Abstract:
This is a coverage of the boundaries and codes used for the U.S. Geological
Survey National Water-Quality Assessment (NAWQA) Program Study-Unit
investigations for the conterminous United States, excluding the High Plains
Regional Ground-Water Study.
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). A "Study
Unit" is a major hydrologic system in which NAWQA studies are focused. Study
Units are geographically defined by a combination of ground- and surface-water
features (Gilliom and others, 1995).
As part of the NAWQA program, Study-Unit investigations were planned for 60
areas throughout the Nation to provide a framework for national and regional
water-quality assessments (Leahy and others, 1990). The 60 planned Study-Units
were divided into three groups of 20. Each group would be intensively studied
on a rotational basis with 20 studies beginning in fiscal year 1991 (FY 1991
runs from October 1990-September 1991), 20 more studies beginning in fiscal
year 1994 (October 1993-September 1994), and the final 20 studies beginning in
fiscal year 1997 (October 1996-September 1997). Each study cycle would span 10
years. In 1996, the number of Study-Units was scaled back to 59 when two of the
original 60 Study Units combined. Also, because of budgetary restraints, some
of the original planned Study Units have been scheduled to start later than
originally planned and others have not even been scheduled to start yet.
This coverage contains the boundaries for the 57 Study Units within the
conterminous United States, excluding the High Plains Regional Ground
Water-Study, which was conceived in late 1997. The coverage also includes the
name, starting date, and NAWQA standard abbreviation of each Study Unit plus
various codes to help display the data. This data set is used primarily to
display the location of NAWQA Study Units and for analysis of data at the
national scale. It is not recommended for either local or regional analysis due
to the small scale of most of the features.
This coverage can be used in conjunction with other NAWQA datasets including
the point coverage of NAWQA Trace Element Sampling Sites (NAWQA_TE) and the
point coverage of NAWQA Nutrients Sampling Sites (NAWQA_NU). Detailed
information on these two coverages can be found in their respective metadata.
Originally, Study-Unit boundaries in this coverage were composed of 1:
2,000,000-scale hydrologic unit boundaries (Allord, 1992) and state boundaries
(Negri, 1994). As the NAWQA project has progressed and Study-Unit
Investigations have gotten underway, many Study-Unit boundaries have been
modified. In addition, Study Units have enhanced their boundary coverages with
features at higher resolutions. As these modifications are made, Study Units
submit their new boundary coverages to National Synthesis teams, who are
responsible for summarizing the results from all of the Study Units, and the
changes are incorporated into this coverage. As a result, this coverage is
composed of linear features at various scales (for example, 1: 100,000, 1:
250,000), but the majority remain at the 1: 2,000,000 scale.
The original version of this coverage was generated by the the USGS
Cartographic and Publishing Program (CAPP) in Madison, Wisconsin, in the fall
of 1991. The procedures used to create this coverage are described below. Each
NAWQA Study Unit was asked for a description of their boundary definition. Once
this information was gathered, CAPP created the coverage by extracting digital
features from the 1: 2,000,000 Hydrologic Unit boundaries coverage and the 1:
2,000,000 state boundaries coverage. Since the majority of Study-Unit
boundaries are defined from hydrologic unit boundaries, most of the features
were directly copied from the Hydrologic Units coverage. An exception to this
was the boundary defining the Georgia-Florida Coastal Plain Study Unit where
the northern boundary was defined by the northern edge of the Florida Aquifer.
To incorporate this boundary into the coverage, the aquifer boundary was
digitized from the U.S. Geological Survey's "Ground-Water Atlas of the United
States", HA-730 (G) (Miller, 1990). In November 1991, responsibility for
maintaining the coverage was transferred to NAWQA's National Synthesis staff.
Major milestones in the development of the coverage and various revisions to
the coverage are listed under the Lineage section.
The NAWQA Program has used the coverage for various analyses and displays and
for various published reports, for example, Leahy and Thompson (1994) and
Gilliom and others (1995).
The coverage is reviewed by one of the NAWQA National Synthesis GIS staff
members prior to release. Related_Spatial_and_Tabular_Data_Sets:
Alaska (Cook Inlet) and Hawaii (Oahu) NAWQA Study-Unit boundaries are
maintained in separate data sets.
The High Plains Regional Ground-Water Study boundary is in a separate data set.
Cook, Oahu, and High Plains study boundaries should be used with this data set
to give the full picture of NAWQA Study Units nationwide.
[Summary provided by EPA] 
