National Water-Quality Assessment (NAWQA) Program
April 14, 2000 (Revision log)
(See Note below)
Results from the first cycle of National Water-Quality Assessment Program (NAWQA) water-quality data collection during 1992-1996 include analyses of 8 nutrient species and 11 other inorganic constituents from about 2,200 samples of ground water and from about 8,500 samples of streams and rivers in 20 of the nation's major watersheds (NAWQA study units). The characteristics of the NAWQA nutrients data are described below, followed by viewable samples of the data and downloadable data files. The data set used for national synthesis is from the February 1998 compilation.
OVERVIEW OF SAMPLING DESIGN AND CONSIDERATIONS FOR DATA INTERPRETATION
NAWQA studies are based on a complex sampling design that targets specific land use and hydrologic conditions in addition to assessing the most important aquifers and large streams and rivers in each area studied. The NAWQA sampling design is described in detail by Gilliom and others (1995).
For streams, rivers, and ground water, a major component of the sampling design is to target specific watersheds and shallow ground water areas that are influenced primarily by a single dominant land use (agricultural or urban) that is important in a particular area of the United States. This aspect of the sampling design facilitates summarizing results by agricultural and urban land use settings for both streams and ground water. However, the results from this design require careful interpretation, because this design results in a biased representation of all streams or shallow ground water in agricultural settings. For agricultural land use, the focus was limited to the most important agricultural settings within the first 20 study units. Thus, some agricultural activities that may be important in other parts of the nation may not be included. The user is encouraged to consider the detailed descriptions of the nature and limitations of the stream and ground water data that are provided.
DATA FILES are tab-delimited ASCII files that contain a header row of column names followed by rows of data for easy import into spreadsheet or software programs. The data files are not intended for viewing in a browser; they are designed for you to download. However, small extracts of each file show the header lines and a few records that are viewable in a browser. Each data file is accompanied by a file explaining the data. Downloadable EXPLANATION FILES accompany the data files so you will have the field definitions and explanation for each data file.