Planning for Water-Quality Sampling

Planning for Water-Quality Sampling Tables
  1. Data-quality objectives associated with data-information objectives for occurence of constituents
  2. Data-quality objectives


The quality of water in streams and rivers commonly is determined by selected chemical and physical analysis of water samples collected to represent the water body. Factors to be considered in selecting a sampling method include: (1) the accuracy of sampling necessary to satisfactorily represent the water-quality constituents of interest so that the specific sampling or data-information objectives can be achieved, and (2) the costs of alternative sampling methods.

The principal sampling methods for determining water quality in flowing water can be classified as (1) surface grab sampling in which samples are collected in an open container from a single point at, or near, the water surface, and (2) cross sectionally integrated, flow-weighted composite ("integrated") sampling using depth-integrating, nozzled samplers that fill isokinetically. Isokinetic sampling means there is no change in stream velocity as the water enters the sampler intake.

This manual contains sampling methods or techniques that can be selected on the basis of various DQOs as defined in tables 1 and 2. For this manual, these DQOs are associated with data-information objectives linked to constituent groups and where they occur in the water column or phase (table 1). The data-information objectives are defined by the purpose for sampling water quality such as determining compliance with applicable water-quality standards, identifying trends, and so forth. For example (referring to table 1), if nutrients are the constituent group to be examined or sampled for, and gross detection (G) is the data-information objective, then the phase sampled should be W (whole water, which includes dissolved constituents and suspended sediment), and (referring to table 2) DQO I is the key to determining the suggested sampling method. Table 2 defines DQO I as gross detection at a point in a river/stream/lake/reservoir and indicates the sample collector should obtain a qualitative grab sample. Table 2 also includes some of the benefits and limitations of the various sampling methods qualified by factors such as costs, potential for contamination, and appropriate quality-control (QC) investment.

Table 1. Data-quality objectives associated with data-information objectives for occurrence of constituents
[D, dissolved; W, whole water (includes dissolved constituents and suspended sediment); TMDL, total maximum daily load; SS, total suspended sediment; SVOC, semivolatile organic compounds]
Constituent group Data-information objective Phase or occurrence of constituent Data-quality objective
(table 2)
Major inorganics/common ions (including total dissolved solids and specific conductance) PDWS1, SWQS2, trends D, W II or IV
Nutrients and biochemical oxygen demand G3, load (TMDL) W I
Trends D, W II or IV
Trace metals and other minor elements PDWS1 W II, III, IV, or V
AL4, HH5 D III or V
Load W or D and SS IV, V, custom SS
SVOCs AL4, load, HH5 W or SS V or custom SS
Soluble pesticides PDWS1, AL4, HH5 D III or V
Load D V
Bacteria G3 W I
Field parameters (temperature, pH, alkalinity, and dissolved oxygen) G3


Organochlorines AL4, load, HH5 W or SS V or custom SS
Suspended sediment Load, AL4 W or SS IV
Toxicity AL4, SWQS2, G3 W II or IV
  1. Public drinking water standards.
  2. Surface-water-quality standards.
  3. Gross detection/screening.
  4. Aquatic life protection.
  5. Human health protection.
  6. Contact recreation use.
Table 2. Data-quality objectives
No. Data-quality objective
Sampling method Benefits and limitations
accuracy level
Sample type
I. Gross detection at a point in a river/stream/lake/reservoir Qualitative Grab Very inexpensive; real-time; limited regulatory use; high potential for environmental contamination; requires least rigorous quality-control measures
II. Detection at a point in a river/stream/lake/reservoir ppm1 Grab Less costly than DQO III, IV, and V; high potential for environmental contamination; applicable for dissolved phase and bacteria at well-mixed sites; requires more rigorous quality-control measures than DQO I but less rigorous than DQO III, IV, and V
III. Low-level detection at a point in a river/stream/lake/reservoir ppb2 Grab More costly than DQO I and II; low potential for environmental contamination; applicable for dissolved phase and bacteria at well-mixed sites; requires more rigorous quality-control measures than DQO I, II, and IV
IV. Detection in a representative cross section of a river/stream/lake3/reservoir3 ppm1 Cross sectional4 More costly than DQO I, II, and III; high potential for environmental contamination; applicable for any phase and bacteria at any site; requires more rigorous quality-control measures than DQO I and II but less rigorous than DQO III and V
V. Low-level detection in a representative cross section of a river/stream/lake3/reservoir3 ppb2 Cross sectional Most expensive; low potential for environmental contamination; applicable for any phase and bacteria at any site; requires the most rigorous quality-control measures
  1. Parts per million (ppm) accuracy is equivalent to milligrams per liter.
  2. Parts per billion (ppb) accuracy is equivalent to micrograms per liter.
  3. Representative lake or reservoir samples should be obtained by collecting depth-integrated samples with a thief-type or pumping sampler.
  4. Cross sectional as used in this manual denotes a depth-integrated sample composited from various cross sections of the stream channel.

Before sampling begins, a sampling plan should be designed to address the objectives of a water-quality project or program. A sampling plan should include specifics about sampling locations or sites, methods and techniques, number of samples, kinds of samples including: volume of water, filtered or whole, preservatives and holding times, number and kinds of quality assurance/quality control (QA/QC) samples, and desired DQOs. Water-quality sampling can be expensive and time-consuming. Sampling plans help assure that sampling results are error free and meet the objectives of the water-quality project or program. For official guidelines and sampling plan components required by various State and Federal agencies, the user should consult those agencies.

Additional resources available to assist personnel with water-quality sampling plans and execution include but are not limited to:

  1. Federal Interagency Sedimentation Project (FISP) is an independent, interagency project created to unify the research and development of activities of Federal agencies involved in fluvial sediment studies. Research conducted by FISP has expanded to include development of sample-analysis methods, development of automatic in-place analyzers, and techniques and equipment for sampling water quality in streams and rivers. Equipment and techniques of FISP are the standards used by most Federal, State, and local governments and private organizations collecting sediment samples in the United States. The FISP catalog can be accessed electronically on the World Wide Web at
  2. The National Water-Quality Monitoring Council's purpose is to coordinate and provide guidance and technical support for the voluntary implementation of recommendations presented in "The Strategy for Improving Water-Quality Monitoring in the United States," by government agencies and the private sector (Intergovernmental Task Force on Monitoring Water Quality, 1995). The USGS and USEPA serve on the National Council with the U.S. Department of Commerce/National Oceanic and Atmospheric Administration, the Tennessee Valley Authority, the U.S. Army Corps of Engineers, the U.S. Department of Agriculture, the U.S. Department of Energy, and the U.S. Fish and Wildlife Service. The National Council is the permanent successor to the ITFM.
  3. World Wide Web pages of Federal agencies such as USGS Office of Water Quality and USEPA Office of Water, Office of Wetlands, Oceans, and Watersheds may be accessed electronically at and

Site Selection

If previous sampling sites can be reactivated and used in current plans, they should be considered. Historical water-quality data from these previous sites can provide useful data to the current data-collection effort. If a sampling plan calls for new water-quality sampling sites to be selected, a number of factors should be considered. New sampling sites should be positioned at or near USGS or IBWC gaging stations whenever possible so that stream discharge can be related to water-quality constituents. If no gaging stations are near the chosen site, discharge measurements should be made at the time of sampling. Consider whether samples can be obtained throughout the entire year at all discharges. If the site is inaccessible during parts of the year, it might not be suitable for the project or program needs. Sampling sites in a river should be located upstream from a confluence in sections where the channel is smoothest, straightest, accessible, and uniform in depth. Avoid locating sites directly above or below confluences or point sources to minimize problems with backwater or poorly mixed flows. Determine if the river or stream is homogenous at the proposed site by measuring temperature, pH, dissolved oxygen (DO), or conductivity at regular intervals and depths across the channel to test the degree of mixing. Because most water bodies are not completely homogenous, the representativeness of samples depends on the equipment and collection method used. Consider the influence of errors that might be encountered during sampling because of turbulence, velocity gradients, and other physical factors that affect the water-sediment mixture. To determine the best site for sample collection, one must consider which site will produce the most representative sample with the least amount of error introduced in obtaining that sample. More details about establishment of a new sampling site are included in the Preparing for Water-Quality Sampling section.

A representative sample is one that accurately reflects the chemical composition and the biological and physical characteristics of the whole stream at the sampling point at an instant. Representative samples also reflect changes that occur in ambient water passing a sampling point. Therefore, enough samples must be distributed in time and space to represent those changes.

Once the sampling sites have been determined, the sample collector or project chief must conduct the research and communication to gain legal access to the site(s). Permission to legally access the site and/or erect any construction to facilitate sampling should be obtained in formal writing. This ensures that all owners or operators of the property have communicated their acceptance of the intent to visit and sample on a predetermined basis as outlined in the sampling plan and agree to any necessary construction. Legal access for some Federal, State, or county officials or designated contractors can range from verbal consent to administrative search warrants. Check with your agency regarding its guidelines or requirements on this topic. Establish a good relationship with property owners by offering a copy of the sampling plan and analysis results. Be sure to notify local environmental and health agencies of sampling plans and activities and solicit their advice and expertise. Their local and technical expertise could provide valuable insight.

When sampling international and transboundary rivers along the United States-Mexico border, personnel should always contact the IBWC for directions and requirements for sampling along the international boundary. In some areas along the border, safety issues can be a concern, and the U.S. Border Patrol should always be contacted when devising a sampling plan. Often they are the most knowledgeable about local conditions that could affect the safety of the sampling personnel. All precautions should be taken to ensure sampling personnel can accomplish their job in a safe manner.

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