Design of the National Water-Quality Assessment Program:

Occurrence and Distribution of Water-Quality Conditions

SURFACE-WATER STUDY DESIGN

Table 2. Relations among measurable water-quality characteristics and selected water-quality issues [primary characteristics for each issue; secondary characteristics useful for evaluating each issue]
Water-Quality issues
Water-quality characteristics	Path- ogens	Nutrient enrich- ment	Trace elements	Pesti- cides	Indus- trial organics	Sus- pended sed- iment	Salinity	Temper- ature	Acidity	Dis- solved oxygen	Eco- logical condi- tion
Field
Streamflow
Dissolved Oxygen
pH and Alkalinity
Specific Conductance
Temperature
Habitat Characteristics
Biological Communities
Laboratory
Suspended Sediment
Major Constituents
Dissolved Solids
Major ions and metals
Nutrients
Organic carbon
Trace elements
Organic contaminants
Dissolved pesticides
Hydrophobic pesticides
Polychlorinated biphenyls
Other semi- volatile organic contaminants
Volatile organic contaminants
Biological
Bacteria
Chlorophyll
Species identification

Overview of Approach

Table 3. Water-quality characteristics measured in National Water-Quality Assessment surface-water components. [ characteristics that are part of the national design for all Study Units; characteristics that are assessed in selected Study Units to variable degrees]
	Study Components
Water-quality characteristics	Water-Column Studies	Bed-Sediment and Tissue Studies	Ecological Studies
Field
Streamflow
Dissolved Oxygen
pH and Alkalinity
Specific Conductance
Temperature
Habitat Characteristics
Biological Communities
Laboratory
Suspended Sediment
Major Constituents
Dissolved solids
Major ions and metals
Nutrients
Organic carbon
Trace elements
Organic contaminants
Dissolved pesticides
Hydrophobic pesticides
Polychlorinated biphenyls
Other semivolatile organic contaminants
Volatile organic contaminants
Biological
Bacteria
Chlorophyll
Species identification

Water-Column Studies focus on assessing physical and chemical characteristics, which include suspended sediment, dissolved solids, major ions and metals, nutrients, organic carbon, and dissolved pesticides, and relating these characteristics to hydrologic conditions, sources, and transport. These studies also involve selected studies of other water-quality conditions, such as dissolved oxygen and pathogenic bacteria, where they are likely to be important, and include selective investigations of hydrophobic organic contaminants or trace elements if the results of Bed-Sediment and Tissue Studies indicate they are important.

Bed-Sediment and Tissue Studies are the primary means by which trace elements and hydrophobic organic contaminants are initially assessed in NAWQA. Concentrations and their areal distribution are assessed to identify sources and potential needs for more detailed study. Data for bed sediment and tissue levels at common sites are used for an initial assessment of biological availability.

Ecological Studies evaluate the effects of physical and chemical characteristics of water and hydrologic conditions on aquatic biota and how biological and habitat characteristics differ among Environmental Settings in Study Units. The aquatic ecological community of a stream is potentially affected by all the water-quality conditions assessed during the Water-Column and the Bed-Sediment and Tissue Studies and also by numerous factors that affect physical habitat, such as climate, stream size, riparian vegetation, and geology.

Sampling designs for the three study components rely on coordinated sampling of varying intensity and scope at two general types of sites--Integrator and Indicator. Integrator Sites are chosen to represent water-quality conditions of streams and rivers in heterogeneous large basins that are often affected by complex combinations of land-use settings, point sources, and natural influences. Indicator Sites, in contrast, are chosen to represent water-quality conditions of streams in relatively homogeneous and usually smaller basins associated with specific individual Environmental Settings; for example, a particular combination of land use and geologic setting.

Most Integrator Sites are on major streams with drainage basins that include a substantial portion of the Study Unit area (typically 10-100 percent). In Study Units where it is possible and appropriate to the hydrologic system, one of the Integrator Sites is located at the outlet of the entire Study Unit. A goal is to locate Integrator Sites at key nodes in the drainage network of the Study Unit so that the most significant contaminant sources in the unit are included within at least one Integrator-Site basin. Data from Integrator Sites provide a general check on the persistence of water-quality influences evident at Indicator Sites for specific Environmental Settings and are used for water-budget and contaminant-transport assessments.

Indicator Sites are located at the outlet of drainage basins with relatively homogeneous land-use and physiographic conditions. Generally, the drainage basin of an Indicator Site has more than 50 percent of its area in the targeted Environmental Setting, and the water quality is primarily influenced by the targeted setting. Sites are selected in an attempt to keep stream size, gradient, and geomorphic characteristics within a restricted range for each Study Unit or for major regions within the Study Unit. Most Indicator-Site basins range from 50-500 km² and are chosen to be as large and representative as possible while still mainly being comprised of the targeted setting.

Special types of Indicator Sites are reference and point-source sites. Reference Indicator Sites are located downstream of undisturbed drainages that represent the selected physiographic and ecological regions within the Study Unit and are chosen to represent background conditions. Point-source Indicator Sites are located downstream of specific major point sources of contaminants that may have regionally significant water-quality effects. Usually, they are paired with a site upstream of the point source. For example, a point-source Indicator Site may be located downstream from a major sewage-treatment plant for comparison to an Integrator Site upstream. Indicator Sites for point sources are not needed in all Study Units and generally are limited to situations where the point source is uniquely significant and not adequately represented by other sites.

The sampling strategy for Water-Column, Bed-Sediment and Tissue, and Ecological Studies is based on four general levels of sampling intensity at a combination of Integrator and Indicator Sites. The general concept is that a few selected sites have intensive sampling for all water-quality characteristics related to all three components, and progressively more sites are included for more-specialized and less-frequent sampling activity. Table 4 summarizes the integrated sampling strategy for the four levels of sampling intensity--Basic Fixed, Intensive Fixed, Bed-Sediment and Tissue, and Synoptic Sites. The core of the NAWQA sampling design is the Basic Fixed Sites, of which Intensive Fixed Sites are a specialized subset with the most extensive sampling, and to which Bed-Sediment and Tissue and Synoptic Sites are added to enhance spatial coverage. An example of the geographic structure of the fixed-site design for a Study Unit is shown in figure 7.

(Click on image for a larger version, 60K)

Figure 7. Example of distribution of Basic and Fixed Sites for the White River Basin Study Unit, Indiana.

The number of sites listed in table 4 for the various design components and the level of sampling activities represent typical ranges for individual Study Units. Generally, the national average for each type of site is expected to fall within these ranges, even though conditions in some individual Study Units merit a higher or lower number of sites. Similarly, variation of other selected aspects of the strategy outlined in table 4, such as the degree of continuous monitoring, also depends on Study-Unit conditions.

Last modified: 3:25 EDT 22jun98 klf