The NAWQA-DWH water-quality data base is a compilation of 42 NAWQA study units. It is large, about 155,000 samples, with many types of measurements for many different environmental media.
This server provides water quality samples collected at NAWQA (National Water Quality Assessment) sites in the USA---see map of Study Units started in 1991, 1994, and 1997.
OVERVIEW OF SAMPLING DESIGN AND CONSIDERATIONS FOR DATA INTERPRETATION
The National Water-Quality Assessment (NAWQA) Program is designed to describe the status and trends in the quality of the Nation's ground- and surface-water resources and to provide a sound understanding of the natural and human factors that affect the quality of these resources. As part of the program, investigations will be conducted in 42 areas called "study units." These investigations throughout the Nation will provide a framework for national and regional water-quality assessment. Regional and national synthesis of information from study units will consist of comparative studies of specific water-quality issues using nationally consistent information.
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 studies are not designed to produce a statistically representative analysis of national water quality conditions. The NAWQA sampling design is described by Gilliom and others (1995).
For both streams 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 the particular area. 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.
The design does not result in an unbiased 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 areas started in 1991. Thus, some agricultural activities and related pesticide use that may be very important in a particular part of the nation may not be included. For example, the first 20 study areas did not include intensive rice growing areas. On the other hand, a particular pesticide may be important in one or two of the 20 study units, but not in the others, and the averaged results may be misleading in this regard. Another possibility is that use of a particular pesticide is much greater than average in the watersheds and ground-water areas studied, leading to an overestimate of occurrence and concentrations relative to other areas. Similar biases are possible for urban areas as well, but the dominant pesticides used are probably more similar among urban areas than they are among agricultural areas with different crops. Therefore, the NAWQA pesticide data must be used with care. It is important to consider the detailed descriptions of the nature and limitations of both the surface and ground water data that are provided with the data.
Overview of data contained in the NAWQA Data Warehouse
The U.S. Geological Survey (USGS) began its NAWQA (National Water Quality Assessment) program in 1991, systematically collecting chemical, biological, and physical water quality data from study units (basins) across the nation. This data warehouse contains data from the 42 study units. The data warehouse currently contains and links the following data:
Chemical concentrations in water, bed sediment, and aquatic organism tissues
Site, basin, well and network characteristics with many descriptive variables
Daily stream flow information for fixed sampling sites
Ground water levels for sampled wells
4,400 stream sites and 7,000 wells
44,000 nutrient samples and 26,000 pesticide samples as well as 8,000 VOC samples
2,650 samples of bed sediment and aquatic organism tissues
Many of these data came from the USGS National Water Information System--NWIS for NAWQA sites.
Brief explanation of data base
Fundamental codes used:
"Site type" describes whether the site is a surface-water, ground-water, precipitation-monitoring, or other site type.
"Site name" describes the location of the site. Often, the site type can be inferred from the site name.
"Medium code" or "medium description" defines what kind of sample was collected for analysis. This question is particularly important for surface-water sites, which can have samples for several media (water, biological tissue, stream-bottom sediments, suspended sediments, etc.).
"Parameter codes". Water-quality data are stored under parameter codes. Each code is linked to a definition. Parameter-code definitions typically contain information about what was analyzed, what units are associated with the numerical data, and sometimes, how the sample was processed prior to analysis (examples include filtering of water samples, and sieving of sediment samples). Definitions for each retrieved parameter are provided in the heading of each data retrieval.