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Nutrient data for the Nation were collected during Cycle 1 (1992-2002)
in 51 large river basins and aquifer systems, each of which is referred
to as a "Study Unit." Implementation of these studies was phased
to have high-intensity sampling occurring in about one-third of
the study units at any one time. Investigations in the first 20
study units occurred from 1991 to 1995. Investigations began in
a second group of 16 study units in 1994 and continued until 1998.
Data was collected in a third group of study units from 1997 to
2001. Nationally, water samples for nutrient analysis were collected
from 159 urban stream sites and ground-water samples were collected
from 844 urban wells.
Surface water and ground water were analyzed for a suite of nitrogen (N) and phosphorus (P)
constituents including dissolved ammonium plus un-ionized ammonia, dissolved nitrite, dissolved
nitrite plus nitrate as nitrogen, dissolved Kjeldahl nitrogen, consisting of ammonia plus organic
N, total Kjeldahl nitrogen, dissolved orthophosphate, dissolved phosphorus, and total phosphorus.
NAWQA assessment of nutrients in Cycle 2 (2002-2012) will continue
at a subset of surface-water and ground-water sites located in 42
of the Nation's major river basins and aquifer systems.
The stream-sites map and tables include only urban fixed sites.
An urban fixed site is a surface-water site where streamflow is
measured and samples are collected and analyzed for various chemical,
physical, and biological characteristics. Combined, such sites are
used to assess broad-scale spatial and temporal variation at the
sites in relation to hydrologic conditions and environmental setting.
Maps include urban and mixed-urban sites. Mixed urban sites are
stream sampling sites located at an outlet of a drainage basin that
contains multiple land uses, including urban.
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