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The NAWQA 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. For surface waters (streams and rivers) NAWQA focuses on a broad spectrum of attributes and sampling approaches to collect data on (1) hydrology; (2) inorganic constituents (major ions, trace elements, nutrients), physical measurements (suspended sediment, conductance, temperature), radionuclides, and organic contaminants in water; (3) trace elements and organic contaminants in bed material and aquatic biota; (4) ecological information (fish, benthic invertebrate, and algal communities); and stream habitat evaluation.

The program is organized into 60 study units on the basis of hydrologic systems (major river basins and large parts of aquifers). Each study unit conducts 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). 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 effects studies.

Retrospective Analysis and Reconnaissance

Retrospective analysis and reconnaissance efforts provide information to aid in the focus of NAWQA issues and in the design of NAWQA studies. The retrospective analysis is designed to provide a historical perspective on water-quality conditions and biota within a study unit and to assist in the identification of major natural and human factors within that study unit. Analysis of retrospective information on water-quality conditions, biota, and natural and human influences within the study unit also provides baseline information to assist in identification of candidate sampling locations. However, sampling locations are not chosen until a reconnaissance is conducted or an exploration and evaluation of candidate sampling locations are completed.

A reconnaissance consists of a rapid site assessment that includes evaluations of such factors as stream access, stream habitat conditions, proximity of major natural or human stream influences, and methods and equipment appropriate for conducting various types of sampling at that location. A reconnaissance is conducted to familiarize project personnel with watershed features of the study unit and to evaluate candidate locations for subsequent sampling of biological, chemical, and physical characteristics of streams. This subsequent, integrated sampling effort is known as an occurrence and distribution assessment.

Occurrence and Distribution Assessment

The occurrence and distribution assessment is conducted to characterize geographic and seasonal distributions of water-quality conditions in relation to major natural and human features. This assessment is designed to fill crucial gaps in existing data for each study unit. The design of water-quality investigations conducted during the occurrence and distribution assessment represents a balance between maximum flexibility of study units to