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and spatial and temporal resolution to allow for comparisons among study units (national synthesis). The occurrence and distribution assessment serves as a basis for designing field activities to evaluate long-term trends in water-quality conditions as well as for designing source, transport, fate, and effect studies.

Long-term trends and changes in selected water-quality characteristics are assessed by time series of regular measurements or repeated samplings. In many study units, assessments of long-term trends and changes are conducted in a few basins that are chosen to represent selected environmental settings. Locations selected for monitoring of long-term trends and changes are chosen on the basis of results of the retrospective analysis, reconnaissance, and occurrence and distribution assessment.

Source, transport, fate, and effect studies are conducted to test hypotheses and examine specific issues about characteristics and causes of any water-quality degradation. These studies are directed at high-priority water-quality issues for individual study units and the Nation. The results of these studies are accumulated from multiple study units and used to link assessments of water-quality status and trends to specific causes and processes by example and inference. Source, transport, fate, and effect studies are designed by project personnel in individual study units and are conducted at a wide range of spatial and temporal scales.

Occurrence and distribution sampling includes two distinct types of sampling sites: basic fixed sites and synoptic sites. Basic fixed sites are geographically "fixed" sites at which a broad suite of chemical constituents, along with continuous discharge measurements and ecological surveys, is measured over relatively long time periods (generally, 6 months to 3 years, depending on the constituent). Basic fixed sites form the basis for long-term trend, transport, and integrated physical, chemical, and biological studies within and among cycles of the NAWQA Program. Synoptic sites are typically nongaged sites where one-time collections of a limited number of chemical and biological measurements are made with the objective of answering questions regarding source, occurrence, or spatial distribution.


Ecological surveys characterize biological communities (fish, algae, and benthic invertebrates) and stream habitats at basic fixed sites chosen to represent combinations of major natural and human engendered factors thought to significantly influence water quality nationally and within the study unit. The communities and habitat conditions associated with a site are characterized within a defined length of the stream referred to as the "sampling reach." This approach provides a common spatial scale upon which to assess community and habitat characteristics.