Guidelines for field measurement of water quality
Contents
1. 19 6 0 3 B Direct field measurement procedures 24 Inline flow flowthrough cell chamber procedure 24 Downhole in situ procedure 24 6 0 3 C Subsample field measurement procedures 26 6 0 4 Selected references 28 6 0 5 Acknowledgments 29 Chapter A6 Field Measurements Info Version 2 0 10 2008 2 INFO Illustrations 6 0 1 Flow chart showing in situ field measurement procedures for surface water 15 6 0 2 Flow chart showing subsample field measurement procedures for conductivity pH and alkalinity of surface water 16 6 0 3 Sketch of a manifold and flowthrough cell chamber used during well purging and sample collection 20 6 0 4 Example of a field form for recording measurements during well purging 23 6 0 5 Flow chart showing field2. 214 Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 Select sampling method Allow sensors to equilibrate with ambient water Test calibrate and check field instruments Still water Flowing water 1 Select one or more vertical section s EWI EDI Divide cross section Divide cross section 2 Lower sensors to the depth into minimum of 10 into minimum of 4 appropriate to meet study needs equal width incre equal discharge incre 3 Monitor field measurement readings ments At each ments unless condi increment tions are appropriate for 1 equal discharge 1 Locate the mid vertical at centroid of point flow At each incre 2 Lower sensors to ment middepth of the 1 Locate the centroid vertical 2 Lower sensors to 3 Monitor field middepth of the measurement vertical readings 3 Monitor field mea surement readings Report stabilized value of the median of the last 5 or more values and the time of measurement Still water Report median of values measured after readings stabilize Equal discharge increment method EDI 1 Record median measurement at each vertical 2 Report mean value of the medians of verticals to get a discharge weighted value Equal width increment method EWI 1 Record median measurement at each vertical 2 Report mean or median value if appropriate of the medians of verticals can be used in area weighted calculation
3. gt i gt i i i gt V gt U i gt i i U V i V i i gt i i i i gt V gt L gt i i i Figure 6 0 2 Subsample field measurement procedures for conductivity pH and alkalinity of surface water INFO 17 GROUND WATER 6 0 3 Field measurements in ground water must accurately represent the physical and chemical properties of water in the formation aquifer at the time of data collection This normally requires purging the well of standing water while monitoring field measurements The purge procedures applied can however depend on the type of well from which sample water will be withdrawn for example a public supply domestic or monitor well well construction and aquifer characteristics and study objectives The final field measurement values to be recorded for the ground water site for that date are determined toward the end of the purging process Consult NFM 4 2 for additional guidance on well purging and USGS protocols and standard procedures for ground water withdrawal at wells TECHNICAL NOTE Well purging removes standing water from a well Purging reduces chemical and biochemical artifacts caused by well installation inadequate well development well construction materials
4. the scientific and technical knowledge practical field experience and valuable contributions of D A Evans and Jacob Gibs USGS technical reviewers and of the following scientists are gratefully acknowledged C W Anderson USGS Michael Lizotte YSI Inc Michael Sadar Hach Inc and R J Wagner USGS I M Collies and L J Ulibarri USGS provided editorial and production support which improved the quality of this report Chapter A6 Field Measurements Info Version 2 0 10 2008
5. Survey Hydrologic Instrumentation Facility March 1992 p 7 8 Hydrologic Instrumentation Facility 1992 pH in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility September 1992 p 8 Hydrologic Instrumentation Facility 1992 Conductivity in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility December 1992 p 12 13 Hydrologic Instrumentation Facility 1993 pH in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility March 1993 p 10 14 Hydrologic Instrumentation Facility 1992 Hydrolab s H2O in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility March 1993 p 4 5 Hydrologic Instrumentation Facility 1993 Dissolved oxygen in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility September 1993 unnumbered insert Hydrologic Instrumentation Facility 1994 Temperature measurement tests in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility September 1994 p 3 7 10 12 Hydrologic Instrumentation Facility 1994 Turbidity meter tests in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility June 1994 p 11 13 Kearl P M Korte N E and Cronk T A 1
6. are encouraged to contact instrument manufacturers for answers to technical questions about the operation of their instrument Apparent conflicts between USGS protocols and a manufacturer s instructions should be identified and resolved by consulting the local or regional water quality specialist or the USGS Office of Water Quality Field instruments constantly are being improved or replaced using newer technology that often is unique in some aspect to the manufacture of the instrument Personnel should be knowledgeable about emerging technologies that can improve the quality and efficiency of their field activities Make field measurements only with properly calibrated instruments Calibration is required at the field site for many but not all instruments This requirement depends on the technology employed by the instrument consult the manufacturer s instructions Review the instrument log book s before leaving for the field site to ensure that problems previously encountered have been resolved and that the appropriate instrument and site maintenance were performed Backup instruments and batteries should be readily available and in good working condition Test meters and sensors before leaving for the field If the instrument or measurement is new to you practice your measurement technique with a mentor who has current experience and is up to date with USGS field method protocol and procedures Performance Be aware of
7. at the field site Expertise Collecting data for USGS studies requires training practice and a knowledge of required and recommended protocols Training Field personnel are to enroll in USGS water quality classes Fundamental training for performing water quality field activities in accordance with USGS protocols and standard procedures includes a 2 week Field Water Quality Methods class QW1028 Other foundational training includes Water Quality Principles QW1022 Statistical Methods for Environmental Data Analysis QW1075 Quality Control Sample Design and Interpretation QW2034 and Water Quality Toolbox for NWIS Users QW1297 These classes provide a practicum for the knowledgeable execution of field and office project activities and help to ensure proper management and valid interpretation of the data being collected National Field Quality Assurance Program NFQA All field personnel including non USGS employees who collect field measurement data that are entered into NWIS are to participate annually in the NFQA Program http nfqa cr usgs gov NFQA overview html accessed 8 22 08 which evaluates the proficiency of personnel in measuring pH SC and alkalinity or ANC Equipment A variety of single parameter and multiparameter field measurement instruments are available that use various technologies to measure the same water property and that require differing calibration maintenance and measurement methods
8. govern the number of verticals used and applies also to streams in which flow is divided for instance in a braided channel 4 Locate the midpoint of the first vertical at a distance of one half of the selected increment width from the edge of the water Locate other measurement verticals at the centers of the remaining increments EXAMPLE In a stream 60 ft wide that has been divided into 15 increments of 4 ft each the first measurement vertical would be 2 ft from the water s edge and subsequent verticals would be at 6 10 14 ft and so forth from the starting point at water s edge RULE OF THUMB In streams 5 ft wide or greater a minimum of 10 equal width increments generally is recommended In streams less than 5 ft wide use as many increments as practical but they must be equally spaced and a minimum of 3 inches in apart When making field measurements 1 Select either the in situ or the subsample method and follow the instructions in 6 0 2 B or 6 0 2 C respectively In situ method Measure at the midpoint of each equal width increment Using submers ible sensors measure at mid depth within the vertical Subsample method Collect an isokinetic depth integrated sample at the midpoint of each equal width increment emptying each sample into a compositing device Use of the correct sampling equipment is critical to execute this method successfully standard samplers cannot meet is
9. iV i U v i gt gt L i V i gt i Li v i gt i i gt iV gt v i i i i gt i gt U v i i gt V gt i gt i i gt i iV i v gt i i gt i v gt i gt i i i L i i V 7 Swirl or stir gently U i i gt i gt i ii i v i U gt i gt V V i gt U 7 i gt i V i i gt i L gt 4 7 gt i gt i L gt i v i i gt i v iVi gt i V iV i i ii i gt i v gt i v filtered samples 3 i i iV i gt i gt gt V i i iV gt i V i Vi V i gt gt gt iV i 2 i iV
10. measurement procedures for ground water using downhole and flowthrough cell chamber systems 25 6 0 6 Flow chart showing subsample field measurement procedures for conductivity pH and alkalinity of ground water 27 Tables 6 0 1 Stabilization criteria for recording dir ect field measurements 5 6 0 2 Example of field notes for a discharge weighted conductivity measurement 12 6 0 3 Example of field notes for an area weighted conductivity measurement 14 he citation for this section 6 0 of NFM 6 is as follows ilde F D 2008 Guidelines for field measured water quality properties ver 2 0 U S Geological urvey Techniques of Water Resources Investigations book 9 chap A6 section 6 0 October vailable only online from http pubs water usgs gov twri9A T W S a Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 3 GUIDELINES FOR FIELD MEASURED 6 0 WATER QUALITY PROPERTIES By Franceska D Wilde This section summarizes general information guidelines and standard procedures that apply to the direct determination of water properties that are measured onsite Procedures at sites where water quality data are being transmitted continuously are beyond the scope of this chapter 1 FI
11. of the center centroid of flow increment 3 the median conductiv ity would have been reported as 155 S cm if conductivity had been measured near the left edge o f the water increment 1 the c onductivity would have been reported as 185 S cm Table 6 0 2 Example of field notes for a discharge weighted conductivity measurement ft sec feet per second ft feet ft2 square feet ft3 sec cubic feet per second S cm microsiemens pe 25 degrees Celsius LEW left edge of water not available REW right edge of water Equal Percent Mean Width of Depth of Area of Increment discharge of flow in velocity increment increment increment discharge increment increment in ft sec in ft in ft in ft 2 in ft3 sec LEW 0 1 20 2 0 22 5 7 125 250 2 20 2 2 11 10 4 114 250 3 20 2 3 9 12 0 109 250 4 20 3 9 5 12 8 64 250 5 20 3 4 10 7 4 74 250 REW 0 r centimeter at Median conductivity in S cm 185 170 155 155 150 Calculation of conductivity mean of median conductivity measurements 815 163 S c m 5 Equal Width Increments EWI The stream cross section is divided into increments of equal width Knowledge of the streamflow distribution in the cross section is not required In situ field measurements are made at the midpoints of each increment
12. sampling and field or other conditions that could introduce bias to the determination of field measurement values Execute field measurements in a manner that avoids or minimizes bias from data collection activities Check instrument precision and accuracy variability and bias periodically while at a field site precision and accuracy may vary depending on the instrument used sampling conditions and the expertise of personnel The USGS standard procedure is to allow sensors that are calibrated in the field to equilibrate to the temperature of the water being monitored to the extent possible before making field measurements as is appropriate for the instrument in use Calibration buffers and standards also should be brought to ambient sample temperature before the instrument sensor s are field calibrated Chapter A6 Field Measurements Info Version 2 0 10 2008 8 INFO Sensors have equilibrated adequately when instrument readings have stabilized that is when the variability among measurements does not exceed an established criterion The USGS criteria for stabilized field readings are defined operationally in table 6 0 1 for a set of about five or more sequential measurements The natural variability inherent in surface water or ground water at the time of sampling generally falls within the stability criteria given in table 6 0 1 and reflects the accuracy that should be attainable with a properly calibr
13. subsequently collected for chemical analyses Puls and others 1991 Kearl and others 1992 Puls and Powell 1992 Test and calibrate field instruments Select downhole or flowthrough chamber system DOWNHOLE IN SITU FLOWTHROUGH CELL CHAMBER Lower sensors and pump intake to selected depth U Install pump in monitoring well or any additional lines and plumbing at a supply well with a pump U Hook up well and flowthrough cell chamber inline U Install sensors in flowthrough chamber cell unless a multiparameter sonde is being used Turn on pump and adjust flow to desired purge rate record rate and time purging began Allow sensors to equilibrate at purge rate U After two or more well volumes are purged and before final five or more readings are made adjust flow rate to be used for sampling flow flow must be sufficient for dissolved oxygen measurements see 6 2 U For pH divert flow from chamber and record measurement when water is quiescent Redirect flow to chamber for next set of readings Record and monitor sequential sets of field measurement readings during withdrawal of final well volume Extend purge time Document difficulty in field notes Are stabilization criteria being met table 6 0 1 NO YES Record at least 5 measurements at intervals of 3 to 5 minutes or more Report the median of the last 5 or more readings and the time of measurement Adjust flow to desired purge rate and record rate a
14. will be made or subsamples collected Streamflow distribution can be based on the long term discharge record for the site or on a discharge measurement made just prior to sample collection RULE OF THUMB Divide the stream into a minimum of four increments More increments could be needed for a stream site that is poorly mixed Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 11 To divide the cross section into increments of equal discharge The following comprises a summary of some steps needed in applying the equal discharge increment method and should not be used without an understanding of the method and the detailed instructions as given in NFM 4 1 3 1 Visually inspect the stream from bank to bank observing velocity width depth distribution and the apparent distribution of sediment and aquatic biota in the cross section Note the location of stagnant water eddies backwater reverse flows areas of faster than normal flow and piers or other obstructions 2 If the channel and the control governing the stage are stable historical streamflow data can be used to determine the measurement locations If the channel is unstable or if no historical data are available make a discharge measurement and preliminary field measurements across the selected section of channel a From the available discharge data either Construct a graph using cumulative discharge or cu
15. 992 Suggested modifications to ground water sampling procedures based on observations from the colloidal borescope Ground Water Monitoring Review v 12 no 2 p 155 160 Lapham W W Wilde F D and Koterba M T 1997 Guidelines and standard procedures for studies of ground water quality Selection and installation of wells and supporting documentation U S Geological Survey Water Resources Investigations Report 96 4233 110 p Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 29 Puls R W and Powell R M 1992 Acquisition of representative ground water quality samples for metals Ground Water Monitoring Review v 12 no 3 p 167 176 Puls R W Powell R M Clark D A and Paul C J 1991 Facilitated transport of inorganic contaminants in ground water part II Colloidal transport Ada Oklahoma Robert S Kerr Laboratory U S Environmental Protection Agency Report EPA 600 m 91 040 12 p Rantz S E and others 1982 Measurement and computation of streamflow Volume 1 Measurement of stage and Volume 2 Computation of discharge U S Geological Survey Water Supply Paper 2175 v 1 p 1 284 v 2 p 285 631 Reilly T E and Gibs Jacob 1993 Effects of physical and chemical heterogeneity of water quality samples obtained from wells Ground Water v 31 no 5 p 805 813 U S Geological Survey 1978 Sediment chap 3 in U S Geological Survey National handbook of recommended met
16. Area weighted concentrations can be computed from these measurements table 6 0 3 Subsample field measurements are made in discrete samples that usually are withdrawn from a composite sample collected using an isokinetic sample and isokinetic depth integrating method The volume of the isokinetic sample must be proportional to the amount of discharge in each increment and measurements in subsamples taken from the compositing device result in discharge weighted values Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 13 To divide the cross section into increments of equal width The following comprises a summary of some steps needed in applying the EWI method and should not be used without an understanding of the method and the detailed instructions given in NFM 4 1 3 1 Visually inspect the stream from bank to bank observing velocity width depth distribution and the apparent distribution of sedment and aquatic biota in the cross section Note the location of stagnant water eddies backwater areas of faster than normal flow and piers or other obstructions 2 Determine stream width using a tagline or from station markings on bridge railings or cableways 3 Divide the section into increments of equal width based on flow and stream channel characteristics along the cross section field measurement variability from the cross section profile and data objectives for the study This interval width will
17. ELD MEASUREMENTS2 Determinations of physical or chemical properties that must be measured onsite as close as possible in time and space to the medium being sampled Onsite measurement is necessary to preserve sample integrity and ensure data accuracy for the following field measurements water temperature dissolved oxygen DO concentration specific electrical conductance SC pH reduction oxidation potential redox ORP alkalinity and acid neutralizing capacity ANC 3 and turbidity 4 In addition guidance is provided in section 6 8 for the use of multiparameter instruments instruments for which the individual field measurement sensors are bundled in a sonde and deployed to the water body for in situ measurements QUALITY ASSURANCE 6 0 1 Adherence to standard U S Geological Survey USGS quality assurance protocols is mandatory and quality control procedures are to be incorporated into every USGS water quality data collection effort Quality assurance of the data collected includes the timely and accurate documentation of field information in electronic and paper records second or third party auditing of such records consistent and conscientious use of procedures and protocols to ensure sample integrity and data quality and training in measurement techniques and the collection of quality control data Quality control data for field measurements includes records of replicate measurements 1 USGS guidelines for continuous monitoring
18. INFO 1 GUIDELINES FOR FIELD MEASURED 6 0 WATER QUALITY PROPERTIES By Franceska D Wilde Page 6 0 Guidelines for field measured water quality properties INFO 3 6 0 1 Quality assurance 3 6 0 1 A Records 4 6 0 1 B Criteria for performing field measurements 6 6 0 2 Surface water 9 6 0 2 A Summary of surface water sampling methods 9 Equal discharge increments EDI 10 Equal width increments EWI 12 6 0 2 B In situ field measurement procedures 15 6 0 2 C Subsample field measurement procedures 16 6 0 3 Ground water 17 6 0 3 A Summary of well purging protocols
19. NFO 9 SURFACE WATER 6 0 2 Field measurements must accurately represent the physical or chemical properties of the surface water being studied In order to collect data that represent water conditions at the time of sampling it is necessary to correctly locate the point s of measurement select equipment appropriate to site conditions and study needs and use appropriate methods to make accurate field measurements Properties of water temperature DO concentration and Eh must be measured directly within the water body in situ Other properties such as pH conductivity and turbidity often are measured in situ but may be measured in a subsample of a composite or grab sample as appropriate for the study Summary of Surface Water Sampling Methods 6 0 2 A Standard USGS procedures for locating points of sample collection and sample collection methods for surface water sampling are detailed in NFM 4 1 This section 6 0 2 A provides an abbreviated description of surface water isokinetic sampling methods and should not be used without a detailed understanding of the method as given in NFM 4 1 3 The quality and interpretation of the field measurement data collected depend also on the equipment with which samples are withdrawn composited and subsampled Guidance for equipment selection is detailed in NFM 2 1 1 Familiarity with the USGS protocols and standard procedures prescribed in these chapters of the National Field Manual is the responsibi
20. Single parameter instruments include for example stand alone temperature sensors DO SC and pH meters with or without temperature sensors and turbidimeters for example nephelometers The protocols and procedures that pertain to these instruments are a primary focus of NFM 6 1 through NFM 6 5 Single parameter instruments are selected according to project needs and convenience or when required because site conditions do not allow use of a multiparameter instrument or for the purpose of instrumenting a flowthrough cell or chamber Multiparameter instruments have measurement sensors that fit into the body of a sonde Measurements using the sonde can be made either by submersing the sensor sonde in surface water or ground water or by pumping a sample inline from its source to an airtight cell chamber into which the sonde fits Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 7 Field teams must determine if the instruments and methods to be used will produce data of the type and quality required to fulfill study needs as well as USGS requirements Check the appropriate section in this chapter for the required and recommended methods and equipment instructions for measurement and quality control procedures and guidelines for troubleshooting and data reporting Become familiar with the instructions and precautions provided by the manufacturers of the instruments to be used Field personnel
21. alue shown for about five or more measurements Temperature Thermistor thermometer Liquid in glass thermometer 0 2 C 0 5 C Conductivity SC 100 S cm gt 100 S cm 5 percent 3 percent pH meter displays to 0 01 0 1 to 0 2 pH unit3 Allow 0 3 pH units if drifting persists or if measurement is in low conductivity 75 S cm water DO4 Amperometric sensors Optical luminescent method sensors 0 2 mg L 0 2 mg L Turbidity5 6 100 FNU or other turbidity unit gt 100 FNU or other turbidity unit 0 5 turbidity unit or 5 of the measured value whichever is greater 10 percent6 1 Eh is not considered to be a routine or direct field measurement see NFM 6 5 Alkalinity and acid neutralizing capacity determinations require a titration procedure and are not considered direct measurements 2 Refer to NFM 6 8 for similar criteria when using multiparameter instruments For continuous monitors consult Wagner and others 2006 3 Select pH sensor criteria based on precision and accuracy listed for the sensor being used 4 Amperometric sensors Note that the calibration criterion when DO is measured by a continuous monitor can be extended to 0 3 mg L Optical Luminescent sensors The criterion for luminescent method sensors is biased conser vatively owing to the differing technologies that are employed among the various manufacturers of these sensors and current paucity of field
22. annular space between the casing and the borehole wall using knowledge of the borehole diameter 3 In monitor and other wells with downhole access lower a submersible pump followed by a water level sensor to the desired location of the pump intake The final pump intake position always is located at the point of sample collection Lower the equipment slowly and smoothly to avoid stirring up particulate matter Position the pump intake between 3 ft 0 9 meter m below the static water surface and a minimum distance above the top of the open screened interval of 10 times the well diam eter for example 20 in for a 2 in well diameter if the sample is to be integrated over the entire screened or open area of the aquifer The location of the intake depends on the study objective well construction and the type of equipment used Unless the intended sampling interval can be isolated adequately locating the intake at a point within a well screen or open borehole will result in extracting a sample that includes water from the entire screened or open interval Varljen and others 2006 Reilly and Gibs 1993 The water level sensor should be a maximum of 1 ft 0 3 m below the static water sur face 4 Start the pump Gradually increase and or adjust the pumping rate to limit drawdown t o between 0 5 and 1 0 ft 0 15 to 0 3 m or as determined by field practice If the final intake position is above the screened or o
23. ated instrument Surface water Allow at least 60 seconds or follow the manufacturer s guidelines for sensors to equilibrate with sample water Take instrument readings until the stabilization criteria in table 6 0 1 are met Record the median of the final three or more readings as the value to be reported for that measurement point section 6 0 2 Ground water Start recording measurements after sensors have equilibrated with purge water Take instrument readings until the stabilization criteria in table 6 0 1 are met and the required number of well volumes of ground water have been purged Record the median of the final five or more readings as the value to be reported for that site section 6 0 3 For sites at which variability exceeds the criteria shown in table 6 0 1 Allow the instrument a longer equilibration time and record more measurements To determine the value to be reported for that measurement point either use the median of the final five or more measurements recorded or apply knowledge of the site and professional judgment to select the most representative of the final readings RULE OF THUMB For field analyses that are made on subsamples such as alkalinity check your precision in the field at least every tenth sample by making the measurement three times using separate sample aliquots from the same sample volume Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 I
24. codes xls Well volume V 0 0408 HD 2 ____ gallons Purge volume n V ____ gallons V volume of water in well in gallons D inside well diameter in inches H height of water column in feet n number of well volumes to purge Well volume is 0 16 gallons per foot for a 2 in diameter casing Figure 6 0 4 Example of a field form for recording measurements during well purging modified from NFM 4 Figure 4 12 Chapter A6 Field Measurements Info Version 2 0 10 2008 24 INFO 6 0 3 B Direct Field Measurement Procedures Direct field measurements are those that are performed either a above ground by pumping water from the well inline directly to a sensor instrumented flowthrough cell or chamber inline flow measurement procedure performed using either single parameter instruments within a flowthrough chamber or a multiparameter instrument or b downhole using individual submersible sensors or a multiparameter sonde in situ measurement procedure Ambient ground water temperature SC DO pH Eh and turbidity can be determined using either procedure fig 6 0 5 but use of the in situ procedure can have more limitations and to avoid sample contamination should not be used if samples for laboratory or other offsite analyses wi ll be collected Inline flow Flowthrough Cell Chamber Procedure A flowthrough cell chamber is an airtight transparent vessel with a pressure relief valve and with either 1
25. data Spectrophotometric method Stabilization is not applicable to the spectrophotometric method 5 Multiparameter instruments used for most USGS turbidity applications contain single beam infrared wavelength turbid ity sensors and are reported in FNU Check the Excel spreadsheet at http water usgs gov owq turbidity_codes xls to determine the appropriate turbidity unit of measure consult NFM 6 7 for detailed guidance on turbidity measurement and instrumentation 6 In high turbidity conditions especially when collecting data during storms lengthening the averaging period to help smooth out the signal assuming this is an option for the instrument in use or increasing the time period between measurements is recommended Chapter A6 Field Measurements Info Version 2 0 10 2008 6 INFO 6 0 1 B Criteria for Performing Field Measurements Field measurements should represent as closely as possible the ambient physical and chemical properties of the surface water or ground water system at the time of sampling Properties such as temperature DO concentration and Eh must be measured directly in situ in the water body Other properties such as pH conductivity and turbidity may be measured either in situ or from a sample withdrawn from the source depending on the type of equipment selected for field measurements Alkalinity determination requires titration on a measured volume of subsample These properties are to be measured
26. dose zone investigations American Society for Testing and Materials publication code number 03 418192 38 166 p Edwards T K and Glysson D G 1999 Field methods for measurement of fluvial sediment U S Geological Survey Techniques of Water Resources Investigations book 3 chap C2 80 p Fishman M J and Friedman L C 1989 Methods for determination of inorganic substances in water and fluvial sediments U S Geological Survey Techniques of Water Resources Investigations book 5 chap A1 545 p Hem J D 1086 Study and interpretation of the chemical characteristics of natural water U S Geological Survey Water Supply Paper 2254 263 p Hoopes B C ed 2004 User s manual for the National Water Information system of the U S Geological Survey USGS U S Geological Survey Open File Report 2004 1238 262 p Horowitz A J Demas C R Fitzgerald K K Miller T L and Rickert D A 1994 U S Geological Survey protocol for the collection and processing of surface water samples for the subsequent determination of inorganic constituents in filtered water U S Geological Survey Open File Report 94 539 57 p Hydrologic Instrumentation Facility 1991 Hydrolab H2O in Instrument News Stennis Space Center Miss U S Geological Survey Hydrologic Instrumentation Facility December 1991 p 7 9 Hydrologic Instrumentation Facility 1992 pH in Instrument News Stennis Space Center Miss U S Geological
27. ecords for surface water and ground water respectively For every field visit instrument performance and the field measurement data are transcribed onto paper and or electronic field and laboratory request forms by the data collector Much of the documentation process should be completed at the field site Records should be checked independently by a field partner and or data administrator Additional field forms such as chain of custody or land use forms may be required depending on project needs USGS personnel are encouraged to use the Personal Computer Field Form PCFF instead of the paper field notes form to increase efficiency and decrease transcription errors A separate log book must be maintained for each field instrument and into which are recorded instrument repair maintenance and calibration history The log book travels with the instrument The pages of the instrument log books should be pre numbered consecutively Do not skip or tear out pages Water resistant paper is recommended Log books are available to USGS personnel through the One Stop Shop item Q609FLD Use a b lue or black indelible ballpoint pen to write o n paper forms and in log books If recording in a Rite in the Rain log book or on other water resistant surface an all weather or bullet pen is recommended Use of a pencil is not acceptable felt tipped pens for example Sharpie should not be used and could compromise the quali
28. ed to the wellhead so as to bypass holding tanks backflow pressure tanks flow meters or chemical treatment systems Monitor wells At wells that allow downhole access it is necessary for the study to determine the specific depth interval at which measurements will be made or from which water will be withdrawn for field measurement determinations Again selection of the sampling depth interval depends on how the well was constructed in addition to the sampling design of the study Record the depth at which the pump intake is set within a given screened or open interval The standard purging protocol usually is appropriate Consult NFM 4 2 before field work begins the summary given in section 6 0 3 A does not provide all the information needed to ensure adequate performance Either direct measurement or subsample procedures can be used at monitor wells depending on study needs and sampling objectives If samples for laboratory analysis w ill be collected however use of the flowthrough cell chamber system inline flow procedure is recommended in order to avoid sample contamination from use of a downhole se nsor system or bailer 5 The terms flowthrough cell and flowthough chamber are synonymous although cell is more commonly used by manufacturers of such equipment 6 Supply wells designed for public supply systems domestic supply or commercial or agricultural use Info Version 2 0 10 2008 U S Geological Su
29. ement Using sub mersible sensors measure at mid depth or multiple depths within the vertical Repeat at each vertical The value recorded at each vertical represents the median of values observed within approximately 60 seconds after sensor s have equilibrated with stream water Subsample method Collect an isokinetic depth integrated sample at the centroid of each equal discharge increment emptying the increment sample into a compositing device Measure field parameters either in the sample collected at each increment or in a subsample taken from the composite of all the increment samples 2 The final field measurement value is the mean of the in situ or individual increment sample value for all the EDI verticals in the section the composite subsample yields a single value Note for pH use the median value or compute a mean pH using the method described in section 6 4 3 A Enter data on field forms Chapter A6 Field Measurements Info Version 2 0 10 2008 12 INFO EXAMPLE Table 6 0 2 provides an example of how mean conductivity measured in situ is calculated using the EDI method In the example the correct value for the discharge weighted mean conductivity is 163 S cm calculated from 815 divided by 5 the sum of the recorded median values divided by the number of median measurements This mean value should be entered into the QWDATA database as a calculated value Note that at the midpoint
30. ensors containing sonde of a multiparameter instrument to which the sample is pumped directly from the well to the measurement chamber c A multiparameter instrument without the flowthough cell on the sonde deployed downhole in the well d Selected single parameter meters with sensors allowing for downhole submergence e Single parameter meters the measurement is made with each sensor immersed in separate subsamples collected from a discrete volume sampler Options a and b generally are preferred and constitute what is termed in this report as the Inline flow Procedure Options c and d constitute what is termed in this report as the Downhole in situ Procedure Option e is termed in this report as the Subsample Procedure 2 Check the identification number on the well and well depth to verify that this is the well intended for data collection or sampling Review the well s record of water levels drawdown and field measurement variability if available and prepare the necessary field forms for example the well purge record national or study field form and chain of custody record Record the type of equipment being used 3 Lay plastic sheeting around the well to prevent contaminating the equipment Unlock the well housing or top of the protective casing and remove the well cap Set up the field measurement system paying attention to the sequence in which sensors are inserted into the flowthr
31. g measurement sensors downhole downhole or in situ procedure or 3 by subsampling water collected from a discrete volume sampler for example bailer and making measurements using individual instruments subsample procedure Inline flow and in situ procedures comprise direct measurement methods section 6 0 3 B in contrast to the subsample method section 6 0 3 C For most field applications using inline flow procedures the flowthrough cell chamber system is recommended and most efficient RECOMMENDED Use a positive displacement submersible pump for ground water withdrawal Use inline flow flowthrough cell chamber procedures for field measurements The measurement system selected can be constrained by the type of well being sampled and its accessibility Supply wells6 Downhole access for a pump or sensors might be limited or not possible at public or domestic supply wells and water withdrawal may be located at or near the well head or from a tap for drinking water studies The flowthrough cell chamber field measurement system inline flow procedure should be used in this case The standard purging protocol may not be appropriate Consult Exceptions to the standard purging procedure in NFM 4 2 Identify equipment permanently installed in or at the well such as a pump that can affect the logistics and quality of the field measurement The field measurement system should be connect
32. hods for water data acquisition p 3 1 to 3 100 U S Geological Survey variously dated National field manual for the collection of water quality data U S Geological Survey Techniques of Water Resources Investigations book 9 chaps A1 A9 available online at http pubs water usgs gov twri9A Varljen M D Barcelona M J Obereiner James and Kaminski David 2006 Numerical simulations to assess the monitoring zone achieved during low flow purging and sampling Ground Water Monitoring and Remediation v 26 no 1 p 44 52 Wagner R J Boulger R W Jr Oblinger C J and Smith B A 2006 Guidelines and standard procedures for continuous water quality monitors Station operation record computation and data reporting U S Geological Survey Techniques and Methods 1 D3 available only online at http pubs water usgs gov tm1d3 Accessed August 28 2008 Ward J R and Harr C A 1990 Methods for collection and processing of surface water and bed material samples for physical and chemical analyses U S Geological Survey Open File Report 90 140 71 p ACKNOWLEDGMENTS 6 0 5 This version of NFM 6 0 dated October 2008 represents an update of the September 2006 version The information contained in this report is based largely on the scholarship and field expertise of D B Radtke one of the original authors of this and other chapters of the National Field Manual for the Collection of Water Quality Data In addition
33. ictate 2 Select the EDI or EWI method to locate points of measurement refer to NFM 4 1 3 to select and execute the appropriate method If stream depth and velocities along the cross section are relatively uniform use EWI If stream depth and velocities along the cross section are highly variable use EDI In a small usually less than 5 feet ft in width and well mixed stream a single point at the centroid of flow may be used to represent the cross section The stream is considered well mixed if the variability of field measurement values noted in the cross sectional pro file is considered negligible for example within the stabilization criteria given on table 6 0 1 plus best professional judgment of what is reasonable for the field site The centroid of flow is defined as the point in the increment at which discharge in that increment is equal on both sides of the point Equal Discharge Increments EDI The stream cross section is divided into increments of equal discharge Field measurements can be made in situ at the centroid of each increment or by collecting an isokinetic depth integrated sample at the centroid of each increment and determining the value either of each sample or of a composite of the samples These methods result in data that are discharge weighted Edwards and Glysson 1999 Knowledge of streamflow distribution in the cross section is required to select verticals at which measurements
34. ity rinse with deionized water 5 Insert sensor s into measurement vessel U Wait for sensors to equilibrate to sample temperature U Don t let sensors touch bottom or sides of vessel 6 Swirl or stir gently to mix sample U Minimize streaming potential or vortex keep sensor out of vortex U For pH do not stir samples with conductivity less than 100 S cm U When using magnetic stirrer use smallest stir bar 7 Record field measurement and method used on field form U Record median value of stabilized readings table 6 0 1 U If readings do not stabilize extend number of measurements and record median of at least 5 or more sequential readings U If there is a constant trend toward lower or higher values record the first value the range of values and the time period observed 8 Repeat process from steps 4 through 7 on two or more subsamples from the same sample volume to document precision Rinse sensors and equipment thoroughly with deionized water Discard sample to waste in accordance with applicable regulations Chapter A6 Field Measurements Info Version 2 0 10 2008 28 INFO 6 0 4 SELECTED REFERENCES ASTM International 2005 D6452 99 2005 Standard guide for purging methods for wells used for ground water quality investigations ASTM International publication accessed May 5 2005 from http www astm org ASTM International 1992 Standards on ground water and va
35. ld measurements across the stream must be known An exploratory field measurement profile across a section of a stream can be used to estimate the magnitude of variation along the cross section A field measurement profile can be useful also especially at new or poorly docu mented sites to determine which isokinetic method equal discharge increment EDI or equal width increment EWI should be used for sampling Chapter A6 Field Measurements Info Version 2 0 10 2008 10 INFO The final points of measurement are determined according to whether the EDI or EWI method will be used Make individual measurements at a number of equally spaced verticals along the cross section and at multiple depths within each vertical Alternatively consult previous records for the site To locate measurement points 1 Check the cross sectional profile data of the stream site to determine the variability of discharge per unit width of the stream and of field measurement values across the section Make individual measurements at a number of equally spaced verticals along the cross sec tion and at multiple depths within each vertical or consult previous records for the site Make in situ 6 0 2 B field measurements for the profile Field measurement profiles of stream variability are needed for low and high flow condi tions and should be verified at least every 2 years or as study objectives d
36. leak proof ports compression fittings to accommodate individual sensors or 2 a multiparameter instrument Several types of flowthrough cell chamber systems are available and can be designed for a specific measurement or simultaneous measurements for example see NFM 6 2 2 for the description of a flowthrough cell for the spectrophotometric determination of DO concentration Figure 6 0 3 is a diagram of a flowthrough cell system and figure 6 0 5 shows the steps for inline flow measurement of field parameters using a flowthrough cell chamber system All sensors must be calibrated before use as described in the relevant sections of this chapter sections 6 1 through 6 8 and according to the manufacturer s guidelines for the instrument in use When using a flowthrough cell chamber system install the cell chamber inline from the pump a nd as close to the wellhead as possible Keep the cell chamber field measurement instruments and tubing off the ground shaded from direct sunlight and shielded from wind Keep the tubing as short as possible For a system for which sensors are to be installed in the chamber insert the DO sensor directly downstream from the chamber inflow and install the pH sensor downstream from the conductivity sensor Turn on the pump direct initial flow to waste to avoid introducing sediment into the chamber For a multiparameter instrument system place the sonde into the cell provided by the manufac
37. lity of all USGS water quality field personnel Normally the point or points at which field measurements are made correspond to the location s at which samples are collected for laboratory analysis NFM 4 1 3 The decision for whether grab sampling methods or isokinetic sampling methods will be used is based on the characteristics of the water body to be sampled or monitored and on study objectives Still water conditions are found in storage pools lakes and reservoirs Field measurements often are made in situ at multiple locations and depths The location number and distribution of measurement points are selected according to study objectives Measurements made at discrete depths through the vertical water column must not be averaged or reported as a median value that represents the entire vertical Report the value selected to represent each point measured in the vertical as individual stations or distinguish measurements in that vertical by assigning a unique time to each measurement Flowing water conditions are found in perennial and ephemeral streams The location and the number of field measurements depend on stream conditions and study objectives NFM 4 1 Generally a single set of field measurement data is used to represent an entire stream cross section at a sampling site and can be useful when calculating chemical loads To obtain data representative of the section the variability of discharge and fie
38. measurement values for each field property against the criteria for field measurement stabilization table 6 0 1 Determine final field measurement values If the criteria are being met report the median value If criteria are not being met consult the study requirements and objectives Extend the purge time if readings still do not stabilize report the median value of the last five or more sequential measurements and document problems encountered and how they were resolved To purge a supply well assuming that a flowthrough cell chamber will be used A supply well that is in regular service and that is pumping continuously or that has been operating long enough to have removed three casing volumes of water within several hours of sample collection does not require removal of three well volumes Before withdrawing a sample in this case flush one tubing volume of sample water through the tubing and monitor measurement values 1 Determine or estimate and record the depth to static water level fig 6 0 4 Instructions for water level measurement are given in NFM 4 appendix A4 B 2 Calculate and record the well volume fig 6 0 4 3 Select the location and method of tubing hookup to the well and connect sample tubing as close as possible to the wellhead There must be no water storage tanks holding or pressurization tanks or chemical disin fection or water softening systems connected inline bet
39. median can be reported instead of the mean but be sure to document this on the field form and in the final data report if a parameter code is not available in NWIS for median values Table 6 0 3 Example of field notes for an area weighted conductivity measurement ft feet LEW left edge of water ft2 square feet SC conductivity specific electrical conductance S cm microsiemens per centimeter at 25 degrees Celsius not available REW right edge of water Section number Cumulative percent of flow in section Vertical location in ft from LEW Width of section in ft Depth of vertical in ft Area of section in ft2 Median SC S cm Product of median SC and area LEW 0 0 1 2 2 4 1 0 4 0 150 600 2 4 6 4 2 0 8 0 145 1 160 3 6 10 4 2 6 10 4 145 1 508 4 10 14 4 3 2 12 8 140 1 792 5 16 18 4 3 5 14 0 135 1 890 6 22 22 4 4 0 16 0 130 2 080 7 28 26 4 4 5 18 0 130 2 340 8 34 30 4 5 4 21 6 125 2 700 9 42 34 4 6 0 24 0 125 3 000 10 50 38 4 5 7 22 8 125 2 850 11 62 42 4 5 1 20 4 125 2 550 12 76 46 4 4 6 18 4 125 2 300 13 88 50 4 3 5 14 0 125 1 750 14 96 54 4 1 4 5 6 135 756 15 99 58 4 1 0 4 0 140 560 REW 100 60 27 836 130 S cm Calculation of SC sum of values in last column divided by the total cross sectional area
40. mulative percent of total discharge plotted against the cross sectional width or Determine EDI sections directly from the discharge measurements note sheet b If profile values of pH conductivity temperature and DO differ by less than 5 percent and show that the stream is well mixed both across the section and from top to bottom a single measurement point at the centroid of flow can be used to represent field measurement values of the cross section 3 From the graph or measurement notes determine the number and locations of EDIs and the centroids the far midpoints of those increments EXAMPLE If five increments will be used select points of measurement by dividing the total stream discharge by 5 to determine increment discharge in this case each EDI equals 20 percent of discharge The first vertical is located at the centroid of the initial EDI the point where cumulative discharge equals 10 percent of the total discharge The remaining 4 centroids are found by adding increment discharge to the discharge at the initial EDI centroid The far midpoints also need to be calculated The EDI centroids will correspond to points along the stream cross section where 10 30 50 70 and 90 percent of the total discharge occur When making field measurements 1 Select either the in situ or subsample method and follow the instructions in 6 0 2 B or 6 0 2 C In situ method Go to the centroid of the first equal discharge incr
41. nd time purging began U Divert initial water to waste U Connect chamber in line from pump U Adjust flow to chamber and eliminate backpressure allow sensors to equilibrate Figure 6 0 5 Field measurement procedures for ground water using downhole and flowthrough cell chamber systems Chapter A6 Field Measurements Info Version 2 0 10 2008 26 INFO 6 0 3 C Subsample Field Measurement Procedures Subsamples or discrete samples are aliquots of sample collected from a nonpumping sampling device such as a bailer a thief sampler or a syringe sa mpler Measurements of field parameters made in discrete or nonpumped samples are more vulnerable to bias from changes in temperature pressure turbidity and concentrations of dissolved gases t han measurements using inline flow or in situ measurement procedures Subsamples must not be used for reported measurements of temperature DO Eh or turbidity Subsamples can be used to determine ambient ground water conductivity pH and alkalinity and other carbonate species Subsample procedures must not be used for any field measurement determination if the ground water conditions are in reducing anoxic waters unless the sample is decanted and measurements are made within a chamber or glove box filled with an inert gas for example ultrapure nitrogen or argon gas Figure 6 0 6 shows the steps for measurement of field parameters on a bailed sample Use bailers ha
42. not be used when measuring DO Eh or other properties sensitive to oxygen contamination or volatilization When the depth to water is greater than 250 ft and or a large volume of water must be purged a dual pump system can be used position a submersible pump downhole and a centrifugal pump at the surface see NFM 4 2 for additional information An inflatable packer system set above the pump can be used to reduce the volume and time required for purging this can be especially useful in deep wells or if regulations require that purge water be contained Bailers Bailers are not recommended for purging because they have limited use for routine field measurement determinations and also are inefficient for well purging The bailed sample must be decanted to obtain subsamples for field measurements exposing the sample to atmospheric conditions Thus water subsampled from a bailer sample may not be used for measurements of DO concentration Eh and water temperature The action of the bailer being deployed downhole can introduce atmospheric gases and particulates into the water column also resulting in biased turbidity and other f ield measurements Chapter A6 Field Measurements Info Version 2 0 10 2008 18 INFO Ground water is sampled for field measurements in one of three ways 1 by pumping formation water inline to a flowthrough cell or chamber5 containing measurement sensors inline flow procedure 2 by deployin
43. of selected field measurements are described in Wagner and others 2006 2 The term field measurements as used in this National Field Manual is synonymous with field properties and field parameters terms that are used commonly in environmental water quality literature 3 Alkalinity ANC is a field analysis not a direct field measurement however the analysis is performed routinely during the same field trip in which the other field measurement data are collected In this section the term alkalinity is used when referring either to alkalinity or acid neutralizing capacity 4 Each of these field measurements is discussed in detail in the following sections of this chapter of the National Field Manual temperature section 6 1 dissolved oxygen section 6 2 specific electrical conductance section 6 3 pH section 6 4 reduction oxidation potential section 6 5 alkalinity and acid neutralizing capacity section 6 6 and turbidity section 6 7 Chapter A6 Field Measurements Info Version 2 0 10 2008 4 INFO 6 0 1 A Records Record keeping is the responsibility of all field personnel Electronic and paper records must be established and maintained for each uniquely identified sampling locatio n to permanently document field activities measurement readings instrument calibration and any other information needed to meet programmatic or regulatory requirements see NFM 4 1 1 and 4 2 1 for detailed information that relates to r
44. okinetic requirements when stream velocity is less than 1 5 feet second NFM 2 1 1 A 2 Record a value for each field measurement at each vertical The value recorded represents the stabilized values observed within approximately 60 seconds after the sensor s have equilibrated with the stream or subsample water Chapter A6 Field Measurements Info Version 2 0 10 2008 14 INFO EXAMPLE Table 6 0 3 provides an example of an area weighted median measurement for conductivity measured in situ In the example the area weighted median conductivity equals 130 S cm To calculate an area weighted median multiply the area of each increment by its corre sponding field measurement sum the products of all the increments and divide by total cross sectional area Note that if the conductivity reported were selected at mid depth of the vertical of centroid of flow section 10 it would have been reported as 125 S cm if the conductivity reported were near the left edge of the water it would have been reported as 150 S cm The final field measurement value normally is calculated as the mean of the values recorded at all EWI increments resulting in an area weighted mean Note for pH take the median value or to calculate a mean pH be sure to use only the method described in section 6 4 3 A Alternatively for EWI if the area weighted median best represents integrated stream chemistry then the
45. or infrequent pumping ASTM International 2005 Purging also serves to rinse and condition sampling equipment with the ambient ground water to be sampled Ground water can be withdrawn from wells using submersible or above ground pumps or bailers or similar discrete volume samplers for example syringe or Kemmerer samplers Well type and construction sampling objectives and target analytes and site conditions can constrain the equipment selected which method of ground water withdrawal will be used and how it will be employed NFM 2 1 2 provides detailed information on equipment for ground water withdrawal at wells and sample collection Pumps The pumping mechanism can affect the degree to which certain measurements represent true ground water properties For example pumps that introduce turbulence or heat to the water column can result in nonrepresentative DO and temperature values The pump should produce a smooth solid stream of water with no air or gas bubbles and without pump cavitation during field measurements and sample withdrawal A positive displacement submersible pump is recommended for environmental sampling at depths greater than 28 ft and generally is recommended for use at wells unless the well has a permanently installed pump A peristaltic pump often is a good choice for obtaining samples collected from wells at a depth of 28 ft or less but the stream of water from this or other suction lift pump should
46. ough chamber with respect to water flow if a flowthrough chamber is used Figure 6 0 3 illustrates a typical sample collection manifold through which water is directed either to waste to the flowthrough field measurements cell chamber or to the sample collection chamber To adjust the direction of flow a flow regulating needle valve or ball valve should be kept either full on or full off and should not be used to adjust either the proportion or rate of flow 4 Review the purging history of the well particularly the field measurement data previously recorded This can save time and help determine the procedures and length of time over which the well should be purged Chapter A6 Field Measurements Info Version 2 0 10 2008 20 INFO From the portable pump tubing used for a monitor well or from the garden hose threaded connections for a water supply well Sample collection chamber Extension line Extension line Manifold system Flowthrough cell chamber To waste water drainage Flowthrough cell chamber note sensor placement and direction of flow To waste water drainage EXPLANATION RIGID WALL TEFLON TUBING QUICK CONNECTION FIELD SENSORS flowthrough chamber pH sensor Dissolved oxygen sensor Temperature sensor Specific electrical conductance sensor ANTIBACKSIPHON THREE WAY TEFLON FLOW VALVE FLOW REGULATING NEEDLE VALVE PINCH CLAMP Keep valve either full on o
47. pen interval the final pumping rate should be about 500 to 1 000 milliliters mL per minute Do not exceed 1 ft of drawdown If the pump and intake position are fixed as in a supply well control the rate of flow for field measurements through flow splitting valve s Monitor the water level as purging progresses Ideally drawdown will be at a steady state with the water level remaining above the top of the open or screened interval 5 Purge a minimum of three well volumes or the purge volume dictated by study objectives consult NFM 4 2 3 for detailed guidance on exceptions to the standard three well volume purging procedure Monitor values for field properties sequentially and at regular time intervals throughout purging fig 6 0 4 The frequency of these measurements depends on the purging rate which in turn is a function of well depth and diameter and aquifer transmissivity If required by Federal State or local regulations contain purge water as directed Chapter A6 Field Measurements Info Version 2 0 10 2008 22 INFO 6 As the third or last well volume is purged slowly adjust the purge rate to the pumping rate to be used during sampling if necessary do not halt or suddenly change the pumping or flow rate during the final phase of purging or while sampling Record field measurements at regular time intervals about 3 to 5 minutes apart Check the last three to five or more
48. r full off do not use valve to adjust the proportion or rate of flow FLOW DIRECTION IN MANIFOLD AT DIFFERENT STAGES During initial purge stage During intermediate and final stages While obtaining most field measurements To obtain turbidity samples if sensor is not available for the cell chamber and at end of purge to route flow to a chamber for collection 1 1 2 2 2b 2b 2a 2a DO DO T T pH pH SC SC optional Figure 6 0 3 Sketch of a manifold and flowthrough cell chamber used during well purging and sample collection top view modified from NFM 4 Figure 4 10 Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 21 To purge a monitor well assuming that the well will be pumped and that a flowthrough cell chamber system and water level sensor will be used 1 Measure and record the depth to static water level fig 6 0 4 Instructions for water level measurement are given in NFM 4 appendix A4 B 2 Calculate and record the well volume using the information on fig 6 0 4 Note that the depth to the screened or open interval and the inside casing diameter must be known to calculate well volume Calculate the casing volume using the height of the water column to the bottom of the well instead of the water column height to the top of the screen Include an estimate for the volume of water stored in the
49. rvey TWRI Book 9 INFO 19 Summary of Well Purging Protocols 6 0 3 A As a rule of thumb the standard USGS purge protocol is to remove three or more well volumes of standing water while monitoring water level and the stabilization of routine field measurements as a function of time pumping rate and the volume of water being removed see NFM 4 2 3 Routine field measurements include pH temperature SC DO and turbidity Inherent in the purge procedure is an assumption that stabilization of field properties indicates that the water being discharged from the well represents ambient formation water Field personnel should examine this assumption for each well using their knowledge of the well and aquifer hydraulics Field experience understanding of the effects of hydrologic and geochemical conditions and knowledge of data collection and data quality requirements are necessary to determine the most accurate field value Before purging begins 1 Select and set up the field measurement system that best fits the requirements for the data collection effort Options include a A clear air tight flowthrough chamber holding the sensors of single parameter instruments generally temperature pH DO SC often turbidity and sometimes Eh ORP The flowthrough chamber is connected to tubing through which the sample is pumped directly from the well to the measurement chamber b A clear air tight flowthrough cell chamber fitted to the s
50. s Figure 6 0 1 In situ field measurement procedures for surface water INFO 15 In situ field measurement procedures 6 0 2 B An in situ measurement fig 6 0 1 made by immersing a mutiparameter sonde or one or more single parameter field measurement sensors directly into the water body is used to determine a profile of variability across a stream section In situ measurement can be repeated if stream discharge is highly variable and if measurement points need to be located at increments of equal discharge Note that in situ measurements are point samples and thus are not depth integrated In situ measurement is mandatory for determining temperature DO concentration and Eh In situ measurement also can be used for pH conductivity and turbidity The alkalinity determination can only be performed on a stirred subsample contained in a vessel If field measurements for pH DO and Eh are to be determined in anoxic water the measurements must be made in situ Chapter A6 Field Measurements Info Version 2 0 10 2008 Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 16 INFO 6 0 2 C Subsample field measurement procedures Subsamples discrete samples that have been wit hdrawn from a sample compositing device or point sampler contained in a measurement vessel open to the atmosphere can yield good data for conductivity pH turbidity and alkalinity as long as correct procedures are follo
51. s for each field property against the criteria for field measurement stabilization table 6 0 1 Determine final field measurement values If the criteria are being met report the median value If criteria are not being met consult the study requirements and objectives Extend the purge time if readings still do not stabilize report the median value of the last five or more sequential measurements and document problems encountered and how they were resolved RECORD OF WELL PURGING Date __________________ By ________________________________________ SITE ID ________________________ STATION NAME _____________________________________ HEIGHT OF WATER COLUMN__________________ DEPTH OF WELL_______________________ PUMP INTAKE ft or m below MP Start__________ End ____________________________________ WELL PURGING METHOD AND PUMP TYPE describe ___________________________________ TIME WATER LEVEL below MP LS DRAW DOWN TEMPER ATURE CONDUC TIVITY pH DISSOLVED OXYGEN TURBID ITY APPROX PUMPING RATE HR MIN ft or m ft or m Celsius S cm standard units mg L gpm or L min Circle the unit used MP measuring point LS land surface HR MIN hour and minutes ft feet m meter S cm microsiemens per centimeter at 25 C mg L miligrams per liter gpm gallons per minute L min liters per minute Select the appropriate turbidity unit from http water usgs gov owq turbidity_
52. turer according to the manufacturer s instructions Adjust the flow into the chamber so that a constant stream of water is maintained at the rate required for DO measurements see NFM 6 2 Correct any backpressure conditions tilt the chamber to expel trapped air Downhole In Situ Procedure When deploying sensors or a multiparameter sonde downhole ground water should be pumped to flow upward past downhole sensors in order to obtain values representative of the depth interval being sampled therefore a submersible pump follows the downhole instrument Because of this constraint the downhole method may not be practical at wells with dedicated pumps or when using multiple equipment in small diameter wells Figure 6 0 5 shows the steps for downhole measurement of field parameters Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 25 The depth at which sensors are located depends on study objectives If a sample is to represent ground water that is integrated over the screened interval locate sensors approximately 1 ft above the screened interval in a 2 in diameter well and just below the pump intake Remove downhole sensors from the well before collecting samples for chemical analysis in order to prevent these instruments from affecting sample chemistry Note that the process of removing these instruments and putting the pump back in the well causes disturbances that can affect the quality of samples
53. ty of data for samples for analysis of volatile organic compounds Fill out the forms as completely as possible Erroneous or mistaken entries should be crossed out with one line and initialed Final or reported field measurement values for USGS studies are to be entered into the following parts of the National Water Information System NWIS see NFM 4 for more detailed information QWDATA Contains discrete noncontinuous field measurement data in addition to laboratory analyses GWSI Contains all final noncontinuous nonautomated ground water water level measurements ADAPS Contains automated continuous water data The conventions used for reporting field measurement data are described at the end of each field measurement section of chapter 6 Stabilization criteria for recording direct field measurements are gi ven in table 6 0 1 Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 5 Table 6 0 1 Stabilization criteria for recording direct field measurements plus or minus value shown C degrees Celsius less than or equal to value shown S cm microsiemens per centimeter at 25 C gt greater than value shown unit standard pH unit about DO dissolved oxygen concentration mg L milligram per liter FNU formazin nephelometric unit Standard direct field measurement1 Stablization criteria2 variability should be within the v
54. ving a double top and bottom check valve and a bottom emptying device Field rinse the bailer or other discrete volume sampler with sample water before using To shield the sample from atmospheric contamination make measurements within a collection chamber or a glove box Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 INFO 27 Figure 6 0 6 Subsample field measurement procedures for conductivity pH and alkalinity of ground water Step 1 Test and calibrate field instruments 2 Purge well see text for exceptions and instructions 3 Field rinse precleaned sampler Use clean dirty hands technique Lower sampler smoothly without splashing to desired depth of screened or open interval Raise sampler smoothly at a constant rate keeping lines clean and off the ground Place sampler in holding stand 4 Withdraw subsamples from sampler U If using bailer insert clean bottom emptying device device should fit snugly over collection bottles and or measurement vessels U If a filtered sample is needed filter in line from sampler to bottle vessel U Drain subsample without turbulence into collection bottles or measurement vessel U Rinse collection bottle s three times with sample filtrate for filtered samples fill to brim cap tightly and maintain at temperature of water source until measurement U Rinse sensors stir bar and measurement vessel three times with sample U For alkalin
55. wed The results for pH and alkalinity cannot be considered valid if the water is anoxic Subsample field measurements should be completed as soon as possible after collection minimizing temperature changes and exposure to the atmosphere fig 6 0 2 Remember that measurements of w ater temperature DO concentration and Eh cannot be taken from a subsample and must be made in situ 5 Rinse sensors measurement vessels and other apparatus three times with sample Rinse with deionized water for alkalinity 6 Pour sample into measurement vessel insert sensor s U 7 gt v i i L gt i gt i i i gt i U i i V L i v i i 9 i i gt Vi v i i L gt i v i gt i gt i i V i iV Rinse sensors thoroughly with deionized water V gt gt i gt i gt VV gt Vi gt V gt L i i gt 8 iV v i i gt i i gt Vi i v i v U iV i gt gt i v gt L i i gt gt gt L i gt L i q V i gt
56. ween the pump and the tap faucet to which sample tubing will be connected Select a faucet without an aerator or obtain written permission to remove the aerator replace the aerator after sampling 4 Regulate the flow using a manifold with a needle valve if possible a Open any additional valves or taps faucets to ensure that the pump will operate continuously an d reduce the possibility of backflow stored in ancillary plumbing lines keep these open throughout purging and sample withdrawal b The pump should produce a smooth solid stream of water with no air or gas bubbles and withou t pump cavitation du ring field measurements and sample withdrawal 5 Purge three well volumes or the purge volume dictated by the pumping status of the well or sampling objectives Throughout purging monitor and record field measurement readings fig 6 0 4 If required by Federal State or local regulations contain purge water as directed Info Version 2 0 10 2008 U S Geological Survey TWRI Book 9 ____________________________________________________________________________________ INFO 23 6 As the third or last well volume is purged when the final field measurements are recorded adjust the purge rate to the pumping rate to be used during sampling if necessary Record field measurements at regular time intervals about 3 to 5 minutes apart Check the last three to five or more measurement value
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