sampling in wells known to contain light
or dense non-aqueous-phase liquids (NAPL), because it may misrepresent the risk to human health and may complicate data
interpretation.
6.2.3 Low-flow purging and sampling cannot be performed using grab sampling devices (for example, bailers) or inertial-lift
devices, because these devices can severely agitate the water column in the well, and this typically results in aeration, excessive
mixing of the water column, and artifactual turbidity (see Guide D6634D6634/D6634M).
6.2.4 Low flow purging and sampling could result in longer purge times than other purge methods described in Guide D6452, such
as fixed volume purging and well evacuation purging.
7. Equipment Requirements for Low-Flow Purging and Sampling
7.1 A variety of adjustable-rate submersible pumps (for example, bladder, centrifugal, peristaltic) capable of pumping at low flow
rates can be used for low-flow purging and sampling. Because the purging and sampling processes are joined together into one
continuous operation, the pump selected (see Guide D6634D6634/D6634M) should be appropriate for use both in purging and
sampling the analytes of interest. Equipment accompanying the pump selected should include a power source (for example,
generator, compressor, compressed gas, or battery) and a controller capable of varying the pump discharge within the range of
desired flow rates for purging and sampling (generally 0.1 to 1.0 L/min).
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7.2 Dedicated tubing or dedicated pumps with tubing (those that are permanently installed in the well) are strongly recommended
over portable pumps because they eliminate disturbance to the water column in the well caused by deployment of the pump and/or
tubing prior to each purging event, and result in lower turbidity values, shorter purge times, and lower purge volumes to achieve
stabilized indicator parameter measurements. However, portable pumps and disposable tubing can be used if care is taken to
minimize disturbance to the water column during pump and tubing installation (see Guide D6634D6634/D6634M). In instances
where the pump is at the surface, the tubing should be dedicated to the well and preferably stored in the well. Using dedicated
pumps and tubing reduces the possibility of cross contamination from the previous well sampled (due to desorption of sorbed
analytes), the need to decontaminate the equipment between sampling events, and the loss of analytes due to sorption especially
by new materials.
7.3 The length of tubing fitted to the pump system for purging and sampling should be minimized to reduce the amount of time
1 3
required to bring purge water to the surface. A 6.4- to 9.5-mm ( ⁄4- to ⁄8-in.) outside tubing diameter is typically used. The selection
of tubing materials should be appropriate for the target analytes.
7.4 A means of determining the pumping flow rate is necessary. This can be accomplished using a volume measuring device (for
example, graduated cylinder) and a timepiece capable of measuring seconds, a flow meter, or any other device that will allow an
accurate flow rate determination.
7.5 Low-flow purging and sampling requires periodic water-level measurements. Any water-level measurement equipment that
does not disturb the water column in the well may be used (for example, transducer or electronic water level meter), as long as
it provides the accuracy required by the sampling program (generally 63 mm (0.01 ft)).
7.6 The purging phase of this sampling protocol requires periodic measurements of indicator and operational parameters to
determine when purging is complete and sampling can commence. Indicator parameters and operational parameters of turbidity
and temperature should be measured using a multi-parameter instrument coupled with an in-line flow-through cell. Transparent
flow-through cells are preferred to observe for potential sediment accumulation and/or gas bubbles. Where initial turbidity levels
are high, it may be preferable to measure turbidity ahead of the flow-through cell to prevent the accumulation of solids in the cell
from affecting turbidity readings. A flow diverter located upstream of the cell can be used to collect a sample for measurement
using a portable turbidity meter. The multi-parameter instrument and turbidity meter should be properly calibrated and maintained
in accordance with manufacturer instructions.
7.7 Other equipment and supplies used in low-flow purging and sampling typically include decontamination supplies, sample
bottles, calibration
...