National Water-Quality Assessment (NAWQA) Program

Design of the National Water-Quality
Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

United States Geological Survey Circular 1112
By Robert J. Gilliom, William M. Alley, and Martin E. Gurtz

Water-Column Studies

As shown in table 4, water-column conditions are assessed by three primary sampling strategies employed at selected Indicator and Integrator Sites: Site choices and sampling strategies for Intensive and Basic Fixed Sites are particularly important to the success of the surface-water design for National Synthesis because this is the part of the study design in which all study components are integrated by a nationally consistent strategy in all Study Units.

Basic Fixed-Site Assessment

The Basic Fixed-Site sampling is designed to provide an integrated assessment of the spatial and temporal distribution of general water-quality conditions and the transport of major inorganic constituents of streamwater in relation to hydrologic conditions and major sources. Data from Basic Fixed-Site sampling are the primary source of information for meeting water-column assessment objectives for temperature, salinity, suspended sediment, major ions and metals, nutrients, and organic carbon. Site selection and sampling strategy for Basic Fixed Sites are based on balancing needs and priorities for assessing water-column conditions, constituents in bed sediment and tissues, and ecological characteristics. Although each of these components has some unique requirements, Study Units also have common requirements to represent the ranges of spatial and temporal scales and Environmental Settings.

Site Selection

Each Study Unit typically has three to five Integrator and four to eight Indicator Basic Fixed Sites. The Integrator Basic Fixed Sites are chosen to represent the range of large streams in each Study Unit and include major nodes in the drainage system for large-scale transport analysis. Collectively, the basins of these Integrator Sites usually include all or most of the Study Unit. Ecological Studies require that Integrator Basic Fixed Sites represent as fully as possible the range of habitat and water-quality conditions in the large streams of each Study Unit. Sites thus are selected by balancing the needs for assessing large-scale water-column conditions and large-scale constituent transport with the need for characterizing stream ecology in a representative manner. These decisions are based on an analysis of existing data and field reconnaissance.

Indicator Basic Fixed Sites primarily are chosen to represent each of the Environmental Settings in the Study Unit that are most relevant to streamwater quality and, thus, also are most likely to represent Environmental Settings that most influence the water quality of the downstream Integrator Site. Commonly, the collective basin areas of Indicator Sites may represent only a small percentage of the total Study Unit area. Ecological Studies require that Indicator Basic Fixed Sites adequately represent physical habitat conditions found for streams of similar size in the selected Environmental Settings. Sites should be located so that chemical and hydrologic characteristics measured in the water column are similar throughout the sampling reach required for Ecological Studies. Another factor that affects the selection of Indicator Sites is the correspondence of Indicator-Site basins with ground-water studies. Ground-water Land-Use and Flowpath Studies frequently overlap with indicator basins of Basic or Intensive Fixed Sites (see also "Ground-Water Study Design"). In particular, most flowpath transects that address surface- and ground-water interaction intersect streams that are associated with Basic or Intensive Fixed Sites.

The White River Basin Study Unit illustrates a typical application of the site-selection strategy for Basic Fixed Sites. The geographic distribution of sites is shown in figure 7. The characteristics of the seven Indicator Sites and four Integrator Sites are provided in table 5, which lists the Environmental Settings and basin sizes.

Table 5. Summary of fixed-site design for the White River Basin Study Unit, Indiana.
[Site locations are shown in fig. 7. km2, square kilometer.]
Site Sampling
basin area
Site description

Indicator Sites
Intensive.......... 241 Row-crop agriculture on till plain.
Intensive.......... 44 Urbanized area on till plain.
Basic................ 844 Row-crop agriculture on till plain.
Basic................ 236 Row-crop agriculture on bedrock lowland and plain.
Basic................ 759 Row-crop agriculture and woodland on bedrock lowland and plain.
Basic................ 91 Row-crop agriculture on karst plain.
Intensive.......... 148 Row-crop agriculture on glacial lowland.

Integrator Sites
Basic................ 6,327 Integrator for till plain urban and agricultural areas.
Basic................ 12,409 Integrator for main stem before confluence with East Fork.
Basic................ 12,755 Integrator for East Fork Basin.
Intensive.......... 29,267 Integrator for entire Study Unit.

Sampling Strategy

The water-column sampling strategy at each Basic Fixed Site consists of three types of sampling activities designed to yield a core of consistent data on hydrologic and general water-quality conditions. The three types of sampling activities--continuous monitoring, fixed-interval sampling, and extreme-flow sampling--are conducted for at least 2 years. A third year may be added for some sites because of local situations, such as unusual hydrologic conditions during the first 2 years, high year-to-year variability, or sparse existing data. The purpose for sampling Basic Fixed Sites for 2 years is, in combination with existing data, to assess year-to-year variability and any major differences or similarities between years in seasonal patterns.

Basic Fixed Sites and Intensive Fixed Sites are the only sites in the NAWQA study design where stream-flow and selected additional characteristics are continuously measured. Continuous streamflow data are collected for the entire sampling period to support analyses of relations between water-quality and hydrologic conditions, to estimate transport over time, and for other interpretive analyses. Detailed characterization of hydrologic conditions also is essential to interpretation of Ecological Studies because of the relation of biological communities and processes to streamflow and its variability over time. Characterization of the thermal regime is a particularly important physical characteristic for interpretation of Ecological Studies, and 1 year of continuous temperature data will be collected at each Basic Fixed Site. Specific conductance will be continuously monitored at selected Basic Fixed Sites where local conditions merit.

Fixed-interval sampling is designed to provide a framework of consistent data on concentrations and typical fluxes at all Basic Fixed Sites for a broad range of water-quality characteristics that cannot be continuously monitored. Fixed-interval sampling is the collection of discrete samples at regular intervals of time for laboratory analysis of chemical and physical properties and for associated field measurements. All samples are flow weighted and cross-sectionally integrated by standard USGS methods (Ward and Harr, 1990). Complete descriptions of sample collection and processing methods are provided by Shelton (1994). The minimum and most common sampling frequency is monthly during the minimum 2-year period of operation.

Sampling at each Basic Fixed Site also includes samples at extreme flow conditions (typically four to eight additional samples per year) to supplement interval sampling. Although fixed-interval sampling provides data for the most common flows and concentrations, high and low flows and concentrations that occur less often during the 2-year sampling period have a small chance of being sampled even though they are particularly important. Occurrence of high and low flows and associated constituent transport and concentrations have significance for water quality far beyond the proportion of time during which they occur.

Sample Analyses

Each time a Basic Fixed Site is sampled, field measurements are made, and samples are submitted to the laboratory for analyses of a national target list of suspended sediment, dissolved solids, major ions and metals, nutrients, and dissolved and suspended organic carbon. Table 6 lists analytes in each category. These analytes are selectively augmented by some Study Units as required to meet specific local needs.

Table 6. Analytical strategy for Basic Fxed Sites
Field measurements

Dissolved oxygen
pH and alkalinity
Specific conductance (hourly or daily if local conditions require)
Temperature (hourly for 1 year)

Laboratory analyses

Suspended sediment

Major constituents:
Dissolved solids
Major ions and metals:
Total dissolved
Total dissolved
Organic carbon:

Intensive Fixed-Site Assessment

Intensive Fixed Sites are the part of the fixed-site design where all water-column sampling strategies are employed, as well as all levels of bed-sediment and tissue and ecological sampling. They are the same as Basic Fixed Sites except for the addition of enhanced sampling frequency and addition of dissolved-pesticide analyses for 1 year. As the program evolves, other types of priority contaminants, such as industrial organic contaminants or trace elements, may be added as a focus of sampling at existing or new Intensive Fixed Sites; as a result, design emphasis may change.

The site selection and sampling strategy for Intensive Fixed Sites during the first cycle of Study-Unit Investigations are governed primarily by requirements for assessing dissolved pesticides. Dissolved pesticides tend to occur erratically during seasonal periods when chemical use and hydrologic conditions favor transport. The premise of the Intensive Fixed Site sampling strategy is that relatively high-frequency sampling at a few carefully chosen sites during key seasonal periods initially yields superior information about occurrence and seasonal patterns compared with other design alternatives.

Site Selection

Each Study Unit typically has one to two Integrator and one to four Indicator Intensive Fixed Sites. Integrator Sites selected as Intensive Fixed Sites are downstream of the largest possible areas of the Study Unit that encompass the greatest intensity of pesticide sources. Major undeveloped basins are avoided. Thus, the chosen Integrator Site may be a major tributary that drains an agricultural valley in which a major city is located rather than, for example, the Study-Unit outlet. This strategy maximizes the value of the Integrator Sites for detecting significant contaminant issues not detected at Indicator Sites and enables comparisons of pesticide transport with upstream Indicator Sites. Whenever possible, the most important Indicator Sites are nested within the basin of an Integrator Site.

The Indicator Intensive Fixed Sites within each Study Unit are selected to represent land-use settings that are most critical for addressing pesticide contamination issues (reference sites generally are excluded). Where possible, these sites include the full range of settings for which pesticide issues are most significant. Known problems are not the only focus. For example, a key setting for an Intensive Fixed Site in many Study Units is urban land, even though most total use of pesticides in the Study Unit may be on agricultural land. The most prominent agricultural settings always are included. In some Study Units, only one or two primary agricultural settings are of significant importance, while other Study Units require four or five Indicator Sites even for a minimal design. National-Synthesis priorities for land-use settings and site suitability for Ecological Study requirements also are evaluated in making final choices. Once selected, the basin characteristics of Indicator Intensive Fixed Sites are assessed in greater detail than most other sites as part of describing the Environmental Framework.

Sampling Strategy

The sampling strategy at each Intensive Fixed Site is the Basic Fixed Site strategy with the addition of analyses of dissolved pesticides for all samples and a higher frequency of interval and extreme-flow sampling during selected seasonal periods. Intensive sampling and analyses usually are done for a 1-year period, after which sampling and analyses return to the same strategy as that of Basic Fixed Sites.

Seasonal periods selected for high-frequency sampling are the most critical periods for concentrations and transport of dissolved pesticides and nutrients. These periods are identified from information on chemical-use patterns, seasonal climatic conditions, irrigation practices, and existing water-quality data. Most Study Units have multiple, overlapping seasonal periods that are important for different chemicals or conditions. The periods of high-frequency sampling are chosen to bracket conservatively the hypothesized seasonal conditions for each Intensive Fixed Site. Typically, the high-frequency sampling periods at Intensive Fixed Sites range from 3 to 9 months.

During the high-frequency sampling, fixed-interval sampling typically is weekly or, if the basin is large, biweekly. Some sites are sampled more frequently if short-term fluctuations are an important concern, and some are sampled less frequently if fluctuations are expected to be less extreme, such as for some larger streams and rivers. The fixed-interval sampling is supplemented with extreme-flow samples of equal or greater number than those for standard operation of Basic Fixed Sites.

Sample Analyses

The analytical strategy for Intensive Fixed Site samples requires the analyses described for Basic Fixed Site samples in table 6 and the additional analyses of pesticides listed in table 7.

Table 7. Analytical strategy for Intensive Fixed Sites in addition to the Basic Fixed Site analyses.

[*, Degradation product; some of the analytes listed may be deleted or qualifed depending on method performance for ambient samples.]

Field measurements
Specific conductance (hourly or daily for 1 year)
Laboratory analyses - Dissolved pesticides

Aldicarb sulfone*
Aldicarb sulfoxide*
Carbofuran, 3-Hydroxy*

Chlorophenoxy herbicides:
2,4-D (acid)
Dichlorprop (2,4-DP)
Silvex (2,4,5-TP)

Dacthal (DCPA)

Methyl parathion


Triazine herbicides:
Atrazine, desethyl*




Water-Column Synoptic Studies

Water-Column Synoptic Studies are short-term investigations of water quality during selected seasonal periods or hydrologic conditions. Every Water-Column Synoptic Study is custom designed to provide improved spatial resolution, compared with fixed-site data, for critical water-quality conditions during selected seasonal periods or hydrologic conditions. They also evaluate the spatial distribution of selected water-quality conditions in relation to causative factors, such as land uses and other contaminant sources. Water-Column Synoptic Studies thus supplement information from the fixed sites by targeting specific water-quality conditions for more-detailed characterization.

Water-Column Synoptic Studies are designed to address the highest priority water-quality issues. Prioritization of topics within a Study Unit are based on such factors as potential ecological and human-health effects, areal extent, and duration and recurrence. These factors are determined from the results of the Retrospective Analysis, NAWQA data, land-use patterns, and local perspectives. Priorities for National Synthesis are based on the same general factors as for the Study-Unit scale, but for current synthesis topics and from a national point of view.

The process of choosing and designing Water-Column Synoptic Studies is coordinated among the Study Units. Most high-priority topics in one Study Unit also are priority topics in other Study Units and thus are likely to be regionally or nationally significant. For some issues, coordinated design involves only two or three Study Units; for other issues, it involves most of the 20 active Study Units.

Most Water-Column Synoptic Studies are done in the second and third years of the 3-year intensive data-collection phase after initial results from the first year of sampling can be combined with existing data to guide the study design. Selected Water-Column Synoptic Studies are done earlier, however, if well-defined, high-priority questions arise. Most Study Units will complete two to three Water-Column Synoptic Studies during the first intensive phase.

The strategies for site selection, sampling, and analysis for synoptic studies are issue specific and keyed to hydrologic conditions, times, and places of specific interest for the targeted water-quality issue. Design of Water-Column Synoptic Studies is coordinated with Bed-Sediment and Tissue Studies and Ecological Studies to enhance the interpretation of relations among physical, chemical, and biological water-quality characteristics. For some issues, a single sampling is done at numerous sites distributed throughout the entire Study Unit. Other Water-Column Synoptic Studies may require a short period of frequent sampling at a few sites in only one part of the Study Unit. The following general principles apply to design:

Continue to Bed-Sediment and Tissue Studies, Glossary, or return to Contents

Last modified: 3:35 EDT 22jun98 klf

Maintainer: h2o Webserver Team