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


Over the past two decades, repeated evaluations of available water-quality information have reached the conclusion that the United States needs longterm national water-quality assessment to support effective water policy and management (Wolman, 1971; James and others, 1983; Cohen and others, 1988; National Research Council, 1990). Agreement on how to assess water quality on a national scale, however, has been much more elusive. One view is that a national water-quality assessment should be done by a national statistical design with prescribed rules for the location and timing of sampling and uniform methods and analyses; for example, Van Belle and Hughes (1983). A contrasting view is that each hydrologic system requires a custom-designed assessment that is based on its unique hydrologic features and human influences (U.S. General Accounting Office, 1981). National statistical designs are best suited for producing consistent descriptions of and monitoring large-scale water-quality conditions that are persistent over time. They tend to be poorly suited for assessing conditions that are short lived or unevenly distributed and for explaining causes and effects. Studies that are custom designed for each system can better assess specific water-quality conditions that are variable and short lived and are suited for explaining causes and effects; however, results for different systems cannot be consistently compared, which makes regional- and national-level assessment of patterns and trends in water-quality conditions difficult or impossible.

The National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS) is designed to describe the status of and trends in the quality of the Nation's ground-water and surface-water resources and to link assessment of status and trends with an understanding of the natural and human factors that affect the quality of water. The design is based on balancing the unique assessment requirements of individual hydrologic systems with a nationally consistent design structure that incorporates a multiscale, interdisciplinary approach for ground water and surface water. By linking assessment of status and trends with an understanding of processes and causes that operate at various spatial and temporal scales, NAWQA can contribute most meaningfully to policies and management actions that improve water quality. The program integrates information about water quality at a wide range of spatial scales, from local to national, and focuses on water-quality conditions that affect large areas of the Nation, such as nutrients and pesticides associated with large agricultural regions, or that occur in many small areas, such as nutrients and pesticides associated with urban areas. Hirsch and others (1988) and Alley and Cohen (1991) discussed the conceptual design of NAWQA and its relation to other programs and design alternatives.

The building blocks of the NAWQA Program are Study-Unit Investigations in 60 major hydrologic basins (Study Units) of the Nation (fig. 1) (Leahy and others, 1990). The glossary at the front of this report includes brief definitions of all study components and related key terms used to describe the design, which are indicated throughout the report with capital first letters. The 60 NAWQA Study Units cover about one-half of the conterminous United States, encompass 60-70 percent of national water use and of the population served by public water supplies, and include diverse hydrologic systems that differ widely in the natural and human factors that affect water quality. This selection of Study Units ensures that the most important national water-quality issues can be addressed by comparative studies. The Study Units are divided into three groups, which are intensively studied on a rotational schedule (fig. 2). The first cycle of assessment for each group of 20 Study Units consists of 2 years of initial planning and Retrospective Analysis of existing data, 3 years of intensive data collection and analysis, and 6 years of report preparation and low-level assessment activity before the second cycle of intensive data collection and analysis begins. One-third of the Study Units are in the intensive study phase at any given time, and the decadal cycle is repeated perennially. The first complete cycle of intensive investigations of all 60 Study Units is scheduled to be completed in 2002.

Figure 1

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Figure 1. Study Units of the National Water-Quality Assessment.

Figure 2

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Figure 2. Phased implementation of Study-unit investigations.

The national assessment goals of NAWQA will be accomplished in two main ways. First, the accumulation of consistent and comparable perennial water-quality assessments for 60 of the largest and most significant hydrologic systems of the Nation will stand alone as a major contribution to our knowledge of regional and national water-quality conditions. Second, National Synthesis builds on and expands the findings from individual Study Units by interpreting results from multiple Study Units, as well as existing information from studies of USGS and other agencies and researchers, to produce regional and national assessments for priority water-quality issues. National Synthesis develops comprehensive assessments issue by issue at the national scale by comparative analysis of Study-Unit findings. Two to three major water-quality topics are simultaneously targeted for National Synthesis design and interpretation during 6- to 9-year periods in a rotational schedule with other topics. Pesticides and nutrients are the first two intensively studied National Synthesis topics, which began in 1991; the third topic, which will begin in 1994, is volatile organic contaminants; and plans are being developed for stream ecology.

External coordination at all levels is an integral component of the NAWQA Program. Information exchange and coordination through Study-Unit liaison committees help ensure that the water-quality information produced by the program is relevant to regional and local interests. The liaison committees are comprised of non-USGS members who represent a balance of technical and management interests. Represented organizations will include, as appropriate, Federal, State, interstate, and local agencies; Indian Nations; and universities. In addition, a national Federal/non-Federal advisory subcommittee specifically designated for the NAWQA Program ensures that Federal and non-Federal interests and needs at the regional and national levels are met.

Overview of Study-Unit Investigations

Study-Unit Investigations are designed to meet National Synthesis requirements for consistent and integrated information and Study-Unit requirements for assessing water quality with sufficient flexibility to adapt to local conditions. Each Study-Unit Investigation consists of four interrelated components: Relations among the four components of Study-Unit Investigations are shown in figure 3. The Retrospective Analysis forms the basis for addressing what is already known and what needs to be further investigated with respect to current water-quality conditions, trends and change, and understanding causes and effects. The Occurrence and Distribution Assessment builds on findings of the Retrospective Analysis to complete a broad assessment of current water-quality conditions. Periodic repetition of selected parts of the Occurrence and Distribution Assessment during future intensive study phases is a key part of the Trend and Change Assessment. Results of the Occurrence and Distribution Assessment also are used to identify the most important questions about sources, transport, fate, and effects to be addressed by Case Studies. The Trend and Change Assessment focuses on document-ing longterm trends and changes, results in new questions about causes and effects, and identifies changes that need to be made in the periodic intensive study phases. Case Studies are used to improve understanding of selected questions about sources, transport, fate, and effects that arise from all aspects of NAWQA investigations and often lead to changes in assessment approaches over time. The interaction among the study components centers on the Occurrence and Distribution Assessment, which provides the foundation of data on which other components build.

Figure 3

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Figure 3. Components of Study-Unit Investigations and their interrelations.

Purpose and Scope

This report summarizes the design of the Occurrence and Distribution Assessment component of NAWQA Study-Unit Investigations, which is the primary focus during the first cycle in all 60 Study Units and thus spans the first decade of the program. The summary is presented in four main parts: goals and major design components, Environmental Framework for design, surface-water study design, and ground-water study design. The primary emphasis is on the nature and interrelations of the study components of the surface-water and ground-water designs. This report is one of a series that will describe selected aspects of the NAWQA design to encourage collaborative efforts with others in the water-resources community and to foster continued evaluation and critical review of approaches to national water-quality assessment.

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