1、Designation: D6771 18Standard Practice forLow-Flow Purging and Sampling for Wells and DevicesUsed for Ground-Water Quality Investigations1This standard is issued under the fixed designation D6771; the number immediately following the designation indicates the year oforiginal adoption or, in the case
2、 of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 This practice describes the method of low-flow purgingand sampling used to collect groundwa
3、ter samples from wellsto assess groundwater quality.1.2 The purpose of this procedure is to collect groundwatersamples that represent a flow-weighted average of solute andcolloid concentrations transported through the formation nearthe well screen under ambient conditions. Samples collectedusing thi
4、s method can be analyzed for groundwater contami-nants and/or naturally occurring analytes.1.3 This practice is generally not suitable for use in wellswith very low-yields and cannot be conducted using grabsampling or inertial lift devices. This practice is not suitable foruse in wells with non-aque
5、ous phase liquids.1.4 UnitsThe values stated in SI units are to be regardedas standard. The values given in parentheses are approximatemathematical conversions to inch-pound units that are pro-vided for information only and are not considered standard.1.5 This practice offers a set of instructions f
6、or performingone or more specific operations. This document cannot replaceeducation or experience and should be used in conjunctionwith professional judgment. Not all aspects of this practice maybe applicable in all circumstances. This ASTM standard is notintended to represent or replace the standar
7、d of care by whichthe adequacy of a given professional service must be judged,nor should this document be applied without consideration ofa projects many unique aspects. The word “standard” in thetitle means only that the document has been approved throughthe ASTM consensus process.1.6 This standard
8、 does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety, health, and environmental practices and deter-mine the applicability of regulatory limitations prior to use.1.7 This interna
9、tional standard was developed in accor-dance with internationally recognized principles on standard-ization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recom-mendations issued by the World Trade Organization TechnicalBarriers to Trade (TBT) Com
10、mittee.2. Referenced Documents2.1 ASTM Standards:2D653 Terminology Relating to Soil, Rock, and ContainedFluidsD5088 Practice for Decontamination of Field EquipmentUsed at Waste SitesD5092 Practice for Design and Installation of GroundwaterMonitoring WellsD5521 Guide for Development of Groundwater Mo
11、nitoringWells in Granular AquifersD5608 Practices for Decontamination of Sampling and NonSample Contacting Equipment Used at Low Level Radio-active Waste SitesD5903 Guide for Planning and Preparing for a GroundwaterSampling EventD5978 Guide for Maintenance and Rehabilitation ofGroundwater Monitoring
12、 WellsD6089 Guide for Documenting a Groundwater SamplingEventD6452 Guide for Purging Methods for Wells Used forGroundwater Quality InvestigationsD6517 Guide for Field Preservation of GroundwaterSamplesD6564 Guide for Field Filtration of Groundwater SamplesD6634 Guide for Selection of Purging and Sam
13、pling De-vices for Groundwater Monitoring WellsD6725 Practice for Direct Push Installation of PrepackedScreen Monitoring Wells in Unconsolidated AquifersD6911 Guide for Packaging and Shipping EnvironmentalSamples for Laboratory Analysis1This practice is under the jurisdiction of ASTM Committee D18 o
14、n Soil andRock and is the direct responsibility of Subcommittee D18.21 on Groundwater andVadose Zone Investigations.Current edition approved Sept. 1, 2018. Published September 2018. DOI:10.1520/D6771-18.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Ser
15、vice at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428
16、-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommendations issued by the World Trade Organization Technic
17、al Barriers to Trade (TBT) Committee.1D7069 Guide for Field Quality Assurance in a GroundwaterSampling EventD7929 Guide for Selection of Passive Techniques for Sam-pling Groundwater Monitoring Wells3. Terminology3.1 Definitions:3.1.1 For common definitions of terms about soil and rockand the fluids
18、contained in them, refer to Terminology in D653.3.2 Definitions of Terms Specific to This Standard:3.2.1 artifactual turbidityparticulate matter that is notnaturally mobile in the groundwater system and can beintroduced to the subsurface during drilling or wellconstruction, sheared from the target m
19、onitoring zone duringpurging of the well, or produced by exposure of groundwater toatmospheric conditions (abbreviated definition from D653).3.2.2 blank-casing waterwater in the casing interval of amonitoring well above or below the well screen that is assumedto not represent formation quality water
20、 because it is lesssusceptible to ambient well flushing and is potentially stagnant.3.2.3 drawdown Lvertical distance the ambient (non-pumping) water level is lowered due to continuous removal ofwater from the well.3.2.4 flow-through cellvessel through which purge wateris transported in order to con
21、tact sensors for continuousmeasurement of indicator and operational parameters.3.2.5 flow-weighted average concentrationsingle analytevalue that reflects a mixture proportional to the flow rate andrespective concentrations of groundwater entering the screeninterval.3.2.6 indicator parameterschemical
22、 properties (oxygen,oxidation-reduction potential, specific conductance, and pH)measured to determine when the discharge water is consideredto represent a flow-weighted average concentration of theformation water.3.2.7 operational parametersphysical properties (waterlevel, turbidity, and temperature
23、) measured to determinewhether pumping operations have introduced potential sam-pling biases.3.2.8 optimum pumping rate L3/Twell-specific pumprate used to minimize the purge time required before samplingwhile also minimizing changes to the ambient groundwaterflow conditions and operational parameter
24、s.3.2.9 pumping water level Lfree or unconfined waterelevation during purging and sampling.3.2.10 screen volume L3quantity of water contained inthe screened interval of a monitoring well.3.2.11 stabilizationcondition that occurs when changes inindicator and operational parameter values are maintaine
25、dwithin a specified range over a selected number of consecutivereadings and it appears the readings will continue to remainwithin that specified range during subsequent readings.4. Summary of Practice4.1 General ObjectiveUnder ambient conditions, theamount of groundwater flow through a monitoring we
26、ll screenis dependent on the local hydrogeological conditions and welldesign (for example, well diameter, screen length, sand pack).If a well is constructed, developed, and maintained properly,hydraulic communication normally exists between the forma-tion and well under ambient conditions (1).3With
27、adequatehydraulic communication and ambient aquifer flow, the com-position of the formation water and pre-pumping well watermay be very similar (Guide D7929). However, purging meth-ods are commonly applied to assure the collection offormation-quality water. Indicator parameters (for example,dissolve
28、d oxygen and specific conductance) can be monitoredto assess changes in the composition of the discharge water asformation water is drawn into the well, mixes with existingwell water, and displaces the pre-existing water in the screenedinterval during purging. If the well is purged at a rate thatres
29、ults in substantial changes (that is, stress) to the ambientflow conditions, as can be shown by increases in operationalparameters (drawdown and turbidity), the quality of formationwater entering the well screen can be altered. The low-flowpurging and sampling method was developed to collect repro-d
30、ucible samples that are considered to represent a flow-weighted average of the formation water while minimizingchanges to the ambient flow conditions (2).4.2 Minimizing Hydraulic StressPumping that inducesexcessive drawdown and/or groundwater inflow velocitiesthrough the well screen can result in sa
31、mpling biases associatedwith screen dewatering, water column aeration, artifactualturbidity, and/or mixing of blank-casing water into thescreened interval. The magnitude of these effects at a givenpumping rate are dependent on the well design and near-wellhydrogeological conditions (for example, gra
32、dient and hydrau-lic conductivity). Since the amount of hydraulic stress andrelated sampling biases that can occur at a given pumping ratevaries for each well, the overall goal of low-flow purging andsampling is to minimize hydraulic stress by reducing thepumping rate to the extent practical. Typica
33、lly pumping rateson the order of 0.1 to 1.0 L/min can be used to minimizechanges to ambient flow conditions while preserving thequality of formation water entering the well (2), althoughhigher rates can be used if appropriate.4.3 Sample CompositionGroundwater samples collectedby this method are cons
34、idered to represent a flow-weightedaverage of the formation water entering the screened intervalbased on the stabilization of indicator parameters (3-7). Thevertical distribution of the inflow rate through the well screenvaries according to the vertical distribution of permeablematerials in the surr
35、ounding formation and the presence ofvertical head gradients (if any). Some degree of vertical mixingoften occurs within the well under ambient flow conditions (4,8). During pumping, the mixed pre-pumping well water isincorporated with groundwater that enters the well screen andadvances toward the p
36、ump intake. The purge time needed toachieve stabilization of indicator parameters is dependent onthe well design, the degree of in-well mixing, vertical hetero-geneity of surrounding formation materials, and stratification3The boldface numbers in parentheses refer to a list of references at the end
37、ofthis standard.D6771 182of the formation water quality (if any) entering the well screen.These factors control the volume of water to be purged. Wherethe composition of formation water entering the well screeninterval is relatively homogenous and/or is similar to thepre-pumping well water (as signa
38、led by the stabilization ofindicator parameters), a sample collected by low-flow purgingand sampling reflects an acceptable mixture of the formationand pre-pumping well water (3, 4).5. Significance and Use5.1 Method ConsiderationsThe objective of most ground-water sampling programs is to obtain samp
39、les that are similarin composition to that of the formation water near the wellscreen. The low-flow purging and sampling method uses thestabilization of indicator parameters to determine when thepump discharge is considered to represent a flow-weightedaverage of the formation water. Measurements of
40、operationalparameters are used to determine potential sampling bias (forexample, artifactual turbidity and increased temperature) thatmay have been introduced by pumping operations and toensure that the sample is representative of formation water. Thelow-flow purge rate minimizes lowering of the amb
41、ientgroundwater level and thereby minimizes potential entrainmentof blank-casing (and potentially stagnant) water above orbelow the screen into the screened-zone of the well. Thissampling method assumes that the well has been properlydesigned and constructed as described in Practices D5092 andD6725,
42、 adequately developed as described in Guide D5521,and has received proper well maintenance and rehabilitation asdescribed in Guide D5978 (see Note 1).NOTE 1This Standard is not intended to replace or supersede anyregulatory requirements, standard operating procedure (SOP), qualityassurance project p
43、lan (QAPP), ground water sampling and analysis plan(GWSAP) or site-specific regulatory permit requirements. The proceduresdescribed in this Standard may be used in conjunction with regulatoryrequirements, SOPs, QAPPs, GWSAPs or permits where allowed by theauthority with jurisdiction.5.2 Applicabilit
44、yLow-flow purging and sampling may beused in a monitoring well that can be pumped at a constantlow-flow rate without continuously increasing drawdown in thewell (2). If a well cannot be purged without continuouslyincreasing drawdown even at very low pumping rates (forexample, 50 100 mL/min), the wel
45、l should not be sampledusing this sampling method as described in this standard; apassive sampling method, as described in Guide D7929, maybe considered as an alternative.5.3 Target AnalytesLow-flow purging and sampling canbe used to collect samples for all categories of aqueous-phasecontaminants an
46、d naturally-occurring analytes. It is particu-larly well suited for use where it is desirable to sampleaqueous-phase constituents that may sorb or partition toparticulate matter, because the method minimizes the potentialfor artifactual turbidity compared with high flow/high volumepurging using a pu
47、mp, bailer, or inertial-lift device (9-12).6. Benefits and Limitations of Low-Flow Purging andSampling6.1 Benefits:6.1.1 Purging and sampling at a low-flow rate providesmore accurate and reproducible samples of the formation-quality water than high flow/high volume purging and sam-pling methods by m
48、inimizing hydraulic stresses on the ambientflow conditions that may introduce one or more of thefollowing biases to the sample (12, 13):6.1.1.1 Artifactual TurbidityArtificially elevated turbiditylevels induced by pumping rates that entrain colloidal sizedparticles that are immobile under ambient fl
49、ow conditions canresult in increased concentrations of contaminants that aresorbed or partitioned on those colloids (for example, metalsand some organics);6.1.1.2 Artificial aeration, or oxygenation, of the watercolumn from percolation and/or cascading of water down thesand pack and well screen, respectively, when the well israpidly dewatered. Water column aeration can also result fromagitation by the sampling device. These processes can result inthe loss of volatile organic compounds and dissolved gases, aswell as chemical changes associated with oxygen
