The occurrence and distribution assessment characterizes geographic and seasonal distributions of water-quality conditions in relation to major natural and human sources of constituents. This assessment is designed to fill crucial gaps in existing data for each NAWQA study unit. The design of water-quality investigations conducted during the occurrence and distribution assessment represents a balance between study-unit flexibility, to target issues of local importance, and national consistency in relation to parameters measured, sampling approaches, and spatial and temporal resolution to allow for comparisons among study units. The occurrence and distribution assessment serves as a basis for designing field activities to evaluate long-term changes in water-quality conditions and studies of sources, transport, fate, and effects.
Occurrence and distribution sampling includes two distinct types of sampling locations: basic fixed sites and synoptic survey sites. Basic fixed sites are geographically " fixed" sites at which sampling for a broad suite of chemical constituent s, along with continuous discharge measurements and ecological surveys, is conducted. Basic fixed sites form the basis for long-term trend and transport assessments, as well as integrated physical, chemical, and biological studies within and among cycles of the NAWQA Program. Synoptic surveys are conducted generally as one-time collections of a limited number of constituents, with the objective of answering questions concerning the sources, occurrence, and spatial distributions of constituents within a study unit.
Assessments of long-term trends and changes in selected water-quality characteristics are designed from the results of the retrospective analyses, reconnaissance, and occurrence and distribution assessments. In many study units, assessments of long-term trends and changes are conducted at a restricted number of fixed sites in a few basins chosen to represent selected environmental settings.
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 among study units enable the linkage of broad 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.
Ecological surveys characterize biological communities (algae, benthic invertebrates, and fish) and stream habitats at locations chosen to represent combinations of major natural and human 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 biological communities and habitat characteristics. Each sampling reach is characterized using a combination of qualitative and quantitative algal samples. The character of periphyton microhabitats present in the sampling reach determines the types of sampling devices and collection methods used for collecting representative algal samples.