Supplemental_Information: The National Water-Quality Assessment 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 (Leahy and others, 1990). The building blocks of the national assessment are
investigations in major hydrologic systems of the Nation, referred to as "Study Units". The
study units are divided into three groups. Each group of study units is intensively studied
on a rotational schedule of three-year periods. The first group began in fiscal year 1991
(FY 1991 ran from October 1990-September 1991), the second in fiscal year 1994 and the third
in fiscal year 1997. This data set includes major surface- water sampling sites from the
first two groups of study units. Sampling sites within the NAWQA program are grouped into
"networks". A surface-water sampling network is characterized by the type of sampling
activity, such as what constituents are sampled and (or) the sampling frequency. This data
set includes sites in a fixed network and (or) bed sediment and tissue network. A NAWQA
fixed site is a site on a stream, at which streamflow is measured and samples are collected
for temperature, salinity, suspended sediment, major ions, nutrients, and organic carbon, to
assess the broad-scale spatial and temporal character and transport of inorganic
constituents of stream water in relation to hydrologic conditions and environmental settings
(Gilliom and others, 1995). Samples are collected regularly at fixed sites for at least two
years. All fixed sites are considered "basic fixed sites" but some are referred to as
"intensive fixed sites" if the sampling frequency increased during selected seasonal periods
and if pesticide samples were collected regularly for 1 year (Gilliom and others, 1995). A
NAWQA bed sediment and tissue site is a site where bed sediment and (or) tissue samples are
collected and analyzed for trace elements and hydrophobic organic contaminants in streambed
sediment and in the tissue of aquatic organisms. Bed sediment and (or) tissue samples are
typically collected once at a site, however some sites are resampled. Some of these sites
are within NAWQA's major bed sediment and tissue networks, while some are not. The two major
bed sediment and tissue networks are the Occurrence survey and the Spatial Distribution
survey. The Occurrence survey focuses on what hydrophobic contaminants are present, while
the Spatial Distribution survey focuses on these contaminants through broader areal
sampling. Though these surveys constitute separate bed sediment and tissue networks, no
differentiation is made in this data set. However, a bed sediment and tissue site in either
the Occurrence survey or the Spatial Distribution survey is differentiated from remaining
bed sediment and tissue sites that are not part of these surveys. These remaining sites are
either part of another bed sediment and (or) tissue network, or part of a synoptic network
at which bed sediment and (or) tissue samples were collected. A synoptic network consists of
a single or multiple sites sampled to investigate specific conditions during selected
seasonal or hydrologic periods. In this data set, synoptic sites at which bed sediment and
(or) tissue samples are collected are considered to belong to a bed sediment and tissue
network. The major surface-water sampling sites in the 1997 NAWQA study unit starts will be
added to this data set in the near future. The NAWQA home page is at
<
http://water.usgs.gov/nawqa/nawqa_home.html>. A map showing the extent of all NAWQA
study units, with links to individual study unit information, is available at
<
http://water.usgs.gov/nawqa/nawqamap.html>. For additional information about the
NAWQA program please see: Gilliom, R.J., Alley, W.M., and Gurtz, M.E., 1995, Design of the
National Water-Quality Assessment Program: Occurrence and Distribution of Water-Quality
Conditions, U.S. Geological Survey Circular 1112. U.S. Geological Survey, Sacramento,
California. Accessible online at: <
http://water.usgs.gov/pubs/circ1112/>. Leahy, P.P.,
Rosenshein, J.S., and Knopman, D.S., 1990, Implementation plan for the National
Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 90-174. U.S.
Geological Survey, Reston, Virginia, 10 p. Leahy, P.P. and Thompson, T.H., 1994, National
Water-Quality Assessment Program: U.S. Geological Survey Open-File Report 94-70, 4 p.,
Accessible online at <
http://water.usgs.gov/nawqa/NAWQA.OFR94-70.html>. Any use of
trade, product, or firm names is for descriptive purposes only and does not imply
endorsement by the U.S. Government.