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This web site contains the Federal Geographic Data Committee-compliant metadata (documentation) for digital data produced for the North Carolina, Department of Environment and Natural Resources, Public Water Supply Section, Source Water Assessment Program. The metadata are for 11 individual Geographic Information System data sets. An overlay and indexing method was used with the data to derive a rating for unsaturated zone and watershed characteristics 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 for susceptibility to contamination. For ground-water supplies, the digital data sets used in the assessment included unsaturated zone rating, vertical series hydraulic conductance, land-surface slope, and land cover. For assessment of public surface-water intakes, the data sets included watershed characteristics rating, average annual precipitation, land-surface slope, land cover, and ground-water contribution. Documentation for the land-use data set applies to both the unsaturated zone and watershed characteristics ratings. Documentation for the estimated depth-to-water map used in the calculation of the vertical series hydraulic conductance also is included.
Overlay and index methods for rating the unsaturated zone and watershed characteristics 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 for susceptibility to contamination. Factors that influence the vulnerability of public ground water and surface water supplies to contamination were selected and assigned ratings on a scale of 1 to 10, covering the range of values in North Carolina. These factors then were assigned weight to reflect their relative influence on the perceived inherent vulnerability and reliability of the data (Eimers and others, 2000).
Factors selected for rating the vulnerability to contamination of the unsaturated zone are vertical series hydraulic conductance, land-surface slope, land cover, and land use. Vertical series hydraulic conductance measures the capacity of unsaturated material to transmit water. Land-surface slope influences whether precipitation runs off land surfaces or infiltrates into the subsurface. Land cover describes the physical overlay of the land surface, which influences the amount of precipitation that runs off or infiltrates into the subsurface. Land use describes activities occurring on the land surface that influence the potential generation of nonpoint-source contamination.
In order to develop the unsaturated zone vulnerability rating, an estimated depth-to-water map was created. The estimated depth-to-water map was necessary for the calculation of the vertical series hydraulic conductance values. A documentation file was created describing methods and formulas that were applied to generate this data set.
Factors selected for rating the watershed characteristics upstream from surface-water intakes are average annual precipitation, land-surface slope, land cover, land use, and ground-water contribution. The average annual precipitation represents the amount of water available for transport in a watershed. Land-surface slope, land cover, and land use have similar influences on watershed characteristics as those identified for the unsaturated zone. Ground-water contribution represents the part of streamflow that is derived from ground water.
The values for each factor were obtained from Geographic Information System (GIS) data layers stored as raster data sets. These raster data sets have 30-meter by 30-meter cells, and each cell is assigned a value based on the characteristics of the factor within that cell. Only one data set was created for the land-use factor because identical ratings were applied for the unsaturated zone rating and watershed characteristics rating. The values for each factor were assigned a weight, then the weighted values were combined to create the final vulnerability values for the unsaturated zone and watershed characteristics ratings.
A structured documentation file (known as "metadata") for each data set has been created. The documentation files comply with the Federal Geographic Data Committee (FGDC) Content Standards for Digital Geospatial Metadata (Federal Geographic Data Committee, 1994). The FGDC-compliant metadata files contain descriptions of the data sets and include narrative sections describing the procedures used to produce the data sets in digital form. The metadata also include references of the sources used to create the data set.
This page provides background context for the 11 digital data sets created for the North Carolina Source Water Assessment Program (SWAP), and is the access point for all associated metadata files.
The FGDC-compliant metadata for the 11 data sets are linked below. The digital data are not currently online because of space limitations. The digital data sets can be requested through the distribution contact identified in the metadata.
Eimers, J.L., Weaver, J.C., Terziotti, Silvia, 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.
Federal Geographic Data Committee, 1994, Content standards for digital geospatial metadata (June 8): Washington, D.C., Federal Geographic Data Committee, 78 p.
Metadata is also available as a Plain Text file.
Ground-water contribution classes is one of 11 data sets developed for the North Carolina Source Water Assessment Program. These data are used to rate the susceptibility of public water supplies in North Carolina to contamination.
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 Federal Safe Drinking Water Act (SDWA) Amendments of 1996 emphasize pollution prevention as an important strategy for the protection of ground-water and surface-water resources. This new focus in the SDWA promotes the prevention of drinking water contamination as a cost-effective means of ensuring reliable, long-term, and safe drinking water sources for public water-supply systems (North Carolina Department of Environment and Natural Resources, 1999a). Specifically, Section 1453 of the SDWA Amendments requires that States develop and implement a Source Water Assessment Program (SWAP) to delineate source water areas, inventory potential contaminants in these areas, and determine the susceptibility of each public water supply to contamination. The agency charged with the task of susceptibility assessment in North Carolina is the Public Water Supply Section (PWSS) of the Department of Environment and Natural Resources. The U.S. Geological Survey (USGS) is directed under the Clean Water Action Plan, funded by Congress in 1999, to assist States with water-quality monitoring and susceptibility determinations.
The inherent vulnerability rating is a measure of the potential for contaminants within a delineated source area to reach the ground-water or surface-water supply. The inherent vulnerability of a ground-water source of public water supply is determined by combining an aquifer rating and an unsaturated zone rating (North Carolina Department of Environment and Natural Resources, 1999a). The inherent vulnerability of a surface-water source of public water supply is determined by combining a watershed classification, intake location, raw water quality (water plant data), North Carolina Division of Water Quality Use Support rating, and watershed characteristics rating (North Carolina Department of Environment and Natural Resources, 1999a). In cooperation with the PWSS, the USGS developed methods to rate unsaturated zones for public ground-water systems and watershed characteristics for public surface-water intakes. All other components of inherent vulnerability were compiled by the PWSS.
Overlay and index methods for rating susceptibility to contamination of the unsaturated zone for ground-watersuppplies, 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.
The identification of factors, development of ratings for each, and subsequent assignment of weights were based on (1) a literature search, which included examination of potential factors and their effects on the drinking-water quality; and (2) consultation with experts in the fields of hydrology, geology, forestry, agriculture, and water management. The relative rating of the unsaturated zone and watershed characteristics combines hydrologic data with expert knowledge to assess the vulnerability of water supplies to contamination.
Factors selected for rating the inherent vulnerability of the unsaturated zone to contamination are vertical series hydraulic conductance, land-surface slope, land cover, and land use. Vertical series hydraulic conductance measures the capacity of the unsaturated material to transmit water. Land-surface slope influences whether precipitation runs off land surfaces or infiltrates into the subsurface. Land cover describes the physical overlay of the land surface, which influences the amount of precipitation that runs off or infiltrates into the subsurface. Land use describes activities occurring on the land surface that influence the potential generation of nonpoint-source contamination.
Factors selected for rating vulnerability to contamination of the watershed upstream from surface-water intakes are average annual precipitation, land-surface slope, land cover, land use, and ground-water contribution. The average annual precipitation represents the mass of water that becomes available for transport in a watershed. Land-surface slope, land cover, and land use have similar influences on watershed characteristics as those identified for the unsaturated zone. In the cases of land-surface slope and land cover, the ratings for watershed characteristic vulnerability are the opposite of unsaturated zone vulnerability to contamination (i.e. more infiltration or ponding produces a higher vulnerability to ground- water, but less to surface-water sources.) Ground-water contribution represents the part of streamflow that is derived from ground-water discharge.
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 component of inherent vulnerability 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.
* Richard Burns, Watershed and Forest Hydrologist, U.S. Forest Service, [U.S. Department of Agriculture] * Ron Coble, Professional Geologist (Retired USGS) * Ed Holland, Orange County Water and Sewer Authority * Beth McGee, Clean Water Management Trust Fund * Ted Mew, Groundwater Section, Division of Water Quality, North Carolina DENR * Joe Rudek, Environmental Defense Fund * Henry Wade, Pesticides Section, North Carolina Department of Agriculture * Steve Zoufaly, Division of Water Quality, North Carolina DENR
The authors also thank the USGS report review team for their review of the metadata products: Stephen J. Char, Jason M. Fine, Michael L. Strobel, Douglas A. Harned and Rebecca J. Deckard.
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)
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 23.816 24.496
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.
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Page Last Modified: Friday, 04-Nov-2022 12:53:33 EDT
