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Data format: Raster Dataset File or table name: gwcontrib Coordinate system: State Plane Coordinate System 1983 Theme keywords: Ground-water, Vulnerability, Surfacewater, Unsaturated zone, Groundwater, Watershed, Rating |
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Abstract:
Ground-water contribution classes is one of 6 data sets updated for the
North Carolina Public Water Supply, Source Water Assessment Program.
These data are used to rate the susceptibility of public water supplies
in North Carolina to contamination.
The original dataset was produced in 2000. This version is updated to
reflect new data sources.
This data set represents the influence that ground-water has on surface
waters. This area of influence has been defined as the area within 1000
meters of any stream or waterbody in North Carolina. The portion of
surface water derived from ground water influences surface-water
quantity and quality.
The ratings for the susceptibility to contamination of the unsaturated
zone were applied within the area of influence. High ratings for
ground-water contribution indicate a higher inherent vulnerability
to the unsaturated zone, and since it contributes to the rating of
watershed characteristics, also indicates a higher vulnerability for
watersheds. |
Metadata elements shown with blue text are defined in the Federal Geographic Data Committee's (FGDC) Content Standard for Digital Geospatial Metadata (CSDGM). Elements shown with green text are defined in the ESRI Profile of the CSDGM. Elements shown with a green asterisk (*) will be automatically updated by ArcCatalog. ArcCatalog adds hints indicating which FGDC elements are mandatory; these are shown with gray text.
Ground-water contribution classes is one of 6 data sets updated for the North Carolina Public Water Supply, Source Water Assessment Program. These data are used to rate the susceptibility of public water supplies in North Carolina to contamination. The original dataset was produced in 2000. This version is updated to reflect new data sources. This data set represents the influence that ground-water has on surface waters. This area of influence has been defined as the area within 1000 meters of any stream or waterbody in North Carolina. The portion of surface water derived from ground water influences surface-water quantity and quality. The ratings for the susceptibility to contamination of the unsaturated zone were applied within the area of influence. High ratings for ground-water contribution indicate a higher inherent vulnerability to the unsaturated zone, and since it contributes to the rating of watershed characteristics, also indicates a higher vulnerability for watersheds.
This data set is to be used in a hydrologic analysis with other data sets to rate the unsaturated zone for public ground-water supplies and watershed characteristics for public surface-water supplies in North Carolina. For ground-water supplies, the factors used to rate susceptibility to contamination include vertical hydraulic conductance, land-surface slope, land cover, and land use. The selected factors used to devise ratings for surface-water supplies' susceptibility to contamination are average annual precipitation, land-surface slope, land cover, land use, and ground-water contribution.
The 1996 Amendments to the Safe Drinking Water Act require that each state prepare a source-water assessment for all public water supplies. States are required to (1) delineate source areas supplying wells and surface-water intakes, (2) inventory potential contaminant sources within the delineated source areas, and (3) determine the susceptibility of wells or intakes to the inventoried potential contaminant sources. In North Carolina, the lead agency charged with this task is the Public Water Supply Section (PWSS) of the Division of Environmental Health, Department of Environment and Natural Resources (DENR). North Carolina's Source Water Assessment Program (SWAP) provides assessments of each public drinking water intake in North Carolina. These assessments provide a relative susceptibility rating calculated using state-wide data. The U.S. Geological Survey (USGS) cooperated with the PWSS to provide components of the inherent vulnerability rating that was used with the contaminant rating to determine the overall susceptibility rating of source water supplies. To assist the PWSS with rating ground water inherent vulnerability, the USGS compiled the unsaturated zone rating based on the following characteristics: vertical hydraulic conductance, depth to water, land surface slope, land use and land cover. The watershed characteristic rating includes average annual precipitation, land surface slope, land use, land cover and groundwater contribution. The SWAP program has planned for updates to the initial ratings as conditions change and new data sources become available. The initial data used to calculate the unsaturated zone ratings and watershed characteristics ratings were compiled from sources from the late 1990's. Critical data layers have been updated since the ratings were calculated. These improved data layers need to be used in the ongoing update of the susceptibility ratings. Overlay and index methods for rating susceptibility to contamination of the unsaturated zone for ground-water suppplies, and watershed characteristics for surface-water supplies were derived for use by the State of North Carolina in assessing more than 11,000 public water-supply wells and approximately 245 public surface-water intakes. Factors that influence the inherent vulnerability of ground water and surface water were selected and assigned ratings on a scale of 1 to 10. These factors were then assigned weight to reflect their relative influence on inherent vulnerability and the reliability of the data. The values for each factor were obtained from geographic information system (GIS) data layers that were transformed into ARC/INFO raster data sets known as grids. These raster data sets have 30-meter by 30-meter cells, and each cell is assigned a weighted- factor value. Limitations -- The overlay and index methods of unsaturated zone and watershed characteristics ratings are broad-stroke methods that assess vulnerability on the basis of expert opinion. The methods aslo have limitations in the age and scale of the hydrologic and geographic data. But the most significant limitation of the methods used is that no statistical confirmation of the results have been performed. GROUND-WATER CONTRIBUTION- Management of surface-water resources has traditionally focused on surface water alone. However, surface water and ground water compose one system. Streamflows are partly derived from underlying aquifers, particularly when the aquifer's upper boundary is the water table. During dry conditions, the portion of streamflow coming from ground water is known as base flow, sometimes referred to as sustained fair-weather flow. In periods of extended dry conditions, flows observed in the stream channel consist only of base flow (and non-natural stream inputs such as wastewater discharge). During storm events, the portion of streamflow coming from ground water is known as subsurface stormflow (Freeze and Cherry, 1979; Winter and others, 1998). The ground water contribution factor is included in the assessment of watershed characteristics to address the influence that ground water has on surface-water quantity and quality. In this study, ground-water contribution is a factor in areas within about 1,000 feet of streams. By restricting ground-water contribution to an area of about 1,000 feet on either side of streams, this factor emphasizes subsurface stormflow and local ground-water flow. Ground-water contribution is not considered to have originated outside of this buffer, though this component of ground-water flow is of increasing importance in a stream experiencing low-flow conditions. Inclusion of a ground-water contribution factor addresses the influence that ground water has on the vulnerability of surface waters to contaminant transport from ground water discharge. This metadata can not fully document the methods used in the project to assess ground-water contribution to surface waters. See Eimers and others (1999) for a detailed description of the methods. The unsaturated zone ratings are applied to an area about 1,000 feet on either side of streams. Ground-water contribution ratings for these areas in watersheds upstream from surface-water intakes are calculated exactly as for unsaturated zone ratings, but to normalize these values from 1 to 10 the unsaturated zone rating for each contributing cell is divided by 10. In areas outside of the 1000 foot buffer zone, the ground-water contribution is zero. Processing Steps- The ground-water contribution component was derived from applying the unsaturated zone ratings within a 1,000 feet (304.80061 meter) area around streams and waterbodies in the National Hydrographic Dataset (NHD) distributed by the U.S. Geological Survey and the U.S. Environmental Protection Agency. First, all stream segments that were assigned a reach were extracted from the NHD. The reaches create a dendritic network representing the centerlines of any lakes or double-sided streams. Ocean or estuary HU's were processed separately to remove ocean shoreline reaches. 305 meter polygons were drawn around either side of all streams using the Arc/Info "BUFFER" command. The streams were processed within 8-digit hydrologic cataloging unit areas. Occasionally the buffered areas extended over the ridgelines defined by the cataloging units. The areas that extended over the cataloging unit boundaries were removed. Next, the buffered streams were adjusted to include waterbodies that overlapped the buffered stream areas that were not considered "SEA" polygons. This insured that the middle of lakes wider than 305 meters are included in the analysis of ground-water contribution. Finally, the unsaturated-zone rating grid was applied to the buffered zones within North Carolina. Ground-water contribution category ratings. > Unsaturated zone value Rating > > 0 (No contribution) 0 > 1 (<= 10) 1 > 2 (> 10 to 20) 2 > 3 (> 20 to 30) 3 > 4 (> 30 to 40) 4 > 5 (> 40 to 50) 5 > 6 (> 50 to 60) 6 > 7 (> 60 to 70) 7 > 8 (> 70 to 80) 8 > 9 (> 80 to 90) 9 > 10 (> 90 to 100) 10 > SELECTED REFERENCES: Freeze, R. A., and Cherry, J. A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall, Inc., 604 p. Winter, T. C., Harvey, J. W., Franke, O. L., and Alley, W. M., 1998, Ground water and surface water - a single resource: U.S. Geological Survey Circular 1139, 79 p. Eimers, J. L., Weaver, J. C., Terziotti, S., and Midgette, R. W., 2000, Methods of rating unsaturated zone and watershed characteristics of public water supplies in North Carolina: U. S. Geological Survey Water-Resources Investigations Report 99-4283, 31 p. North Carolina Department of Environment and Natural Resources, 1999, North Carolina source water assessment program plan: Raleigh, North Carolina Department of Environment and Natural Resources, Division of Environmental Health, Public Water Supply Section, [variously paged]. DISCLAIMER: Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this Federal Geographic Data Committee-compliant metadata file is intended to document the data set in nonproprietary form, as well as in ARC/INFO format, this metadata file may include some ARC/INFO-specific terminology.
Ground Condition
Not to be used for site specific analysis
3916 Sunset Ridge Road
The authors are grateful to colleagues in the Public Water Supply Section of the Division of Environmental Health, North Carolina Department of Environment and Natural Resources (DENR) for their assistance in this collaborative project: thanks to Robert Midgette, Protection and Enforcement Branch Head; Amy Axon,hydrogeologist - Source Water Protection; and David Hammermann, Geographic Information Systems Specialist. The authors also thank the USGS report review team for their review of the update procedures: Jeanne Robbins, Surface-water Specialist; Melinda Chapman, Ground-water Specialist; Mary Giorgino, Water-quality Specialist; and Douglas Harned, ALBE-National Water Quality Assessment Project Chief.
visually verified against original source
Not applicable for raster data.
Complete for NC. Not clipped to inidividual water supply watersheds.
Based on various scales of data from 1:100,000 scale data
unsaturated zone ratings
hydrography
reference for ground water/surface water interaction
reference for ground water/surface water interaction
methods for applying rating
Extract the routes from the NHD streams in North Carolina.
Extract the waterbodies from the NHD
Buffer the streams and waterbodies by 304.80061
Union the streams and waterbodies
Create a grid with null values outside of the polygons and a value of 100 inside the polygons
Use the buffered stream grid to mask the Unsaturated Zone grid to create a grid with values inside the buffered zones.
Calc buffered areas outside of North Carolina to the median value of unsaturated zone within buffered areas (48)
Calc null values to 0. Set mask to areas with buffered streams.
Use the old gwcontrib grid as a mask, and create a new gwcontrib layer from the updated unsaturated zone grid. Use merge to place new gwcontrib values in the NC portion, and keep the values outside of NC.
Metadata imported.
Internal feature number.
ESRI
Value is rating of 0 for no contribution, or between 10 and 84 for unsaturated zone rating. Count is the number of 30 meter by 30 meter cells that have the value associated with them. > >GWCONTRIB.VAT: > >COLUMN ITEM NAME WIDTH OUTPUT TYPE N.DEC ALTERNATE NAME > 1 VALUE 4 10 B - > 5 COUNT 4 10 B - > Min is the minumum value (0) Max is the maximum value (84) Mean is the average value STDV is the standard deviation > >GWCONTRIB.STA: > >COLUMN ITEM NAME WIDTH OUTPUT TYPE N.DEC ALTERNATE NAME > 1 MIN 8 15 F 3 > 9 MAX 8 15 F 3 > 17 MEAN 8 15 F 3 > 25 STDV 8 15 F 3 > > >list gwcontrib.sta >Record MIN MAX MEAN STDV > 1 0.000 84.000 25.383 26.272
none
3916 Sunset Ridge Rd.
Contact via email
Although these data have been used by the U.S. Geological Survey, U.S. Department of the Interior, no warranty expressed or implied is made by the U.S. Geological Survey as to the accuracy of the data. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey in the use of this data, software, or related materials.
3916 Sunset Ridge Road