Guidelines For Field-measured 6.0 Water-quality Properties

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Guidelines For Field-measured 6.0 Water-quality Properties

Transcript Of Guidelines For Field-measured 6.0 Water-quality Properties

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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 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

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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 field-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 direct 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

The citation for this section (6.0) of NFM 6 is as follows:
Wilde, F.D., 2008, Guidelines for field-measured water-quality properties (ver. 2.0): U.S. Geological Survey Techniques of Water-Resources Investigations, book 9, chap. A6, section 6.0, October, available only online from http://pubs.water.usgs.gov/twri9A/

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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 waterquality data are being transmitted continuously are beyond the scope of this chapter.1
FIELD 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 acidneutralizing 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 waterquality 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 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).

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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 location 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 records 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 blue or black indelible ballpoint pen to write on 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 quality 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 given in table 6.0–1.

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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; >, 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 value shown for about five or more measurements)

Temperature: Thermistor thermometer Liquid-in-glass thermometer
Conductivity (SC): 100 S/cm > 100 S/cm
pH: (meter displays to 0.01)
DO4 Amperometric sensors Optical/luminescent-method sensors
Turbidity5, 6  100 FNU (or other turbidity unit) > 100 FNU (or other turbidity unit)

± 0.2°C ± 0.5°C
± 5 percent ± 3 percent
± 0.1 to 0.2 pH unit3 Allow ± 0.3 pH units if drifting persists, or measurement is in low-conductivity (~75 µS/cm) water, or for continous monitor.
± 0.2 mg/L (± 0.3 mg/L for continous monitor) ± 0.2 mg/L (± 0.3 mg/L for continous monitor)
± 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 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.

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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 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 nonUSGS 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.
 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.

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 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 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 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 fieldcalibrated.

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 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 calibrated 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.

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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 responsibility 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 grabsampling 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 field 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.

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— 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 dictate. 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 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.

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