NA Not available.Z Less than 1.

Source: U.S. Geological Survey, national-level data, unpublished; state-level data,level data in Water-Data Report, annual series prepared in cooperationwith the state governments.

http://h2o.usgs.gov/

*NASQUAN (National Stream Quality Accounting Network

During the past 30 years, the U.S. Geological Survey (USGS) has operated two national stream water-quality networks, theHydrologic Benchmark Network (HBN) and the National Stream Quality Accounting network (NASQAN). These networkswere begun in the 1960s and 1970s, respectively, to document stream water-quality conditions and trends for watershedsthroughout the United States and to improve our understanding of the effects of the natural environment and human activities onwater quality. The enactment of significant environmental legislation during these decades, including the Water Quality Act of1965 and the Clean Water Act of 1972 and its subsequent amendments, increased the need for comprehensive water-qualitymonitoring data at all levels of government. The networks were designed to address these needs at the national and regionallevels by providing systematically-collected information on the quality of the nation's rivers (Leopold, 1962b; Briggs and Ficke,1975).

Since their inception, the national networks have provided water-quality data for numerous watersheds with diverse climatic,physiographic, and cultural characteristics, which has benefited efforts to investigate the causes of spatial and temporalvariations in water quality (Smith and others, 1987a; Smith and Alexander, 1983). The watersheds range from the relativelysmall, minimally disturbed basins in the HBN to the larger, more culturally-influenced drainage basins of NASQAN. During theoperation of the networks, considerable changes have occurred in terrestrial and atmospheric sources of water pollutants in theUnited States brought about, in part, by regulatory actions. Examples include changes in fertilizer use, atmospheric sources ofsulfur and nitrogen, and wastewater discharges from municipal sewage treatment plants. The national networks have providedsome of the best available information for investigating the influences of these pollutant sources on national and regional waterquality (e.g., Smith and others, 1987a; Smith and Alexander, 1986; Kramer and others, 1986; Lins, 1986; Smith and others,1993). In addition, national network data have been used to quantify trends in stream water quality (e.g., Lettenmaier andothers, 1991; Smith and others, 1993), estimate the rates of chemical flux from watersheds (e.g., Alexander and others,1996a; Peters, 1984), and investigate relations between water quality and streamflow (e.g., Smith and others, 1982) andbetween water quality and various physical characteristics of the watersheds (Biesecker and Leifeste, 1975; Peters, 1984).Numerous state and local investigations of stream water quality have also used NASQAN and HBN data to examine streamwater-quality conditions and trends (e.g., Schertz, 1990; Schertz and others, 1994; Hay and Campbell, 1990; Hainly andRitter, 1986).

The national networks have provided stream measurements for a relatively consistent and comprehensive set of waterproperties. Approximately 85 physical, chemical, and biological properties have been analyzed during more than 60,000 visitsto 679 stream locations. Relatively consistent sampling and analytical methods have been used at national network sites duringtheir operation. The water-quality data reflect sampling over a wide range of streamflow conditions. As of 1995, expendituresfor the two national networks have totaled approximately $95 million, including $80 million for NASQAN and $15 million forthe HBN.

Documentation and quality-assurance information on network operations and historical records of sample-collection agencies,laboratories, analytical methods, and estimates of laboratory measurement error are available to assist in the proper use of thestream water-quality data. This information is especially useful for documenting changes in network operations and samplingand analytical methods that may affect the use and interpretation of the ambient water-quality data. Much of this information iscontained in USGS publications and previously unpublished USGS internal technical memoranda. Estimates of laboratorymeasurement error (i.e., variability and bias) have been previously published as part of the Blind Sample Program, an externalUSGS laboratory quality- assurance program (Maloney and others, 1994). These data have been used to systematicallyassess the accuracy of USGS laboratory methods and national network stream water-quality data (see for example, Maloneyand others, 1994; Alexander and others, 1993). *