Design of the National Water-Quality
Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

Water-Column Studies

• Basic Fixed-Site assessments characterize the spatial and temporal distribution of general water-quality and constituent transport in relation to hydrologic conditions and contaminant sources.
• Intensive Fixed-Site assessments characterize seasonal and short-term temporal variability of general water quality and constituent transport and determine the occurrence and seasonal patterns in concentrations and transport of dissolved pesticides.
• Water-Column Synoptic Studies investigate the geographic distribution of selected water-quality characteristics in greater detail during specific seasons and in relation to sources.

Basic Fixed-Site Assessment

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.

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.

Intensive Fixed-Site Assessment

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.

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.

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.

Water-Column Synoptic Studies

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:

• Water-quality issues that are most relevant during low-flow conditions usually require many sites but only one or very few samples per site.
• Water-quality issues that are most relevant during runoff events or unsteady flow conditions commonly require frequent sampling at study sites during events or for a limited period of time. Intense sampling is usually feasible for only a few sites.
• Water-Column Synoptic Study designs commonly include all or most Basic and Intensive Fixed Sites so that the short-term findings from synoptic sampling can be related to longer time scales and a greater number of measured characteristics.
• Usually, Water-Column Synoptic Study sites are located to ensure balanced spatial coverage at the desired resolution in the target geographic area and near selected stream junctions to facilitate mass-balance analysis for the sampling period.
• Analytical requirements are sometimes narrowly defined by the specific objective for some synoptic studies. A broadened analytical approach, however, is often used to take advantage of the sampling visit to assess related issues.

Last modified: 3:35 EDT 22jun98 klf