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


Basic Fixed Sites--
Sites on streams at which streamflow is measured and samples are collected to assess the broad-scale spatial and temporal character and transport of inorganic constituents of streamwater in relation to hydrologic conditions and environmental settings. Data from these sites 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. Three types of water-column sampling activities--continuous monitoring, fixed-interval sampling, and extreme-flow sampling--are conducted for 2 years during each cycle of investigation. Reach assessments of ecological conditions and bed-sediment and tissue sampling also are conducted at these sites. Study Units usually have 8-12 Basic Fixed Sites.

Bed-Sediment and Tissue Studies--
Assessment of concentrations and distributions of trace elements and hydrophobic organic contaminants in streambed sediment and tissues of aquatic organisms to identify potential sources and to assess spatial distribution. Studies are divided into two phases--the Occurrence Survey and the Spatial Distribution Survey.

Case Studies--
Detailed studies of selected contaminants in selected hydrologic systems to address specific questions that concern the characteristics, causes, and governing processes of water-quality degradation.

Ecological Studies--
Studies of biological communities and habitat characteristics to 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. Ecological Studies have three main components--Fixed-Site Reach Assessments, Intensive Ecological Assessments, and Ecological Synoptic Studies.

Ecological Synoptic Studies--
Short-term investigations of specific ecological characteristics within all or part of a Study Unit to provide improved spatial resolution and representativeness compared with fixed-site sampling, and to evaluate the spatial distribution of selected ecological characteristics in relation to causative factors, such as land uses or contaminant sources and instream habitat conditions. These studies supplement information derived from the more comprehensive data collected at Basic and Intensive Fixed Sites by targeting specific and more narrowly defined seasonal and habitat conditions for ecological characterization at more locations. One to two Ecological Synoptic Studies are constructed in most Study Units.

Environmental Framework--
Natural and human-related features of the land and hydrologic system, such as geology, land use, and habitat, that provide a unifying framework for making comparative assessments of the factors that govern water-quality conditions within and among Study Units.

Environmental Setting--
Land areas characterized by a unique, homogeneous combination of natural and human-related factors, such as row-crop cultivation on glacial-till soils.

Fixed-Site Reach Assessments--
One-time assessments of biological communities and habitat conditions of a stream reach at all Basic and Intensive Fixed Sites.

Flowpath Studies--
Assessments of the spatial and temporal distribution of ground-water quality in relation to ground-water flow and land use in shallow aquifer systems for selected Environmental Settings. Flowpath Studies contribute to understanding the natural processes and human factors that control the evolution of ground-water quality along flowpaths through the saturated zone and to evaluate the degree and water-quality significance of interaction between ground water and streams. Typically, one to two Flowpath Studies are undertaken in each Study Unit during the first NAWQA cycle, with most in Land-Use Study areas.

Indicator Sites--
Stream sampling sites located at outlets of drainage basins with homogeneous land use and physiographic conditions. Basins are chosen to be as large and representative as possible while still encompassing primarily one Environmental Setting (typically 50-500 km2).

Integrator Sites--
Stream sampling sites located down-stream of drainage basins that are large and complex and often contain multiple Environmental Settings. Most Integrator Sites are on major streams with drainage basins that include a substantial portion of the Study Unit area (typically 10-100 percent).

Intensive Ecological Assessments--
Annually repeated Fixed-Site Reach Assessments for at least three reaches for 3 or more years at selected Basic and Intensive Fixed Sites.

Intensive Fixed Sites--
Basic Fixed Sites with increased sampling frequency during selected seasonal periods and analysis of dissolved pesticides for 1 year. Most Study Units have one to two integrator Intensive Fixed Sites and one to four indicator Intensive Fixed Sites.

Land-Use Studies--
Investigations of the concentrations and distribution of water-quality constituents in recently recharged ground water (generally less than 10 years old) associated with the most important regionally significant Environmental Settings of land use and hydrogeologic conditions in each Study Unit. For each study, usually a combination of 20-30 shallow existing and observation wells are sampled. Two to four studies typically are completed in each Study Unit during the first cycle of NAWQA.

National Synthesis--
Synthesis of results from all Study Units with information from other programs, agencies, and researchers to produce regional and national assessments for priority water-quality issues.

Occurrence and Distribution Assessment--
Assessment of the broad-scale geographic and seasonal distributions of water-quality conditions for surface and ground water of a Study Unit in relation to major contaminant sources and background conditions. This assessment is the largest and most important component of the first intensive study phase in each Study Unit.

Occurrence Survey--
The first phase of study of trace elements and hydrophobic organic contaminants in streambed sediment and tissues of aquatic organisms to determine which target constituents are common and important to water-quality conditions in each Study Unit. Typically, all Basic and Intensive Fixed Sites and 5-10 additional sites are sampled.

Retrospective Analysis--
The review and analysis of existing water-quality data to provide a historical perspective on the water quality in the Study Unit, to assess strengths and weaknesses of available information, and to evaluate initial implications for water-quality management and study design.

Spatial Distribution Survey--
Extension of the Occurrence Survey for bed sediments and tissues to improve geographic coverage with particular emphasis on assessment of priority constituents identified in the Occurrence Survey. Occurrence Survey results, combined with existing data, govern the analytical strategy and the geographic distribution of sampling sites. The combined data from the two phases of sampling, typically collected from 20-30 sites, provide a basic description of spatial distribution of trace elements and hydrophobic organic contaminants for each Study Unit and support initial evaluation of sources and biological availability for selected constituents.

Study Unit--
A major hydrologic system of the United States in which NAWQA studies are focused. Study Units are geographically defined by a combination of ground- and surface-water features and usually encompass more than 10,000 km2 of land area. The NAWQA design is based on an assessment of 60 Study Units, which collectively cover a large part of the Nation, encompass the majority of population and water use, and include diverse hydrologic systems that differ widely in natural and human factors that affect water quality.

Study-Unit Investigation--
The systematic study of a NAWQA Study Unit. These investigations consist of four main components: Retrospective Analysis, Occurrence and Distribution and Trend and Change Assessments, and Case Studies. Study Units are organized into three groups of 20 that are studied on a rotational schedule, with 3-year intensive study periods repeated once every 9 years.

Study-Unit Survey--
Broad assessment of the water-quality conditions of the major aquifer systems of each Study Unit. The Study-Unit Survey relies primarily on sampling existing wells and, wherever possible, on existing data collected by other agencies and programs. Typically, 20-30 wells are sampled in each of three to five aquifer subunits.

Trend and Change Assessment--
Decadal scale trends and changes in water-quality conditions will be assessed by using a combination of existing historical data, periodic intensive assessments, and selected long-term monitoring strategies.

Water-Column Studies--
Investigations of physical and chemical characteristics of stream water, which include suspended sediment, dissolved solids, major ions and metals, nutrients, organic carbon, and dissolved pesticides, in relation to hydrologic conditions, sources, and transport. These studies also involve selected studies of other water-quality conditions and include selective investigations of hydrophobic organic contaminants or trace elements where their importance is indicated by results of Bed-Sediment and Tissue Studies. Water-Column Studies have three main components--Basic and Intensive Fixed-Site Assessments and Water-Column Synoptic Studies.

Water-Column Synoptic Studies--
Short-term investiga-tions of specific water-quality conditions during selected seasonal or hydrologic periods to provide improved spatial resolution for critical water-quality conditions compared to fixed-site sampling. For the period and conditions sampled, they assess the spatial distribution of selected water-quality conditions in relation to causative factors, such as land use and contaminant sources, through mass-balance analysis of sources and transport. During the first 3-year intensive study period, two to three Water-Column Synoptic Studies are included in most Study-Unit Investigations.

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