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The NAWQA Program sampling design emphasizes a multidisciplinary approach using physical, chemical, and biological tools to provide multiple lines of evidence with which to evaluate water-quality conditions. The Program focuses on a broad spectrum of at tributes and sampling approaches to collect data on (1) benthic invertebrate, fish, and algal communities; (2) stream habitats; (3) water-column measures of inorganic constituents (major ions, trace elements, nutrients), physical characteristics (suspended sediment, conductance, temperature), radionuclides, and organic compounds; (4) trace elements and organic compounds in bed material and aquatic biota; and (5) hydrology. The foundation of the Program's sampling design is the 60 study units distributed across the conterminous United States, Alaska, and Hawaii. Each study unit consists of one or more coupled river-basin aquifer systems encompassing from 3,100 to more than 155,000 km2 (Leahy and Wilber, 1991). Study units conduct water-quality investigations for 4 to 5 years, followed by 5 years of low-level monitoring, with the cycle repeated perennially (Leahy and others, 1990). Activities are staggered so that approximately one-third of the study units are in an intensive data-collection phase each year. Study-unit investigations consist of four main components: (1) retrospective analysis and reconnaissance, (2) occurrence and distribution assessment, (3) assessment of long-term trends and changes, and (4) source, transport, fate, and effect studies.

Retrospective analysis consists of a review and analysis of existing water-quality data (physical, chemical, and biological) within the study unit. This effort provides an historical perspective on water-quality conditions and assists in the identification of major natural and anthropogenic factors that control water quality within the study unit. Analysis of existing information allows project personnel to examine a wide range of environmental-variable combinations associated with specific land areas and provides baseline information to assist in identifying candidate sampling locations within this range of environmental-variable combinations. Sampling locations are chosen following a reconnaissance of candidate sampling locations.

A reconnaissance consists of a rapid visual assessment of a location, including evaluation of stream access, stream habitat conditions, proximity of major natural or anthropogenic stream influences, and methods and equipment appropriate for conducting various types of sampling at that location. The reconnaissance is used by project personnel to gain familiarity with watershed features of the study unit and to evaluate and select candidate locations for subsequent sampling to determine biological, chemical, and physical characteristics of streams. This subsequent, integrated sampling effort is known as an occurrence and distribution assessment.

The occurrence and distribution assessment characterizes geographic and seasonal distributions of water-quality conditions in relation to major natural and anthropogenic features. This assessment fills crucial gaps in existing data for each study unit. The design of water-quality investigations conducted during occurrence and distribution assessment represents a balance between providing study units with the flexibility to address issues of local importance and maintaining the consistency in data collec tion, sampling approaches,