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GLOSSARY OF ASSESSMENT COMPONENTS

(from Gilliom and others, 1995)

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.

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.

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.

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 about 10,000 km² (about 3,860 mi²) 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 Assessment, Trend and Change Assessment, 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, whenever possible, on existing data collected by other agencies and programs. Typically, 20 to 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.

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