Arsenic in Ground Water of the United States
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Frequently-anticipated questions:
- What does this data set describe?
- How should this data set be cited?
- What geographic area does the data set cover?
- What does it look like?
- Does the data set describe conditions during a particular time period?
- What is the general form of this data set?
- How does the data set represent geographic features?
- How does the data set describe geographic features?
- Who produced the data set?
- Why was the data set created?
- How was the data set created?
- How reliable are the data; what problems remain in the data set?
- How can someone get a copy of the data set?
- Who wrote the metadata?
What does this data set describe?
- Title: Arsenic in Ground Water of the United States
- Abstract:
-
This image shows national-scale patterns of naturally occurring
arsenic in potable ground-water resources of the continental
United States. The image was generated from the most recent
arsenic measurement available for each of 31,350 wells and
springs across the United States. Over 20,000 of the water
samples were collected by the U.S. Geological Survey between 1973
and 2001. The remainder of the samples were collected by State
agencies and analyzed by comparable laboratory techniques. The
data set shows a moving 75th percentile, which can also be
described as the maximum arsenic concentration found in 75% of
samples within a moving 50 km radius (the median size of a U.S.
county). In other words, for any given 50-km-radius region in
the data, lower concentrations of arsenic were found in 75% of
sampled wells, while higher concentrations of arsenic were found
in 25% of sampled wells. This is a revised version of the
August 2002 map layer, with a corrected shoreline for Greenland.
- Supplemental_Information:
-
Some of the samples came from individual homeowner wells used,
without treatment, for drinking water. Other samples came from
wells supplying water for irrigation and other purposes. The
data set does not include samples of saline or thermal water; all
the samples represent aquifers used for drinking water supply,
even if the specific well was not used to supply drinking water.
The intent of this study was to describe the quality of the untreated ground-water resource itself, rather than to directly measure the quality of drinking water by monitoring water from taps and other "finished" water. Comparisons of these findings with those from previous drinking-water studies indicate that the concentrations reported here are comparable on a national scale to concentrations in untreated ground water tapped by water utilities across the Nation. These findings also reflect the quality of the ground-water tapped without treatment by homeowners and small community water systems.
Arsenic concentrations are measured in units of micrograms per liter (ug/L), which is equivalent to parts per billion.
The following table describes the display of the 75th percentile of arsenic concentrations in the GeoTIFF image:
Arsenic Color RGB values Concentration ---------------------------------------------------------------- 1 ug/L or less dark green 50.67328 150.3464 0.67328 3 ug/L light green 152 251 152 5 ug/L yellow 255 255 0 10 ug/L orange 255 165 0 50 ug/L or greater red 255 0 0 Insufficient data white 255 255 255 Non-US land grey 204 204 204 Water light blue 132 193 255
Further information on arsenic in ground water is available from the USGS Water Resources Discipline page about Arsenic in ground water in the United States, at <http://water.usgs.gov/nawqa/trace/arsenic/>.The associated world file is included as part of the GeoTIFF. The contents of the world file are:
1000.0000 0.000000 0.000000 -1000.0000 -6086629.0000 4488761.0000
- How should this data set be cited?
U.S. Geological Survey, 200212, Arsenic in Ground Water of the United States: National Atlas of the United States, Reston, VA.
Online Links:
- What geographic area does the data set cover?
- West_Bounding_Coordinate: 166
- East_Bounding_Coordinate: -4
- North_Bounding_Coordinate: 83
- South_Bounding_Coordinate: 15
- What does it look like?
- Does the data set describe conditions during a particular time period?
- Beginning_Date: 1973
- Ending_Date: Nov-2001
- Currentness_Reference: Ground condition
- Ending_Date: Nov-2001
- Beginning_Date: 1973
- What is the general form of this data set?
- How does the data set represent geographic features?
- How are geographic features stored in the data set?
This is a Raster data set. It contains the following raster data types:
- Dimensions 7273 x 10366, type Grid Cell
- What coordinate system is used to represent geographic features?
The map projection used is Lambert Azimuthal Equal Area.
- Projection parameters:
-
- Longitude_of_Projection_Center: -100.00
- Latitude_of_Projection_Center: 45.00
- False_Easting: 0.0
- False_Northing: 0.0
Planar coordinates are encoded using Row and column
Abscissae (x-coordinates) are specified to the nearest 1000.0
Ordinates (y-coordinates) are specified to the nearest 1000.0
Planar coordinates are specified in MetersThe horizontal datum used is None.
The ellipsoid used is Sphere.
The semi-major axis of the ellipsoid used is 6370997.0.
The flattening of the ellipsoid used is 1/1.0.
- How are geographic features stored in the data set?
- How does the data set describe geographic features?
- Arsenic concentration grid cell
- Any of the grid cell elements in the arsenic concentration
raster file.
(Source: U.S. Geological Survey)
- Arsenic concentration grid cell value.
- Each grid cell value is a color definition that represents an
estimate of the 75th percentile of arsenic concentration in
that region. The concentrations were recorded in micrograms
per liter (ug/L; equivalent to parts per billion), and were
converted to colors according to the following table. All
arsenic concentrations greater than 50 micrograms per liter
were recoded as 50 micrograms per liter. There may be
intermediate colors in the image that are not included in this
table.
Arsenic Color RGB values Concentration -------------------------------------------------------------- 1 ug/L or less dark green 50.67328 150.3464 0.67328 3 ug/L light green 152 251 152 5 ug/L yellow 255 255 0 10 ug/L orange 255 165 0 50 ug/L or greater red 255 0 0 Insufficient data white 255 255 255 Non-US land grey 204 204 204 Water light blue 132 193 255
(Source: U.S. Geological Survey)Range of values Minimum: 0 Maximum: 255
Who produced the data set?
- Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
- U.S. Geological Survey
- Who also contributed to the data set?
This image incorporates field data collected by the U.S. Geological Survey, the Maine Department of Health, the Massachusetts Department of Environmental Protection, the Minnesota Pollution Control Agency, the New Hampshire Department of Environmental Services, the Rhode Island Department of Health, the Texas Water Development Board, and the Wisconsin Department of Natural Resources.
- To whom should users address questions about the data?National Water-Quality Assessment Program, U.S. Geological
12201 Sunrise Valley Drive, MS-413
Reston, VA 20192
USA
703-648-5716 (voice)
nawqa_whq@usgs.gov
Why was the data set created?
This map layer is intended to provide national-scale, broad-brush answers to questions such as, "Where in the country might at least 25 percent of wells have high arsenic concentrations?" The data should be displayed and analyzed at scales appropriate for 1:2,000,000-scale data.The data should not be used to infer an arsenic concentration for an individual well. Although the data set shows that concentrations of arsenic are likely to be higher in certain geographic regions relative to others, there can be a high degree of local variability because of factors such as the well’s depth, the type of aquifer used by the well, and local geology and water chemistry. The only way to be certain of the arsenic concentration in the water supplied by any given well is to have the water tested.
No responsibility is assumed by the U.S. Geological Survey in the use of these data.
How was the data set created?
- From what previous works were the data drawn?
- USARSENIC (source 1 of 4)
-
Ryker, S.J., 2001, Mapping arsenic in ground water: Geotimes Newsmagazine of the Earth Sciences v. 46, no. 11, p. 34-36, American Geological Institute, Richmond, VA.
- Type_of_Source_Media: Journal article
- Source_Contribution:
-
National distribution of arsenic concentrations in ground
water.
- Atlas-FCT (source 2 of 4)
-
Service, USDA Forest , and U.S. Geological Survey, 200205, Forest Cover Types: National Atlas of the United States, Reston, VA.
- Type_of_Source_Media: Online
- Source_Contribution: Background and shoreline.
- Atlas-AR200208 (source 3 of 4)
-
U.S. Geological Survey, 200208, Arsenic in Ground Water of the United States: National Atlas of the United States, Reston, VA.
- Type_of_Source_Media: Online
- Source_Contribution: Spatial and attribute information.
- WVS (source 4 of 4)
-
National Imagery and Mapping Agency, 1990, World Vector Shoreline: National Imagery and Mapping Agency, Bethesda, MD.
Online Links:
- Type_of_Source_Media: Online
- Source_Contribution: Shoreline information.
- How were the data generated, processed, and modified?
- Date: 2001 (process 1 of 3)
-
A detailed description of how this map layer was created, and
discussion on interpreting the map layer, may be found in
USARSENIC. Information on sources is included under Data Set
Credit, above.
The area of the United States was divided into a grid using equal-area hexagons 100 kilometers across (the median size of a U.S. county). For areas with a minimum data density of five wells per hexagon, the 75th percentile of arsenic concentration was computed. A moving 75th percentile was then approximated by "smoothing" across hexagon boundaries based on neighboring values.
Data sources used in this process:
- USARSENIC
- Date: 2002 (process 2 of 3)
-
The map layer was projected to a Lambert Azimuthal Equal Area
projection, merged with the background information from Atlas-
FCT, and converted to a GeoTIFF image.
Data sources used in this process:
- USARSENIC
- Atlas-FCT
Data sources produced in this process:
- Atlas-AR200208
- Date: Dec-2002 (process 3 of 3)
-
The coastline of Greenland was replaced by a new, corrected
version derived from World Vector Shoreline (WVS). This image
contains no thematic data for Greenland; the new data were
inserted only to maintain consistency with other National Atlas
data layers.
Data sources used in this process:
- Atlas-AR200208
- WVS
- What similar or related data should the user be aware of?
How reliable are the data; what problems remain in the data set?
- How well have the observations been checked?
- How accurate are the geographic locations?
- How accurate are the heights or depths?
- Where are the gaps in the data? What is missing?
This map layer includes arsenic measurements from ground water in 49 States and Puerto Rico. Arsenic data were not available for the Hawaiian Islands.
- How consistent are the relationships among the observations, including topology?
No tests for logical consistency have been performed on this map layer. The data samples were collected from a variety of agencies over a span of 28 years, but were all processed with comparable laboratory techniques.
How can someone get a copy of the data set?
Are there legal restrictions on access or use of the data?
- Access_Constraints: None.
- Use_Constraints:
- None. Acknowledgment of the National Atlas of the United States and (or) the U.S. Geological Survey would be appreciated in products derived from these data.
- Who distributes the data set? (Distributor 1 of 1)Earth Science Information Center, U.S. Geological Survey
507 National Center
Reston, VA 20192
USA
1-888-ASK-USGS (1-888-275-8747) (voice)
- Contact_Instructions:
- In addition to the address above there are other ESIC offices throughout the country. A full list of these offices is at <http://geography.usgs.gov/esic/esic_index.html>.
- What's the catalog number I need to order this data set?
- What legal disclaimers am I supposed to read?
Although these data have been processed successfully on a computer system at the U.S. Geological Survey, no warranty expressed or implied is made by the U.S. Geological Survey regarding the utility of the data on any other system, nor shall the act of distribution constitute any such warranty. No responsibility is assumed by the U.S. Geological Survey in the use of these data.
- How can I download or order the data?
- Availability in digital form:
Data format: TIFF Network links: <http://nationalatlas.gov/atlasftp.html>
- Cost to order the data: There is no charge for the map layer.
- Availability in digital form:
Who wrote the metadata?
- Dates:
- Last modified: 23-Sep-2005
- Metadata author:
-
Peg Rawson
12201 Sunrise Valley Drive
Reston, VA 20192
USA
703-648-4183 (voice)
atlasmail@usgs.gov
- Metadata standard:
- FGDC Content Standards for Digital Geospatial Metadata (FGDC-STD-001-1998)
