This data set was constructed by combining data created for or from the regional GWA chapters. Minor aquifers that are important local sources of water were mapped in some regions, so the regional maps in the GWA may show more detail than this data set. The juxtaposition of regionally mapped aquifers has led to some instances where an aquifer outcrop or shallow subcrop is bounded by a State line. This is a result of the regional mapping and national categorization methods used and is not meant to imply a hydrogeologic change coincident with a State boundary.
The aquifer outcrop and shallow subcrop boundaries represent broad, regional categories and should not be interpreted as site-specific. Comments regarding the names of aquifers or the hydrogeologic interpretation of the aquifers can be directed to the U.S. Geological Survey, Water Resources Division, Office of Ground Water, Roy Sonenshein, sunshine@usgs.gov, (305) 717-5824.
This data set was used as part of the effort to publish a 1:5,000,000- scale 'Principal Aquifers' map in the National Atlas of the United States of America series of printed maps. The printed map can be considered a representation of this data set with the exceptions of: the smaller scale, slight differences in the coastline due to generalization, base and cultural information, and delineation of the glacial-deposit area.
These data were developed in conjunction with the publication of the GWA. For documentation purposes, areas are referred to by their corresponding GWA chapter letter, or by State. This list shows the relationship between State names and GWA chapters:
HA 730-L Segment 11-Delaware, Maryland, New Jersey, North Carolina, Pennsylvania, Virginia, West Virginia HA 730-M Segment 12-Connecticut, Maine, Massachusetts, New Hampshire, New York, Rhode Island, Vermont
Refer to <http://capp.water.usgs.gov/gwa/gwa.html> for a graphic depiction of the GWA chapter regions, as well as more information about the GWA. Although these spatial data can stand alone, it may be helpful to refer to the printed GWA chapters when using the data. There are three subject areas in which there are significant differences between this data set and the printed chapters: category changes, which are described below; coastline location, and the representation of aquifers in Western Montana, both of which are discussed in the Process Description under Lineage.
Category Changes
Because the GWA regional chapters were written by different authors, there were areas of different interpretations and category delineations, aquifer names, etc., that became apparent when combining the regions. In most cases, the name and delineation of the aquifer is identical between this data set and the GWA printed maps, but there are several areas of differences. In some cases, a single aquifer from a GWA chapter is represented among multiple aquifers in this data set.
The following listings show the differences between aquifer names in the GWA chapters and the aq_name and aq_code used in this data set. See the Entity and Attribute Information section for definitions of the data attributes.
GWA chapter HA 730-G Name from fig 3, page 3 aq_code-aq_name ____________________________________________________________ Sand and gravel aquifer 201-Coastal lowlands aquifer system
Piedmont and Blue Ridge 999-Not a principal aquifer aquifers
Appalachian Plateaus 310-Pennsylvanian aquifers aquifers
Confining unit 999-Not a principal aquifer
PIEDMONT AND BLUE RIDGE AQUIFERS Aquifers in early Mesozoic 308-Early Mesozoic basin basins aquifers
Carbonate-rock aquifers 417-Piedmont and Blue Ridge carbonate-rock aquifers
Crystalline-rock aquifers 999-Not a principal aquifer
Valley and Ridge 505-Valley and Ridge carbonate-rock aquifers carbonate-rock aquifers 416-New York and New England carbonate-rock aquifers
APPALACHIAN PLATEAUS Permian and Pennsylvanian 310-Pennsylvanian aquifers aquifers
GWA chapter HA 730-M Name from fig 10, page M5 aq_code-aq_name ____________________________________________________________ CARBONATE-ROCK AQUIFERS New York and New England 416-New York and New England carbonate-rock aquifers
Valley and Ridge 416-New York and New England carbonate-rock aquifers
SANDSTONE AQUIFERS Mesozoic sandstone and 308-Early Mesozoic basin basalt of the Newark aquifers Supergroup
Lower Paleozoic 309-New York sandstone aquifers
CRYSTALLINE-ROCK AQUIFERS Adirondack 999-Not a principal aquifer
New England 999-Not a principal aquifer
Related Spatial and Tabular Data Sets A data set showing the extent of glacial deposits is included in the online, interactive National Atlas of the United States, but is not available to download. This data set shows the southern limit of glaciation in the United States; areas north of the limit line may contain significant sand and gravel glacial deposits that are important sources of water for local areas.
The final data are being served to the public in the following formats: Arc/INFO Export, or ArcView Shapefile.
Other Online Sources of Information About Aquifers include Aquifer Basics <http://capp.water.usgs.gov/aquiferBasics/index.html> provides information about the various aquifer types; The USGS Office of Ground Water provides information about the ground-water resources of the Nation and ground-water activities of the USGS <http://water.usgs.gov/ogw/>; The Ground Water Atlas of the United States <http://capp.water.usgs.gov/gwa/index.html>describes the location, the extent, and the geologic and hydrologic characteristics of the important aquifers of the Nation.
Individual chapter coverages were appended into one continuous coverage. Arcedit was used for edgematching and to remove duplicate labels.
Coastline Location The 1:2,500,000 principal aquifer maps in the Ground Water Atlas publications used a coastline and international boundary, derived either from the map separates from the National Atlas of the United States of America (published 1970), or from the Digital Line Graph (DLG) data created from the 1970 National Atlas. GWA chapters HA 730-G, -J, and -H used coastline and boundary filmwork photo-mechanically reduced from the 1:2,000,000 National Atlas map separates. The remaining chapters used coastline and boundary data from the DLG's. A revision of the DLG data took place in 1997, after the creation of the chapter aquifer map data.
After the edgematching, a decision was made to replace the coast and international boundary of this national-level aquifer data with one derived from a newer version of 1997 DLG data. State boundary data from the U.S. Geological Survey National Mapping Division was used to provide the coastline and two aquifer boundaries that coincide with State lines. A publicly available version of this data and its metadata (Map Layer Theme States) can be found at <http://nationalatlas.gov/atlasftp.html>.
The existing outline (coastline and international boundary line) was removed, the new outline added to the coverage, and the remaining aquifer contact lines snapped to the new outline. The coverage was re- cleaned, data categories checked onscreen and with plots, and corrected. Line and labeling corrections, such as undershoots, overshoots, and improper labels were corrected in Arcedit.
There are some slight locational differences between this data set and the printed GWA chapters. For example, there are several small differences in the number of small islands in coastal areas and the categorization of these small islands was assigned to either the nearby principal aquifer category or as Not a principal aquifer, depending on the location and categorization of similar small islands in the printed GWA chapters. There are other differences in the categorization of some land/shoreline-related features. For example, an area symbolized as a bay in a GWA chapter may be land in this data set.