INTERAGENCY FIELD MANUAL FOR THE COLLECTION OF WATER-QUALITY DATA

Collecting Water-Quality Samples


PAGE INDEX
 
Collecting Water-Quality Samples Tables
  1. Summary of grab sampling method and preservation, storage, and handling requirements
  2. Summary of cross-sectional depth-integrated sampling method and preservation, storage, and handling requirements

COLLECTING WATER-QUALITY SAMPLES

Sampling Methods

The following sections describe grab sampling and cross-sectional sampling as currently implemented by various State and Federal water-quality monitoring agencies. The protocols most widely accepted at this time, especially when using the parts-per-billion analytical levels, require the use of clean sampling procedures. These sampling procedures help to reduce (to the extent feasible given current resources) the amount of contamination introduced when collecting water-quality samples in the field. "Clean" sampling procedures involve (1) using equipment that is constructed of noncontaminating materials and that has been cleaned rigorously before field work and between field sites; (2) handling equipment in a manner that minimizes contamination; (3) collecting, processing, and handling samples in a manner that prevents contamination; and (4) routinely collecting quality-control (QC) samples.

Clean Hands/Dirty Hands Techniques

"Clean" sampling procedures, including CH/DH techniques, are required when collecting inorganic samples for metals and other trace elements. Clean sampling procedures are recommended for all other sampling, to the extent that is reasonable, but particularly when the target analyte could be subject to contamination from field or laboratory procedures at a level that could exceed DQOs for reporting and interpretation. CH/DH techniques separate field duties and dedicate one individual as "clean hands" to tasks related to direct contact with the sample. These techniques are summarized below:

  1. CH/DH techniques require two or more people working together.
  2. At the field site, one person is designated as "clean hands" (CH) and a second person as "dirty hands" (DH). Although specific tasks are assigned at the start to CH or DH, some tasks overlap and can be handled by either as long as contamination is not introduced into the samples.
  3. Both CH and DH wear appropriate noncontaminating, disposable, powderless gloves during the entire sampling operation and change gloves frequently, usually with each change in task (wearing multiple layers of gloves allows rapid glove changes).
  4. CH takes care of all operations that involve equipment that comes into contact with the sample; for example, CH:
  1. DH takes care of all operations that involve contact with potential sources of contamination; for example, DH:
Metal and Trace Element Sampling

The following field practices are recommended when sampling for metals and trace elements:

  1. Think contamination: be aware of and record the potential sources of contamination at each field site.
  2. Wear appropriate noncontaminating, disposable, powderless gloves.
  1. Use equipment constructed of materials that are relatively inert with respect to targeted analytes. Metal samplers must be epoxy-coated to prevent trace element contamination.
  2. Use only equipment that has been cleaned according to prescribed procedures. See the equipment cleaning procedures in the Preparing for Water-Quality Sampling section of this manual. Sample processing equipment should be kept covered (when not dispensing samples).
  3. Field-rinse the equipment only as directed. Some equipment for some analytes should not be field-rinsed.
  4. Use correct sample-handling procedures:
  1. Collect and process samples in a clean enclosure such as a dedicated water-quality field vehicle or field processing chamber. Metallic objects, dirt, oil residue, engine exhaust, and food can all be sources of contamination. Simple processing chambers can be fashioned from a polyvinyl chloride (PVC) framework and a clear plastic bag.
  2. Filter samples for dissolved trace elements and metals as soon as practical after collection. Use a disposable, tortuous path, capsule filter (effective pore size of 0.45 m). The USEPA uses a Gelman Supor, model no. 12175 (15-mm diameter or larger), or an equivalent model. A variable speed, battery-operated pump fitted with a peristaltic pump head that forces the sample through TygonTM or TeflonTM tubing is recommended. Filtered samples should be preserved with (1+1) ultra-pure nitric acid (HNO3) to a pH of 2.0 or less. Normally, 3 mL of (1+1) acid per liter should be sufficient to preserve the sample. Ultra-pure nitric acid is available in 2-mL polypropylene vials.
  3. Collect a sufficient number of appropriate types of QC samples. QC samples should be reviewed to determine if cleaning procedures are sufficient and contamination has been minimized.
  4. Follow a prescribed order of sample collection and processing.

Specific details regarding grab and depth-integrated sample-collection methods, preservation, storage, and handling requirements are summarized in tables 3 and 4. The basic techniques of obtaining the water samples are outlined in the grab sampling and cross-sectional sampling sections that follow.

Grab Sampling

Grab samples are indicated in table 2 for DQOs I, II, and III. A grab sample, as explained in an earlier section, is collected in an open container from a single point at or near the stream/river/lake/reservoir surface. Grab samples can be collected with a suspended or hand-held polypropylene (NalgeneTM) 5-gal container, disposable bailer, or narrow, open-mouth bottle. If the grab sample is collected by hand-held methods, the sample collector should wade to where the sample will be collected (preferably at the centroid of flow or mid-channel) and immerse a hand-held narrow-mouth bottle. Water samples should be collected before any other work is done at a site. If other work (for example, sediment-sample collection, flow measurement, or biological/habitat assessment) is done before the collection of water samples, a representative sample will be difficult to collect from a disturbed stream. The sample collector should stand downstream of the bottle while it is being filled. Care must be taken to avoid collecting particulates that are resuspended as the result of wading. Various examples of grab samples include dip, discrete, and pump samples. Dip samples typically are collected by dipping the collection container (of appropriate noncontaminating material) into the upper layer of the water body. Discrete or point samples are collected by either (1) lowering a sampler to a specified depth and then collecting a sample by opening and closing the sampler or (2) using a single-stage sampler, which fills when stream stage rises to a predetermined height. Thief-type samplers and some pumps are the samplers most often used to collect samples by method 1. Although these samplers are designed primarily to sample still waters, they can be adapted to slowly flowing water. Single-stage samplers used in method 2 include the U-59 and are useful at stations on flashy streams or at other locations where it is difficult to reach a station to manually collect samples. Pump samples typically are collected with suction lift or submersible pump systems designed to collect water-quality samples. Pump systems can be portable or permanently installed and automated for sampling.

For a routine water-quality sample where near-surface water is representative of the water mass, a water sample can be collected by directly immersing the container beneath the water surface to a depth of 1 ft. A bucket can be used to collect a sample if the mixed surface layer is very shallow or accessible only from a bridge. If a bucket is used, extreme care should be taken to avoid contaminating the sample with debris from the rope and bridge. Care must be taken also to rinse the bucket between stations. In slow-moving rivers, reservoirs, and estuaries, the depth of the mixed layer can be determined from field measurements by locating the thermocline or an abrupt change in specific conductance. In tidally-influenced water bodies, the mixed surface layer is defined as that part of the water column from the surface to the depth at which the specific conductance is 6,000 S/cm greater than the specific conductance at the surface. For mixed surface-layer samples (depth greater than 1 ft), pre-rinse one of the following sampling devices at least once with native water before using: submersible pump tube, Kemmerer, or Van Dorn. A minimum volume of 3 L should be collected from each site. Sample containers do not have to be rinsed with site water. Care should be taken at all times during sample collection, handling, and transport to prevent exposure of the sample to direct sunlight. Information on sample processing, preservation, and holding times for the grab sampling method, including information on dissolved metals in water, is listed in table 3.

Table 3. Summary of grab sampling method and preservation, storage, and handling requirements
[mL, milliliter; L, liter; C, degrees Celsius; conc., concentrated; <, less than; >, greater than; g, gram; FAS, ferrous ammonium sulfate; m, micrometer; CH/DH, clean hands/dirty hands; ft, foot; VOC, volatile organic compound; BTEX, benzene, toluene, ethylbenzene, xylenes; qt, quart; gal, gallon; DIW, deionized water]
Property or constituent Container(s) Sample volume (mL) Preservation Maximum holding time Procedure(s)
Water samples
Routine water sample
(3 containers: 2 unpreserved, 1 preserved with H2SO4)
Alkalinity Quart or 1-L plastic or glass 100 Cool to 4 C, dark 14 days

Label containers before collection with a unique sample identifier number, station location, date, and sample type.

Place an X on the container lid to identify the acidified sample.

Pre-rinsing containers with ambient water is not necessary.

Fill each container with ambient water by submerging container about 1 ft below the surface mid-stream until filled.

Place sample on ice immediately. Acidify the X container as soon as possible.

Place on ice and ship as soon as possible.

*It is preferable that samples be filtered in the field or laboratory as soon as possible.

**If nitrite and nitrate are analyzed by ion chromatography, acidification is not required. For other methods of analysis, preserve with H2SO4 to pH <2 for a holding time of 28 days.

***According to "Standard Methods," samples should be filtered as soon as possible, and filters can be stored frozen for 21 to 30 days. Other authorities state filtered samples can be stored indefinitely.

Total suspended sediment, suspended sediment do. 400 do. 7 days
Chloride (Cl ) do. 100 do. 28 days
Sulfate (SO4 ) do. 100 do. 28 days
Orthophosphate phosphorus (PO4-P) do. 150 do. Filter ASAP*; 48 hours until analysis
Nitrite + nitrate (NO2 + NO3)** do. 150 do. 28 days
Ammonia (NH3) do. 150 1 to 2 mL conc. H2SO4 to
pH <2 and cool to 4 C, dark
28 days
Total phosphorus do. 150 do. 28 days
Total organic carbon do. 100 do. 28 days
Chlorophyll a Quart or 1-L plastic or brown glass 1,000 Cool to 4 C, dark Filter within 48 hours
Pheophytin a Filters may be stored frozen for 30 days***
Nitrite do. 50 Cool to 4 C, dark 48 hours
Total dissolved solids do. 250 Cool to 4 C, dark 7 days
Hardness Quart or 1-L plastic or glass 250 Unfiltered, cool to 4 C, dark,
OR
Filtered 2 mL conc. H2SO4 or HNO3 to pH <2; cool to 4 C, dark
48 hours

 

 

6 months

Nonroutine water samples
Oil and grease Glass jar with TeflonTM-lined lid rinsed with hexane or methylene chloride 1,000 2 mL conc. H2SO4 to pH<2; cool to 4 C, dark 28 days  
Phenols Glass jar with TeflonTM- lined lid 1,000 2 mL conc. H2SO4 to pH<2; cool to 4 C, dark 28 days
Cyanide Quart or 1-L plastic 1,000 2 mL 1:1 NaOH added to pH>12; 0.6 g ascorbic acid if residual chlorine present. Cool to 4 C, dark 14 days
Biochemical oxygen demand Gallon plastic >4,000 Cool to 4 C, dark; add 1 g FAS crystals per liter if residual chlorine present 48 hours
Chemical oxygen demand Quart or 1-L plastic 110 2 mL conc. H2SO4 to pH <2; cool to 4 C, dark 28 days
Metals in water
Dissolved (except Hg) HNO3-cleaned quart plastic bottle 1,000 Filter at sampling site with 0.45-m in-line filter into ultra-pure HNO3 preacidified container to pH <2 6 months

Dissolved metals (includes hexavalent chromium)

Put on powder-free latex gloves using CH/DH technique:
Assemble pump, tubing, and filter.
Immerse intake tubing directly into water 1 ft and pump about 500 mL of ambient water to flush tubing and filter.
Fill precleaned, preacidified container with 600 to 1,000 mL of filtrate leaving some headspace.

Total metals

Put on powder-free latex gloves using CH/DH technique:
Assemble pump and tubing without filter.
Immerse intake tubing directly into water 1 ft
and pump about 500 mL of ambient water to flush tubing.
Fill precleaned, preacidified container with 600 to 1,000 mL of sample leaving some headspace.

Dissolved mercury do. 1,000 Filter at sampling site with 0.45-m in-line filter into ultra-pure HNO3 preacidified container to pH <2 28 days
Total (except Hg) do. 1,000 Preacidified container with 5 mL ultra-pure HNO3 to pH <2 6 months
Total mercury (Hg) do. 600 Preacidified container with 5 mL ultra-pure HNO3 to pH <2 28 days
Hexavalent chromium (filtered) Plastic or glass 600 Cool to 4 C, dark 24 hours; must notify lab in advance
Organics/pesticides in water
Volatile organic compounds (VOC) Two 40-mL VOC vials 80 Cool to 4 C, dark; or 2 to 4 drops HCl to pH<2, cool to 4 C, dark for BTEX 14 days  

Organics

 

Pesticides and herbicides

Organophosphorus pesticides

Organochlorine pesticides

Chlorinated herbicides

 

Semivolatile organic compounds

1-qt glass jar with TeflonTM-lined lid per sample type; must be pre-rinsed with hexane, acetone, or methylene chloride 1,000 Each sample type requires 1,000 mL in a separate container

Cool to 4 C, dark

 

 

If chlorine is present, add 0.1 g sodium thiosulfate

7 days until extraction

Label each container before collection with tag number/unique sample identifier number, station location, date, and "ORGANICS-Organophosphorus Pesticides or Organochlorine Pesticides, or Chlorinated Herbicides" or "SEMIVOLATILES" (depending on sample type).

Fill quart jar(s) to the top. Put in the dark and on ice.

Biological samples
Toxicity in water Two 1-gal glass or plastic 8,000 Cool to 4 C, dark 7 days

Label containers before collection with station location, date, and sample type.

Open cubitainers by pulling apart. Pre-rinsing cubitainers with ambient water is not necessary.

Fill each container with ambient water by submerging container about 1 ft below the surface mid-stream until filled.

Place on ice and ship as soon as possible.

Quality-assurance samples
Field duplicates

Represent the variability introduced during sampling, preservation, and handling. Collected on a 5- to 10-percent basis, depending on specific program requirements.

Collect two sets of routine water samples at the same location, sequentially, using the same methods. The samples are handled, stored, shipped, and analyzed using identical procedures. This applies to all cases of routine surface-water collection procedures, including in-stream grab samples, bucket grab samples from bridges, pumps, and other water- or sediment-sampling devices.

Each set of samples has a separate tag number. Submit both sets of water samples to the same laboratory for analysis; LABEL RFA tag as a DUPLICATE.

Trip blanks

One set of DIW samples is submitted for VOC samples only.

DIW blanks are prepared in the laboratory, transported to the field, and preserved (as required) along with other samples.

The trip blank demonstrates that the containers and sample handling did not introduce contamination.

Metals blanks
Dissolved metals

To assess contamination of dissolved metals in water samples, FIELD SAMPLE BLANKS are submitted to the laboratory for every sampling trip.

Blanks are collected at the last station of a sampling trip or sampling day.

DIW is obtained from the laboratory.

1,000 mL of metals-free DIW that has been drawn through a new filter will be submitted as a blank. Flush tube and filter with 500 to 1,000 mL of metals-free DIW. Routine procedure described for collecting dissolved metals in water will be followed.

Label container with DISSOLVED METALS BLANK and a separate sample tag number (the same RFA tag is used for both dissolved and total metals in water samples); LABEL RFA tag as a BLANK.

Total metals

To assess contamination of total metals in water samples, FIELD SAMPLE BLANKS are submitted to the laboratory for every sampling trip.

Blanks are collected at the last station of a sampling trip or sampling day.

DIW is obtained from the laboratory.

1,000 mL of metals-free DIW that has been drawn through a clean tube will be submitted as a blank. Flush tube with 500 to 1,000 mL of metals-free DIW. Routine procedure described for collecting total metals in water will be followed.

Label container with TOTAL METALS BLANK and sample tag number (the same RFA tag is used for both dissolved and total metals in water samples); LABEL RFA tag as a BLANK.


 
Cross-Sectional Sampling

Cross-sectional sampling (used in this manual to denote cross sectionally integrated, flow-weighted composite) is achieved by using depth-integrating, nozzled samplers that fill isokinetically. An isokinetic sampler operates in such a way that the water-sediment mixture moves into the sampler with no change in speed and direction (velocity) as the water enters the sampler intake. This sampling method ensures that the sediment concentration in the water-sediment mixture in the sampler and the sediment concentration in the stream are equal. An isokinetic sampler is lowered through a vertical (the center of each increment or part of the stream cross section) at a predetermined transit rate. The transit rate (the rate at which the sampler is lowered and raised) is mainly a function of the sampler nozzle diameter, volume of sampler container, stream velocity, and sampling depth. The transit rate must be kept constant during sampler descent and ascent through a vertical. Three different sampling methods based on the use of isokinetic samplers are the single vertical at centroid of flow (VCF) method, the equal-discharge increment (EDI) method, and the equal-width increment (EWI) method. These methods will be summarized in this section. The centroid of flow is the point in the increment at which discharge is equal on both sides. The number of increments needed to collect a discharge-weighted sample at a site is related primarily to how stringent the DQOs are and how well mixed or homogenous the stream is with respect to the physical, chemical, and biological characteristics (variation) of the cross section. Information on sample processing, preservation, and holding times for the cross-sectional depth-integrated sampling method is listed in table 4.

Table 4. Summary of cross-sectional depth-integrated sampling method and preservation, storage, and handling requirements
[mL, milliliter; C, degrees Celsius; N, normality; CH/DH, clean hands/dirty hands; DIW, deionized water; EDI, equal-discharge increment; EWI, equal-width increment; ss, suspended sediment; mg/L, milligram per liter; m, micrometer; L, liter; mm, millimeter; DOC/SOC, dissolved organic carbon/suspended organic carbon; <, less than; >, greater than; BOD, biochemical oxygen demand; g, gram; FAS, ferrous ammonium sulfate; cm, centimeter]
Property or constituent Container(s) Sample volume (mL) Preservation Maximum holding time Procedure(s)
Water samples
Routine water samples
(depth-integrated sample volumes for inorganics and DOC/SOC are composited into splitting device)
Specific conductance, pH, turbidity Unfiltered sample volumes for bottles are dispensed from churn or cone splitters 250 No preservatives/not necessary to chill 6 months

Before field collection, all equipment is cleaned in office/laboratory according to CH/DH techniques given in this manual.

Label bottles with site identification number, station name, date, time, and sample designation code.

Rinse all inorganic sample bottles three times with DIW, then fill halfway with DIW for transport to the field.

Rinse all sampling equipment and splitting devices thoroughly three times with native water before collecting first sample.

Collect required volume of water by appropriate methods (cross sectional (EDI/EWI) or centroid of flow) using CH/DH techniques.

Composite depth-integrated samples into either churn or cone splitter. For streams where ss concentrations are 2,000 mg/L or greater, cone splitter is recommended.

Filter required amounts (with 0.45-m capsule filter preconditioned with 1 L DIW) for dissolved trace elements, nutrients, major ions, and alkalinity.

Place sample on ice immediately. Ship as soon as possible.

DOC/SOC sample is filtered in stainless-steel barrel filter unit using a 47-mm-diameter silver filter pre-conditioned with organic-free DIW. *Filtered SOC volumes vary with concentration of ss as follows: <30 mg/L-filter 250 mL; ss 30 to 300 mg/L-filter 100 mL; ss 300 to 1,000 mg/L-filter 30 mL; and ss >2,000 mg/L-filter 10 mL. Carefully remove SOC silver filter, fold in half with sediments inside, and place in plastic petri dish, record filtrate volume on dish, and place in sealable plastic bag.

Alkalinity, fluoride, chloride, sulfate, vanadium Filtered sample volumes are pumped from the splitting device through capsule filter 500 do. 6 months
Orthophosphate
phosphorus (PO4-P)
Filtered sample volumes dispensed through capsule filter into brown polyethylene bottle 125 Cool to 4 C, dark Ship to laboratory immediately for analysis.
48 hours
Nitrite + nitrate
(NO2 + NO3)
do. do. 28 days
Ammonia (NH3) + organic N do. do. 28 days
Nitrite (NO2) do. do. 48 hours
Ammonia (NH3) + organic N, total phosphorus Unfiltered sample volumes dispensed from splitter 120 1 mL 4.5N H2SO4 and cool to 4 C, dark Ship to laboratory immediately for analysis.
28 days
Dissolved organic carbon (DOC) Pre-baked amber glass bottle, not pre-rinsed 5-120 Cool to 4 C, dark Filter on-site
Suspended organic carbon (SOC) do. 10-250* do. Filters may be stored frozen for 30 days
Nonroutine water samples
Biochemical oxygen demand (BOD) BOD bottle with stopper 300 Cool to 4 C, dark; add 1 g FAS crystals per liter if residual chlorine present 48 hours  
Trace elements in water

Dissolved trace elements

aluminum, antimony, arsenic, barium, beryllium, boron, cadmium, chromium, cobalt, copper, iron, lead, lithium, manganese, molybdenum, nickel, selenium, silver, strontium, uranium, vanadium, zinc

Rinse bottles with 25 to 50 mL of filtered native water 250 Filter at sampling site with 0.45-m capsule filter, acidify with HNO3 TeflonTM ampules 6 months

Clean all equipment using cleaning process given in this manual. Collect sample with the same protocols as used to collect other depth-integrated samples.

Sampling equipment, splitter, and TeflonTM tubing should be soaked for 30 minutes in 5-percent HCl solution.

Filter required sample volume with in-line 0.45-m capsule filter (preconditioned with 1 L of DIW).

Use 25 to 50 mL of filtered sample to rinse trace element bottle (fill to the top of the lower lip of the 250-mL bottle).

Fill designated sample bottle only to top of upper lip of sample bottle for a total volume filter of about 250 mL.

Organics/pesticides in water
Organics Sample is filtered and dispensed into a 1-L baked amber glass bottle 1,000 Cool to 4 C, dark 7 days until extraction

Pre-rinse filter (142-mm plate) with at least 100 mL of native water.

Filter with baked glass fiber filter (0.7-m pore size).

Collect about 1 L filtered sample without pre-rinsing bottle, leaving 2 cm of headspace in bottle.

Determine exact volume of filtered sample by subtracting tare weight from weight of filled sample bottle.

Rinse all equipment/tubing in contact with pesticide sample with pesticide-grade methanol.

Pesticides and herbicides do. 1,000 Cool to 4 C, dark. If chlorine is present, add 0.1 g sodium thiosulfate 7 days until extraction
Quality-assurance samples
Laboratory/field equipment blanks
Laboratory equipment blanks are required for both inorganics (dissolved metals and nutrients) and organic carbon. They are required by the inorganic protocol to be collected at least once per year for equipment that is used to collect low-level samples for inorganic constituents. Laboratory equipment blanks are generated in the laboratory to verify that cleaning and maintenance of equipment is adequate to prevent contamination of native water when collecting an environmental sample. Two field equipment blanks are required for both inorganics and organic carbon, and one is required for pesticides at least once per year. A field equipment blank is a blank solution that is generated under actual field conditions and is subjected to the same aspects of sample collection, field processing, preservation, transportation, and laboratory handling as the environmental samples. Field equipment blanks should be prepared immediately before collecting and processing a native-water sample at a selected site and should be prepared using either inorganic- or organic-free blank water, but not both, at the same site.
Inorganic equipment blanks
Collect two initial samples of source solution, one for each schedule if necessary. After initial rinsing with blank water, fill the sampler and pour the water through the nozzle into a sample bottle for the dissolved trace element blank. Pour the remainder of the blank water from the sampler into the churn splitter; refill the sampler and repeat until the churn contains about 5 L of water; pump an aliquot of blank water from the churn splitter, using the routine pumping system, into a sample bottle for the dissolved trace element pump blank. Pump two aliquots of blank water from the churn splitter through the preconditioned filtration system into two sample bottles for the filtered nutrients and dissolved trace element field equipment blanks. Preserve all samples as required; submit only the final field equipment blank samples (filtered nutrients and dissolved trace elements) to the laboratory and store the remainder of the samples for later analyses, if necessary
Organic equipment blanks
Collect two initial samples (1 L and 125 mL) of source solution, one for each schedule, if necessary. Rinsing of the sampler should simulate as closely as possible the field rinsing that occurs before collection of environmental samples. After initial rinsing, fill the sampler with blank water and pour the water through the nozzle into the glass carboy; refill the sampler and repeat until the carboy contains at least 2 to 3 L of water; pump an aliquot of blank water from the glass carboy through the preconditioned pesticide filtration system for the pesticide equipment blank. Refill the sampler and pour another 2 to 3 L of blank water into the churn splitter; collect an aliquot through the churn spigot and pump through the preconditioned DOC/SOC filtration system; after conditioning the filter with 100 mL, filter an additional 100 mL and submit the filtrate and the filter for the DOC/SOC equipment blank.

 
Single Vertical at Centroid of Flow Method

The VCF method uses one centroid of flow for the stream cross section, and therefore, only one vertical is sampled. Consequently for appropriate use of this method, the cross section should be well mixed vertically and laterally with respect to concentrations of target constituents. The following steps should be followed to use the VCF method:

  1. Measure discharge along the cross section to be sampled. Space the verticals to provide about 25 to 30 subsections or increments.
  2. Locate the centroid of flow (the point in the stream where the discharge is equal on both sides) directly from the discharge measurement sheet OR
  3. Construct an equal-discharge increment (EDI) curve by plotting cumulative discharge or cumulative percentage of discharge against cross-section width (in feet from right streambank). The centroid of flow (as determined by the EDI curve) would be located at the cross-section width where 50 percent of the discharge crosses the plotted line.
    1. If the stream channel is stable at the cross section to be sampled, then EDI curves of cumulative discharge at various stages can be based on historical discharge measurements. The location of centroids can be determined from these EDI curves so that discharge measurements do not have to be made before each sampling. EDI curves require occasional verification by comparison to recent discharge measurements.
    2. Examine the cross section for uniformity of appearance.
  4. Measure the cross-sectional variation of field measurements (temperature, pH, DO, and specific conductance) at sites that have sampling history. Generally these field parameters should be measured at no less than 10 evenly spaced increments about 1 ft below the water surface. Record and review variations along the cross section. If values of field measurements differ by less than 5 percent and show that the stream is well mixed both along the cross section and from top to bottom, a single vertical of flow can be used.
  5. Evaluate data from steps 1-4 to decide if the VCF method is appropriate. If discharge, field-measurement, or chemical-analysis data do not confirm that the cross section is well mixed vertically and laterally, use either the EDI or the EWI method.
  6. If the VCF method is appropriate, go to the "Equal-Discharge Increment Method" section and follow step 3 to select the transit rate and step 4 to collect samples.
Equal-Discharge Increment Method

The objective of the EDI method is to collect a discharge-weighted sample that represents the entire flow passing through a cross section by obtaining a series of samples, each representing equal volumes of stream discharge. The EDI method requires that flow in the cross section be divided into increments of equal discharge. Equal-volume samples are collected at the centroid of each of the EDIs along the cross section. Samples are collected by passing the sampler through a vertical located at the centroid of each EDI. The sample collector should (1) use isokinetic, depth-integrating sampling equipment (Appendix D), (2) use the same size sampler and nozzle at each of the verticals, (3) use the same constant transit rate during each ascent and descent in verticals, and (4) composite the samples from all verticals with the appropriate compositing equipment.

The following steps summarize the EDI method:

  1. Prepare the field site.
    1. Upon arrival, set out safety equipment such as traffic cones and signs. Park the vehicle in a location and direction that will prevent sample contamination from vehicle emissions.
    2. Assemble the equipment needed and set up a clean workspace.
  1. Select the number and distribution of EDIs.

    The number of EDIs selected for a sampling site is governed by factors described in the following steps and should not be determined arbitrarily.

    1. Visually inspect the stream from bank to bank and longitudinally, observing velocity, width, depth distribution, and apparent sediment and aquatic biota distribution along the cross section. Note and document the locations of stagnant water, eddies, backwater, reverse flows, areas of faster-than-normal flow, and piers or other obstructions along the cross section.
    2. Determine the stream width from a tagline or from distance markings on a bridge railing or cableway.
    3. At sites that have little sampling history: measure, record, and review the cross-sectional variation of field measurements (temperature, pH, DO, and specific conductance). Review the magnitude of the variations along the cross section.
    4. Measure discharge along the cross section for sampling or use existing EDI curves drawn from historical discharge measurements (if available).
    5. Determine the volume of discharge that will be represented in each EDI on the basis of DQOs for the study; the variation in discharge, field measurements, and stream-channel characteristics along the cross section; and the volume of sample required for analyses of target constituents.
    6. Divide the cross section into EDIs.

TECHNICAL NOTE: If the stream channel is stable at the cross section to be sampled, graphs of cumulative discharge or percentage of cumulative discharge at various stages can be based on historical discharge measurements. Location of EDI centroids can be determined from these graphs so that discharge measurements do not have to be made before each sampling. Linear interpolation based on discharge can be made between EDI curves. However, these EDI curves require occasional verification by comparison to recent discharge measurements.

  1. Select the transit rate.
    1. Determine the sampling depth and the mean stream velocity at the centroid of each EDI.
    2. Determine the actual transit rate for each centroid that will yield samples of approximately the same volume, using sampling depth, mean stream velocity, and information in Appendix F.

Guidelines for selecting the EDI-sampling transit rate

  1. Collect samples (the procedures are the same whether the sample collector is wading or using a reel-and-cable suspension method).
  1. Process the samples as appropriate. This step includes removing necessary volumes of water from the compositing device for filtering or dispensing into the required sampler bottles. After filling the bottles specified for the scheduled analyses, add the necessary preservatives and package the bottles appropriately for shipment to the laboratory.
  2. Clean the equipment as appropriate.
Equal-Width Increment Method

For the EWI method, the stream cross section is divided into a number of EWIs. Samples are collected by lowering and raising the sampler through the water column at the center of each EWI. The combination of the same constant transit rate used to sample at each vertical and the isokinetic characteristic of the sampler results in a discharge-weighted sample that is proportional to total streamflow.

The following steps summarize the EWI method:
  1. Prepare the field site
    1. Upon arrival, set out safety equipment such as traffic cones and signs. Park the vehicle in a location and direction that will prevent sample contamination from vehicle emissions.
    2. Assemble the equipment and set up a clean workspace.
  1. Select the number and width of EWIs.
    1. Visually inspect the stream from bank to bank and longitudinally, observing velocity, width, depth distribution, and apparent sediment and aquatic biota distribution in the cross section. Note and document the locations of stagnant water, eddies, backwater, reverse flows, areas of faster-than-normal flow, and piers or other obstructions along the cross section.
    2. Determine the stream width from a tagline or from distance markings on a bridge railing or cableway.
    3. At sites that have little sampling history, measure, record, and review the cross-sectional variation of field measurements (temperature, pH, DO, and specific conductance). Review the magnitude of the variations along the cross section.
    4. Determine the width of the increment or the distance between verticals. To determine the width of each equal increment, divide the stream width by the number of verticals necessary to collect a representative sample. The number of increments must be a whole number. Increment width is based on DQOs for the study and on the variation in discharge, field measurements, and stream-channel characteristics along the cross section. For all but very wide and shallow streams, a minimum of 10 and a maximum of 20 verticals usually is sufficient.
  1. Select the transit rate.
    1. Refer to Appendix F for guidelines for determining the isokinetic transit rates for collecting isokinetic, depth-integrated samples. Unless the mean velocity is actually determined, use the trial-and-error method to determine the minimum transit rate.
    2. Locate the EWI that has the largest discharge (the largest product of depth times velocity) by sounding for depth and either measuring or estimating velocity. At the vertical for this increment, use of the minimum transit rate results in the maximum allowable filling of the sampler bottle or bag during one vertical traverse.
    3. Determine the minimum transit rate at this vertical for the size of the sampler nozzle and bottle or bag that will be used.
    4. Approximate the mean velocity (in feet per second) of the vertical by timing a marker as it travels a known distance. Divide the distance (in feet) by the time (in seconds) and multiply by 0.86.
    5. Make sure that the transit rate does not exceed the maximum allowable transit rate to be used at any of the remaining verticals along the cross section. This can be determined by sampling the slowest and shallowest increment with any substantial flow. If the minimum sample volume is not collected at this vertical, then the EWI method cannot be used at this cross section to collect a discharge-weighted sample.
  1. Collect samples (the procedures are the same whether the sample collector is wading or using the reel-and-cable suspension method; the same sampler bottle or bag can be used for all verticals in the cross section).
    1. Move to the first vertical (the midpoint of the first EWI near the edge of the water) and field-rinse the equipment.
    2. Record the start time and gage height.
    3. Lower the field-rinsed sampler at the predetermined constant transit rate until a slight contact is made with the streambed.
  1. Process the samples as appropriate.
  2. Clean the equipment as appropriate.
Processing Cross-Sectional Samples

Cross-sectional samples typically are composited to produce a water-quality sample that is representative of the total stream discharge at the sampling station. This compositing can be accomplished by depositing each sample volume from each vertical into a churn splitter -- a polyethylene container that slowly stirs the composited sample with a polypropylene disk. Because the churn splitter requires 3 to 8 L of composited water, verticals in a narrow stream may need to be sampled more than once. All verticals should be sampled the same number of times to retain the representativeness of the sample to the stream. It is also important to churn while drawing sample aliquots from the splitter. Do not use a churn splitter to composite samples collected for VOCs, organic carbon, oil and grease, pesticides, herbicides, or bacteria, because the plastic components have the potential for adsorbing and contaminating the samples. Instead use glass containers for sampling these constituents with the grab-sampling method.

The choice of using the parts-per-million/milligrams-per-liter protocol or the parts-per-billion/micrograms-per-liter protocol is based on the minimum reporting level (MRL) required in the sampling plan. Most analytical work and most Federal drinking-water regulations are reported in micrograms per liter. The parts-per-billion protocol with its extremely low MRLs requires certain field practices, such as the CH/DH technique, appropriate cleaning/handling of sampling equipment, and rigorous and appropriate QC sampling and planning to ensure minimum contamination occurs during the entire water-quality sampling and processing procedure.

Some constituents require different sampling methods than others. Always check with the laboratory that will perform the analyses about container type and volume, preservation, and holding times. Clear descriptions of selected constituents, preservatives, and holding times should be included in the sampling plan before samples are collected. The current properties and constituents, preservatives, and holding times for both grab and depth-integrated sampling methods are listed in tables 3 and 4.

Samples should be wrapped and packed in containers so that they are received unbroken at the laboratory. Record all identification numbers on the field notes or field logbook. Communicate with the laboratory about the sample delivery time and method of shipment. Follow necessary procedure, complete the required forms, and retain copies so that samples can be tracked after shipment.

Quality-Assurance Plans and Quality-Control Samples

Assuring the validity of environmental data or QA is accomplished by collecting QC samples, reviewing and analyzing QC data, and making adjustments to data-collection procedures on the basis of the results. Because environmental sampling is done in a relatively uncontrolled manner subject to many variables, large sources of error may exist. Therefore, measures of control over the collection of water-quality samples are crucial for answering any questions regarding the validity of the data. QA plans should contain planned and systematic procedures necessary to provide confidence that the data will satisfy established requirements for quality. Detailed information about QA/QC samples can be found throughout the literature and within specific agencies responsible for water-quality data collection. This section gives a review of the types of QC samples commonly required in QA plans.

Blank solutions or "blanks" are solutions (DIW or rinse water) that are laboratory certified to have target constituent concentrations that are less that the method detection limits. These solutions are used to develop specific types of blank samples. Equipment blanks are blank solutions that are processed through all equipment used for collecting and processing an environmental sample. Once analyzed, the blanks indicate the effectiveness of cleaning of field equipment. Equipment blanks should be collected after sampling the station with the highest contamination. Field blanks are containers of DIW that are subjected to the same aspects of sample collection, field processing and preservation, transportation, and laboratory handling as environmental samples. Field blanks are used to check for contamination in the laboratory and for cross contamination during the collection and shipment of samples. Trip or travel blanks are containers of DIW that are put in the same type of bottle as used for environmental samples and are kept with the set of sample bottles both before and after sample collection. These blanks are used to detect contamination that occurs during sample transport or storage or in the laboratory. Replicate samples are used to assess the performance in those parts of the procedure that are replicated. Replicate samples are collected in a manner such that the samples are thought to be essentially identical in composition. Many types of replicate samples are possible, and each might yield slightly different results in a dynamic hydrologic setting. Split replicates sent to separate laboratories are used to ensure that results from the different laboratories are comparable. Split replicates sent to the same laboratory are used to measure the variability of the laboratory. Concurrent or sequential replicates are used to ensure that samples collected with different samplers or by different sampling methods are comparable. Spike samples are samples that have known concentrations of specific constituents added in such a manner as to minimize the change in the matrix of the original samples. Spike samples are used to verify the method performance for either accuracy or recovery. Accuracy data reflect the best results that can be expected at the time the samples were analyzed, and recovery data reflect bias from an environmental sample matrix.

Most QA plans and QC samples take into account the following:


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