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NAWQA assessments of trace elements during Cycle I (1992-2002)
followed a nationally consistent approach in 51 of the Nation's
major river basins and aquifer systems, referred to as "Study Units,"
with 20 Study Units examined during 1992-1995; 16 during 1996-1998;
and 15 during 1998-2001. Nationally, bed-sediment samples were collected
from 133 urban stream sites and fish- and clam-tissue samples were
collected from 133 urban stream sites. Ground-water samples were
collected from 583 urban wells.
NAWQA assessment of trace elements in Cycle II (2002-2012) will
focus on mercury in stream ecosystems and sampling a subset of ground-water
wells sampled in Cycle I.
The stream-sites maps 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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