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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.
This data set represents the ratings applied to the unsaturated zone for inherent vulnerability. It is one of 11 data sets developed for the North Carolina Source Water Assessment Program. The data sets are used to rate the susceptibility of public water supplies in North Carolina to contamination.
The selected factors contributing to the unsaturated zone rating for ground-water supplies include vertical series hydraulic conductance, land-surface slope, land cover, and land use. For ground-water systems, the higher the inherent vulnerability of the unsaturated zone, the higher the rating applied on a scale of 10 to 100.
The purpose of this data set is to rate the susceptibility to contamination of the unsaturated zone for public ground-water supplies.
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-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.
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 also 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.
UNSATURATED ZONE RATING --
The relative unsaturated zone rating is based on a combination of factors that contribute to the likelihood that contaminants from surface and shallow sources will reach the water table by following the path of aquifer recharge. The selected factors, which are represented by GIS spatial-data layers, include vertical series conductance of the unsaturated zone, land-surface slope, land cover, and land use.
The values of each of these four factors are categorized, and the categories are assigned a rating on a scale of 1 to 10. A rating of 1 reflects a low contribution to inherent vulnerability and 10 reflects a high contribution. For example, the rating for land-surface slope is low (1) in areas of high slope (greater than 50 percent) and high (10) in areas of low slope (less than 2 percent) because increased infiltration potential in flatter terrain leads to an increased likelihood of ground-water contamination.
With the exception of land use, these factors describe influences on the physical transport of water, with or without contaminants. The land-use factor is included as a measure of the potential for generating nonpoint-source contamination at land surface. For the purpose of calculating the unsaturated zone rating, each of these four factors is weighted on the basis of importance of the factor in determining vulnerability. Vertical series conductance and land use are weighted more heavily than are land-surface slope and land cover.
For each cell, the product of the factor weights and ratings are summed to determine the total rating for the cell, resulting in a relative numerical unsaturated zone rating that ranges from 10 to 100 for each cell.
Factors contributing to the unsaturated zone rating.
Description Weight Vertical series conductance of the unsaturated zone 3 The harmonic mean of a series of layers of unsaturated material provides a single value for the capacity of the entire sequence of the unsaturated zone to transmit water (with or without contaminants) from the land surface to the water table. Land-surface slope 2 The inclination or slope of the land surface indicates the likelihood that precipitation will infiltrate or run off. Land cover 2 The type of material covering the land surface influences the likelihood that precipitation will infiltrate into the ground or run off the land surface. Land use 3 The type of land use influences the likelihood of potential nonpoint-source contamination.
SELECTED REFERENCES:
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.
Heath, R. C., 1994, Ground water recharge in North Carolina: Groundwater Section, North Carolina Department of Environmental Management, informal report, 52 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.
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; Elizabeth Morey, Hydrogeologist; Gale Johnson, Hydrogeologist; and Rajpreet Butalia, Geographic Information Systems Coordinator.
The authors also thank the following scientists and engineers who provided additional technical review of this work:
* 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.
Value attribute table, INHER_GW.VAT:
COLUMN ITEM NAME WIDTH OUTPUT TYPE N.DEC ALTERNATE NAME 1 VALUE 4 10 B - 5 COUNT 4 10 B -VALUE is rating between 10 and 100.
Summary statistics table, INHER_GW.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 3MIN is the minimum value.
MIN MAX MEAN STDV 12.000 84.000 48.684 9.171
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 13:01:26 EDT
