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Introduction
This report is the companion data report for: Nutrients in Streams and
Rivers Across the Nation — 1992–2001 (Mueller and Spahr, 2006).
The data contained in this report were collected as part of the National
Water-Quality Assessment (NAWQA) Program. Investigations were conducted
in 51 large river basins and aquifer systems, which are referred to as "study
units." Implementation of study-unit investigations were phased so
that high-intensity sampling occurred in about one-third of the study units
at a time. Investigations in the first 20 study units began in 1991, and
stream sampling began in 1992; however, most samples were collected during
water years 1993–95. (Water year is defined as the period from October
through September and is identified by the year in which it ends.) A second
group of 16 study-unit investigations began in 1994, with most of the sampling
completed during water years 1996–98. A third group, consisting of
15 study units, began in 1997 with most of the data collected during water
years 1999–2001. At some sites, additional sampling continued after
the high-intensity time period. Gilliom and others (1995) provide additional
information about study-unit sampling design. Additional information about
the NAWQA program is available at http://water.usgs.gov/nawqa/.
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