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The major impetus for establishing the NASQAN program in 1974 was to develop a baseline water chemistry data set that was long-term and systematically collected throughout the nation. The original network consisted of over 500 stations sampled at monthly intervals. Measured constituents included nutrients, major ions, and suspended sediment. Over time, the program was constrained by budget cuts and corresponding reductions were made in sampling, both in terms of station numbers and sampling frequency. By 1994, the program was limited to quarterly sampling at roughly 275 stations.
Data was collected at uniform time intervals, without concern for the hydrologic patterns of high or low flow, and therefore provide a fairly representative description of conditions on any given day. These data are appropriate for trend detection and can be used for load estimation ONLY if a sufficient number of years are considered together so as to cover a broad range of discharges.
Additional water-quality monitoring was conducted by the Hydrologic Benchmark Network (HBN), which was established in 1963. This program focused on relatively small and minimally-disturbed watersheds. It provides data that is used to evaluate trends in water-quality over time and serves as a control for distinguishing natural variability in small streams from effects induced by human activity.
For the first five years after a major redesign in 1995, the NASQAN program focused on monitoring the water quality of the nation's largest rivers--the Mississippi (including the Missouri and Ohio), the Columbia, the Colorado, and the Rio Grande. During this phase of the program, NASQAN operated a network of approximately 41 stations where the concentration of an expanded range of chemicals, including pesticides and trace elements, were measured in tandem with stream discharge. Stations were chosen at major nodes within the river basin network to provide characterization of large subbasins of these rivers. (Station Selection Criteria)
The sampling strategy was changed to focus on characterizing the variations in chemical and sediment concentrations that occur during a year, particularly the variation that occurs between low and high flows and during different seasons. In this way, NASQAN data can be used to evaluate mass fluxes or loads of constituents to ultimately determine regional source areas for these materials.
In 2001, the NASQAN program entered a five-year special study phase that included significantly decreasing sampling at two basins, the Colorado and Columbia, and redirecting resources to an intensive sampling program at the Yukon River. Sampling continues unchanged in the Mississippi and Rio Grande basins, but only one or two index stations are sampled in the Columbia and the Colorado basins. In the Yukon Basin, fixed-station monitoring to determine constituent fluxes is being supplemented with a series of synoptic cruises. The synoptics are designed to provide baseline data on organic carbon dynamics in response to melting of permafrost in the Arctic.