GROUND-WATER POLLUTION SUSCEPTIBILITY --------------------------------------------------------------------------- Table of Contents --------------------------------------------------------------------------- DOCUMENTATION REPORT FOR GIS DATABASE: GROUND-WATER POLLUTION SUSCEPTIBILITY MAP OF GEORGIA DEPARTMENT OF NATURAL RESOURCES ENVIRONMENTAL PROTECTION DIVISION GEORGIA GEOLOGIC SURVEY Atlanta 1996 DOCUMENTATION REPORT 96-? Table of Contents OVERVIEW GLOSSARY PART A - General Description of the Database 1.0 Identification Information/Project Background 1 2.0 Data Quality Information 1 2.1 Attribute Accuracy 1 2.2 Logical Consistency 1 2.3 Completeness 2 2.4 Positional Accuracy 2 2.5 Lineage 2 3.0 Spatial Data Organization Information 6 4.0 Spatial Reference Information 6 5.0 Entity and Attribute Information 6 6.0 Distribution Information 6 7.0 Metadata Reference Information 8 Appendix A - Signatures PART B - Technical Description of the Database 1.0 Identification Information 1 1.1 Citation 1 1.2 Description 1 1.3 Time Period of Content 2 1.4 Status 2 1.5 Spatial Domain 2 1.6 Keywords 2 1.7 Access Constraints 2 1.8 Use Constraints 2 1.9 Point of Contact 2 2.0 Data Quality Information 3 2.1 Attribute Accuracy Information 3 2.2 Logical Consistency Report 3 2.3 Completeness Report 4 2.4 Positional Accuracy Report 4 2.5 Lineage 4 3.0 Spatial Data Organization Information 13 3.1 Indirect Spatial Reference Method 13 3.2 Direct Spatial Reference Method 13 3.3 Point and Vector Object Information 13 4.0 Spatial Reference Information . 13 5.0 Entity and Attribute Information 14 5.1 Detailed Description 14 6.0 Distribution Information 14 6.1 Distributor 14 6.2 Resource Description 14 6.3 Distribution Liability 14 6.4 Standard Ordering Process 14 6.5 Standard Ordering Process 14 7.0 Metadata Reference Information . 14 7.1 Metadata Date 14 7.2 Metadata Review Date 14 7.4 Metadata Contact 14 7.5 Metadata Standard Name 15 7.6 Metadata Standard Version 15 Appendix B PART C - Digital Files Containing the GIS Database Summary of Contents of Data Cartridge 1 Data Cartridge Jacket GLOSSARY OF GIS TERMS GLOSSARY TERMS GEOGRAPHIC INFORMATION SYSTEM TERMS Arc/Info: a popular GIS software, used by the GGS ASCII: abbreviation for `American Standard Code for Information Interchange. A set of codes for representing alphanumeric information in a format which any computer can read. attribute: a characteristic of a geographic feature. For example, if the geographic feature is a river, then an attributes of the river are the river name, the flow rate, the chemical history, etc. attribute accuracy: a measure of how well the reported characteristics actually match the real-world characteristics of a geographic feature Compact Disk Read Only Memory (CDROM): an optical media which can store 650 Mb of information. completeness: a description of the relation between the items represented in the database and the real world. For example, if a database contains water wells in Georgia, does it contain all the water wells in Georgia, or a defined sub-set of wells. If the database contains a defined subset of wells, does it contain every well in Georgia that meets the definition by which the sub-set was chosen. Content Standards for Digital Geospatial Metadata: a standard developed by the FGDC which specifies the information content of metadata for a set of digital geospatial data. database: a logical collection of interrelated information, managed and stored as a unit on a computer or other storage media. A GIS database includes data about the spatial location and shape of geographic features recorded as points, lines, areas, as well as their attributes. decimal degrees: a unit of measure for geographic coordinates. The conversion formula for decimal degrees is: Decimal Degrees = Degrees + Minutes/60 +Seconds/3600 Department of Natural Resources (DNR): a department of the government of the State of Georgia. digital: refers to the process of conversion of information into machine language so that a computer can read, write, store, and process the information. Digital Line Graph (DLG): computer files from the USGS which contain digital maps of transportation, hydrography, contours, and public land survey boundaries. digital maps: a map is an abstract representation of the physical features of a portion of the Earth's surface graphically displayed on a piece of paper. When that piece of paper is converted into a form which the computer can use, the map is then digital. diskette: a storage medium, usually measuring 3.5 inches in diameter, which is used to store or transfer information from one computer to another. FrameMaker - a popular Unix-based word processing software. Federal Geographic Data Committee (FGDC): an inter-governmental committee established through the Office of Management and Budget and charged wit the responsibility to coordinate various surveying, mapping, and spatial data activities to meet the needs of the Nation. Geographic Information System (GIS): an organized collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information. label points: a single x,y coordinate which identifies an area, and holds a place in the database wherein the area's attribute information is stored. latitude, longitude: a geographic reference system to locate positions on the Earth. Latitude and longitude are angles measured from the Earth's center to the position on the Earth's surface. Latitude measures angles in a north-south direction, while longitude measures angles in the east-west direction. lineage: information about the events, parameters, and source data which constructed a geographic database, and information about the responsible parties. logical consistency: an explanation of the fidelity of the relationships in the database. For example, for a geographic area, do the vector lines which create area boundaries join to encircle the area with no gaps. Or for example, for a particular attribute, are all the value given within a valid range. longitude, latitude: see latitude, longitude metadata: information about the content, quality, condition, and other characteristics of a database MIF: abbreviation for Maker Interchange Format, a file transfer format produced by FrameMaker National Map Accuracy Standards: a set of standards developed by the USGS which states the level of accuracy required for a map product of a particular scale. pkzip: a popular data compression software. positional accuracy: an assessment of how well the reported position of a geographic feature represents the real-world position. United States Geological Survey (USGS): a branch of the United States Department of Interior vector lines: lines which are described by x,y coordinates and are commonly used to represent linear geographic features. Each linear feature is represented as an ordered list of vertices. WordPerfect: a popular word processing software OVERVIEW This document contains a three part documentation report for a Geographic Information System (GIS) database developed, with the Arc/Info software, by the Geologic Survey Branch, Environmental Protection Division of the Georgia Department of Natural Resources. Part A is intended as a general purpose summary of the database. Part B is a more technical section included primarily for the benefit of GIS processors. Part C is a digital product which contains the database and associated documentation. The digital documentation includes a digital copy of the paper publication. This documentation report follows the "Content Standards for Digital Geospatial Metadata" (Standards) developed by the Federal Geographic Data Committee (FGDC). As a convenience, Parts A and B employ the outline/headings contained in the Standards. Part A and Part B are printed paper and Part C is a digital product. Part B meets the technical requirements of the Standards. The digital documentation in Part C contains two digital copies of this publication. One digital copy is stored as an ASCII file. The other digital copy is in Framemaker (MIF) format. Moreover, there are some digital documentation embedded in the database. The embedded documentation was done using the document aml (metadata generation tool). The document aml, can be used to view the embedded documentation. The document aml must be run within the Arc/Info, ver. 7.0 or later, software. The database in Part C consists of an Arc/Info Export file. The digital files are written on 150 megabyte data cartridge tape( media), using the DG/UX ver. 5.4R3.10 tape archive (tar) command. A directory of the files with a short explanation are contained in the ASCII file READ.ME on the 150 megabyte data cartridge tape( media). PART A - GENERAL DESCRIPTION OF THE GIS DATABASE: GROUND-WATER POLLUTION SUSCEPTIBILITY MAP OF GEORGIA 1.0 Identification / Project Background The Georgia Geologic Survey has developed a 1:500,000 scale map(database) which shows the relative susceptibility of the shallow water table aquifer in Georgia to pollution from manmade surface sources. Relative susceptibility was derived by generally following the DRASTIC method developed by the United States Environmental Protection Agency. DRASTIC is a methodology that allows the pollution potential of any hydrogeologic setting to be systematically evaluated, providing a standardized technical basis for environmental decision making using existing data.. Areas within the state of Georgia are classified as having a relatively lower, average, or higher susceptibility to pollution. The pollution susceptibility map was developed using a computer based geographic information system to overlay different natural resource and cultural data bases. The map does not address the pollution susceptibility of an area with man-made modification, nor is the map appropriate for making site specific recommendations. The DRASTIC system involves two major themes: The designation of mappable units, termed hydrogeologic settings, and the superposition of a relative ranking system (see Note 3, Appendix B). The term DRASTIC is an acronym derived from the seven hydrogeologic parameters deemed most influential to pollution susceptibility. They are depth to water(D), net recharge(R), aquifer media(A), soil media(S), topography(T), impact of the vadose zone(I), and hydraulic conductivity(C) of the aquifer. Each factor is incorporated into a relative rating scheme that uses a combination of weights and ratings to produce a numerical value called the DRASTIC Index. The higher an area scores on the index, the more vulnerable or more susceptible the area is believed to be to ground-water pollution. 2.0 Data Quality This section (2.0 - 2.5) contains information about the lineage, positional accuracy, attribute accuracy, logical consistency, and completeness of the database. 2.1 Attribute Accuracy The attributes in this database describe the susceptibility of the surficial aquifer to ground-water pollution in a general sense, relative to other areas of the State of Georgia, and probably should not be considered on an accuracy standard. Rather, think of the attributes in a thematic sense. For example, because of their karstic nature, the areas of limestone outcrop in the Coastal Plain Physiographic Province are rated "high" on the pollution susceptibility scale in comparison to the granitic gneisses of the Piedmont Province, which are rated "low" susceptibility due to their crystalline matrix, with characteristic low permeability. 2.2 Logical Consistency The processor made several quality control checks on the data. First, county based databases were drawn on the computer graphics screen. Individual polygons were selected and the DRASTIC ratings were checked to make sure that ratings for each parameter were assigned correctly according to the attribute code tables. The ratings were manually added to determine if the final score was correct. Also, each of the seven parameters were shaded according to ratings, plotted, and visually inspected for logical consistency (i.e. do the ratings make sense? Are there high susceptibility areas in the middle of low susceptibility areas? If so, why? Are all the limestone units rated as highly susceptible?, do the ratings correspond with what we expect for this region of the state?, etc.) Corrections were made when errors were found, and the data were reprocessed. After it was determined that all seven layers had been coded and processed correctly, the final database was plotted, shading areas of high, average, and low susceptibility. Once again data was visually inspected for logical consistency, and the edges were checked against adjoining counties to make sure that ratings matched across county boundaries. The completed database (hardcopy map) and methodology(ground-water) were reviewed extensively by Dr. Jerry Lineback, Assistant State Geologist of Georgia, Dr. William H. McLemore, State Geologist of Georgia, Tim Hale, District Chief, Water Resources Branch, USGS, Doraville, GA, Bob Faye, Regional Hydrologist, USGS, Doraville, GA, Greg Mayer, Ground-water Specialist, USGS, Doraville, GA, Rick Krause, Assistant District Chief - Ground-water Specialist, USGS, Doraville, GA. In addition to reviewing the rating system and DRASTIC methodology, the hardcopy map was reviewed for logical consistency. In particular Bob Faye, Rick Krause, Greg Mayer, and Dr. McLemore inspected the analysis results in relation to their personal field observations, and expected results in different geologic and physiographic regions of the state. No revisions in methodology or ratings were made as a result of these very helpful reviews. 2.3 Completeness This analysis was completed for the entire state of Georgia. 2.4 Positional Accuracy The 1:500,000 scale map in this atlas is a generalized version of a series of more detailed maps developed for Georgia's eighteen Regional Development Centers (RDC's) and plotted for them at a scale of 1:100,000. It is intended primarily for planning and educational purposes. The pollution susceptibility maps were developed by using a computer based geographic information system (GIS) to overlay different natural resource and demographic data bases. Though published at a scale of 1:500,000, this derivative map is a compilation of data having scales ranging from 1:100,000 to 1:500,000. Because individual map layers are from different sources, a feature from one source may not coincide exactly with the same or related features from another source. Overlay accuracy appears to be about one mile or less in most cases. The one mile limit on accuracy was developed in consultation with the GIS staff of the United States Geological Survey. Areas of less than one square mile are not shown on the 1:500,000 scale map. They are present, however, in the detailed digital datbases. 2.5 Lineage The seven DRASTIC parameters influencing the relative pollution susceptibility of the shallow ground water in Georgia were derived from three existing digital statewide natural resource databases: slope (1:250,000), soils (1:250,000), and geology (1:500,000). All databases underwent quality assurance and quality control procedures before use. For information on the development of these databases, please see Documentation Report 96-3, Documentation Report 96-6, and the STATSGO Data Users Guide - U.S. Dept. of Agriculture Miscellaneous Publication Number 1492. A fourth database, average depth to the water table, was derived from an overlay of the slope and soils data bases. Ratings for aquifer media, impact of the vadose zone media, and hydraulic conductivity of the aquifer were derived from the geology data base. Soil ratings were derived from soil-type descriptions assigned by the United States Soil Conservation Service. Net recharge and topography ratings were derived from the slope data base. All mapping was done utilizing the overlay process of a geographic information system using ARC/INFO software. Development of the seven DRASTIC parameters and numerical values used for mapping the pollution susceptibility of Georgia is described in the following subsections. Depth to Water Depth to the water table was estimated by using an overlay of the slope and soils data bases. Following the DRASTIC methodology, described by Aller and others, (1987), depth to water was assigned a weight of 5. Areas having a slope of greater than 6% were assumed to have a depth to water of 15 feet or more and were assigned the recommended DRASTIC rating of 7 (see Note 4 in Appendix B for discussion of the use of 6% slope as the measure of depth to water). For areas having slopes equal to or less than 6%, depth to water was estimated further by identifying which of such areas had soils associated with a shallow water table and which areas did not. This meant that if an area had a slope of 6% or less and soil associations characterized by "shallow water table", then the depth to water was assumed to be less than or equal to 5 feet, and were assigned the recommended DRASTIC rating of 10. On the other hand, if an area had a slope of 6% or less and a soil association not characterized by a "shallow water table" then the depth to water was assumed to be 5 - 15 feet and was assigned the recommended DRASTIC rating of 9. The ratings, in turn, were multiplied by a weight of five, resulting in DRASTIC numbers of 35, 45, and 50 respectively. Depth to Water DRASTIC DRASTIC Number Rating (weight 5) 0'-5' 10 50 >5'-15' 9 45 >15' 7 35 Recharge The length, shape, and steepness of slope determine the rate of runoff of precipitation. Runoff generally is more rapid on steep slopes than on areas where the ground surface is level or nearly level. In these latter areas, more time is available for precipitation to penetrate into and percolate through the vadose zone down to the water table. Aller and others (1987) estimated a typical net recharge of 4-7 inches for the residual soils overlying crystalline rocks of the Piedmont/Blue Ridge. For the Valley and Ridge and Cumberland Plateau, they reported a typical net recharge of 2-4 inches per year for folded sedimentary rocks of moderate slope (e.g., >6%) and a typical net recharge of 10+ inches per year for low slope valley limestones. All of Aller and others (1987) shallow slope (less than 6%) Coastal Plain settings had a net recharge of 10+ inches. Based upon Aller and others (1987) frequent use of the 6% slope to differentiate areas of higher and lesser recharge as well as their frequent use of 10+ inches per year as a measure of higher and lower net recharge, those portions of Georgia having a slope of less than or equal to 6% were assumed to have a net recharge of 10+ inches per year and were assigned the recommended DRASTIC rating of 9. Areas having a slope of greater than 6% were assumed to have a net recharge of less than 10 inches per year and were assigned a net recharge rating of 8. These ratings when multiplied by the recommended DRASTIC weight of 4, result in DRASTIC numbers of 36 and 32 respectively, for net recharge. Actual net recharge in Georgia probably averages about 6 inches per year (Carter and Stiles, 1983). This means that by following Aller and others (1987) recommendations, actual net recharge is slightly overestimated presenting a slight overestimation of actual pollution susceptibility. Aquifer Media, Impact of the Vadose Zone, and Hydraulic Conductivity In the DRASTIC methodology, lithologic outcroppings are used as surrogates for aquifers. This means that any standard geologic map which depicts the distribution of lithologic units can be used as an "aquifer media" map. According to Aller and others (1987), the vadose zone encompasses those materials between the base of the soil profile and the water table. For unconfined aquifer systems, these would be the same as the "aquifer media". In other words, the DRASTIC methodology allows any standard geologic map, which depicts the distribution of lithologic units, to be used as a measure of the impact of the vadose zone. Ranges of hydraulic conductivity have been established for a number of lithologic units (Freeze and Cherry, 1979). Thus, by knowing the lithology of the aquifer media, an estimate can be made of hydraulic conductivity. All geologic units mapped at a scale of 1:500,000 on the Geologic Map of Georgia (1976) and the limestone units mapped on the Tertiary and Quaternary Formations of Georgia (1948) map were assigned a numerical rating for aquifer media, impact of the vadose zone, and hydraulic conductivity of the shallow aquifer. The DRASTIC ratings ranged from 1-10, according to the pollution potential of the lithologic unit. Following the DRASTIC methodology, ratings assigned to aquifer media and to the hydraulic conductivity of the aquifer were assigned a weight of 3, and the impact of the vadose zone media ratings were assigned a weight of 5. The numerical rating was multiplied by the weight factor to obtain a DRASTIC number for the particular lithologic unit, as follows: Aquifer Impact of Geologic DRASTIC Media Vadose Zone Unit Rating (weight 3) (weight 5) Metamorphic/igneous rock 2 6 10 Weathered metamorphic/igneous 3 9 15 Bedded sedimentary 5 15 25 Massive sandstone 6 18 30 Sand/gravel 8 24 40 Karstic limestone 10 30 50 Geologic Unit Hydraulic DRASTIC Conductivity Number (gpd/ft2) (weight 3) bedded sedimentary 1 - 100 3 metamorphic/igneous 101 - 300 6 weathered metamorphic/igneous 301 - 700 9 massive sandstone 701 - 1000 18 sand/gravel 1001 - 2000 24 karstic limestone 2000+ 30 Soils Soil media DRASTIC ratings ranging from 1 to 10 were assigned to the soils data base according to soil types. The soils parameter was assigned a weight of 2 in conformity to DRASTIC methodology. The ratings were multiplied by the weight of 2, resulting in a DRASTIC soil index of 2 - 20, as follows: Soil Media DRASTIC Soil Number Rating (weight 2) Thin or absent 10 20 Gravel 10 20 Sand 9 18 Peat 8 16 Shrinking Aggregated Clay 7 14 Sandy loam 6 12 Loam 5 10 Silty loam 4 8 Clay loam 3 6 Muck 2 4 Nonshrinking/Nonaggregated clay 1 2 Topography As used in DRASTIC, "topography" means slope. Areas with a slope of less than or equal to 6% were assigned a topography DRASTIC rating of 10 (see Note 5, Appendix B for use of 6% slope for assessing topography). Areas with a slope of greater than 6% were assigned a topography DRASTIC rating of 5. These ratings were multiplied by the weight of 5, resulting in DRASTIC numbers of 10 and 5, respectively, for topography. Interpretation Once the DRASTIC ratings were assigned for the seven parameters, the pollution susceptibility for each hydrogeologic setting was estimated by calculating the DRASTIC Index. The equation for calculating the DRASTIC Index is: DrDw + RrRw + ArAw + SrSw + TrTw + IrIw + CrCw = DRASTIC Index where: r = rating w = weight. Following the calculation of the DRASTIC Index for individual hydrogeologic settings, the range of index scores in Georgia was divided into three parts (see Note 1, Appendix B). Those areas having a DRASTIC Index of less than 141 were considered to have a relatively low pollution susceptibility for Georgia and were mapped accordingly. Areas having an index between 141 and 181 were considered to have average pollution susceptibility and hydrogeologic settings with DRASTIC index values greater than 181 were considered to have relatively high pollution susceptibility. The subdivision of the range of index values found in Georgia into three pollution susceptibility categories is relative, and for that reason, is meaningful only within the state of Georgia. In other words, a hydrogeologic setting with a DRASTIC Index of 135 might be considered to have relatively low pollution susceptibility in Georgia, but might be one of the highest values measured in another state. Alternatively, a setting with an index value of 182 is considered relatively susceptible to pollution in Georgia, but 182 may be considered a low or medium value in another state where pollution susceptible hydrogeologic settings are widespread. 3.0 Spatial Data Organization Information This database defines areas throughout the state according to the relative susceptibility of the surficial aquifer to ground-water pollution. The areas are represented digitally as polygons, each with a label of "high", "average", or "low", accordingly. Areas covered by water (lakes, ponds, etc.) have a label of "water". 4.0 Spatial Reference Information This database is stored in the Albers Conic Equal Area projection with units in meters, to make the data conform with the GIS standards of the Georgia Geologic Survey and the U.S. Geological Survey. Further information on the specific parameters used can be found in Part B. 5.0 Entity and Attribute Information Each polygon in the database has one attribute describing the relative susceptibility of the surficial aquifer to ground-water pollution. The possible values for this attribute are "high", "average", "low", or "water". 6.0 Distribution Information This database is available for distribution from the Georgia Geologic Survey, referenced as GIS-1, Ground-water Pollution Susceptibility Map of Georgia. 7.0 Metadata Reference Information The metadata is incorporated within this publication, DOCUMENTATION REPORT 96-?. Part B, the Technical Section of this publication, meets the "Content Standards for Digital Geospatial Metadata" as defined by the Federal Geographic Data Committee. APPENDIX A - SIGNATURES [Image] PART B - TECHNICAL DESCRIPTION OF THE DATABASE: GROUND-WATER POLLUTION SUSCEPTIBILITY MAP OF GEORGIA 1.0 Identification Information 1.1 Citation 8.1 Originator: Georgia Geologic Survey 8.2 Publication Date: 1992 8.4 Title: DOCUMENTATION REPORT 96-? Ground-water Pollution Susceptibility Map of Georgia 8.5 Edition: Version 1 8.6 Geospatial Data Presentation Form: GIS Database 8.7 Series Name: Ground-water Pollution Susceptibility Map of Georgia 8.8.1 Publication Place: Atlanta, Georgia 8.8.2 Publisher: Georgia Geologic Survey 8.10 Online Linkage: Not Available 1.2 Description 1.21 Abstract: The Georgia Geologic Survey has developed a 1:500,000 scale map (GIS database) which shows the relative susceptibility of the shallow water table aquifer in Georgia to pollution from manmade surface sources. Relative susceptibility was derived by generally following the DRASTIC method developed by the United States Environmental Protection Agency. DRASTIC is a methodology that allows the pollution potential of any hydrogeologic setting to be systematically evaluated, providing a standardized technical basis for environmental decision making using existing data.. Areas within the state of Georgia are classified as having a relatively lower, average, or higher susceptibility to pollution. The pollution susceptibility map was developed using a computer based geographic information system to overlay different natural resource and cultural data bases. The DRASTIC system involves two major themes: The designation of mappable units, termed hydrogeologic settings, and the superposition of a relative ranking system (see Note 3, Appendix B ). The term DRASTIC is an acronym derived from the seven hydrogeologic parameters deemed most influential to pollution susceptibility. They are depth to water(D), net recharge(R), aquifer media(A), soil media(S), topography(T), impact of the vadose zone(I), and hydraulic conductivity(C) of the aquifer. Each factor is incorporated into a relative rating scheme that uses a combination of weights and ratings to produce a numerical value called the DRASTIC Index. The higher an area scores on the index, the more vulnerable or more susceptible the area is believed to be to ground-water pollution 1.2.2 Purpose: This GIS database was developed for the Environmental Protection Division for use in analysis to support EPD's environmental protection programs. 1.3 Time Period of Content 9.1.1 Calendar Date: 1992 1.3.1 Currentness Reference: Trent, Victoria P., 1992. Ground-water Pollution Susceptibility Map of Georgia. Georgia Geologic Survey Hydrologic Atlas 20, one plate. Scale 1:500,000. 1.4 Status 1.4.1 Progress: Complete 1.4.2 Update Frequency: none planned 1.5 Spatial Domain 1.5.1 Bounding Coordinates 1.5.1.2 West Bounding Coordinate: 85 45 00 1.5.1.3 East Bounding Coordinate: 81 00 00 1.5.1.3 North Bounding Coordinate: -35 00 00 1.5.1.4 South Bounding Coordinate: -30 20 00 1.6 Keywords 1.6.1 Theme 1.6.1.1 Theme Keyword Reference: none 1.6.1.2a Theme Keyword: ground-water 1.6.1.2b Theme Keyword: pollution susceptibility 1.6.1.2c Theme Keyword: environmental protection 1.6.1.2d Theme Keyword: DRASTIC 1.6.2 Place 1.6.2.2a Place Keyword: GA 1.6.2.2b Place Keyword: Georgia 1.6.2.2c Place Keyword: USA 1.7 Access Constraints: Distribution constraints, see 1.9 1.8 Use Constraints: Scale 1:500,000 1.9 Point of Contact 10.1.1 Contact Person: Alan Giles 10.1.2 Contact Organization: Georgia Geologic Survey 10.3 Contact Position: Information Geologist 10.4. Contact Address 10.4.2a Address: Agricultural Building, Rm 400 10.4.2b Street: 19 Martin Luther King, Jr Boulevard 10.4.3 City: Atlanta 10.4.4 State: Georgia 10.4.5 Postal Code: 30334 10.4.6 Country: USA 10.5 Contact Voice Telephone: (404) 657-6127 10.7 Contact Facsimile Telephone: (404) 657-9425 10.8 Contact Electronic Mail Address: alan_giles@mail.dnr.state.ga.us 10.9 Hours of Service: 8:00 a.m. - 4:30 p.m. EST 2.0 Data Quality Information 2.1 Attribute Accuracy 2.1.1 Attribute Accuracy Report: The attributes in this database describe the susceptibility of the surficial aquifer to ground-water pollution in a general sense, relative to other areas of the State of Georgia, and probably should not be considered on an accuracy standard. Rather, think of the attributes in a thematic sense. For example, because of their karstic nature the areas of limestone outcrop in the Coastal Plain Physiographic Province are rated "high" on the pollution susceptibility scale in comparison to the granitic gneisses of the Piedmont Province, which are rated "low" susceptibility, due to their crystalline matrix and characteristic low permeability. 2.2 Logical Consistency Report The processor made several quality control checks on the data. First, county based databases were drawn on the computer graphics screen. Individual polygons were selected and the DRASTIC ratings were checked to make sure that ratings for each parameter were assigned correctly according to the attribute code tables. The ratings were manually added to determine if the final score was correct. Also, each of the seven parameters were shaded according to ratings, plotted, and visually inspected for logical consistency (i.e. do the ratings make sense? Are there high susceptibility areas in the middle of low susceptibility areas? If so, why? Are all the limestone units rated as highly susceptible?, do the ratings make sense for what we expect in this region of the state, etc.) Corrections were made when errors were found, and the data were reprocessed. After it was determined that all seven layers had been coded and processed correctly, the final database was plotted, shading areas of high, average, and low susceptibility. Once again data was visually inspected for logical consistency, and the edges were checked against adjoining counties to make sure that ratings matched across county boundaries. The completed database (hardcopy map) and methodology (ground-water) were reviewed extensively by Dr. Jerry Lineback, Assistant State Geologist of Georgia, Dr. William H. McLemore, State Geologist of Georgia, Tim Hale, District Chief, Water Resources Branch, USGS, Doraville, GA, Bob Faye, Regional Hydrologist, USGS, Doraville, GA, Greg Mayer, Ground-water Specialist, USGS, Doraville, GA, Rick Krause, Assistant District Chief - Ground-water Specialist, USGS, Doraville, GA. In addition to reviewing the rating system and DRASTIC methodology, the hardcopy map was reviewed for logical consistency. In particular Bob Faye, Rick Krause, Greg Mayer, and Dr. McLemore inspected the analysis results in relation to their personal field observations, and expected results in different geologic and physiographic regions of the state. No revisions in methodology or ratings were made as a result of these very helpful reviews. 2.3 Completeness Report: This analysis was done for the entire state of Georgia. 2.4 Positional Accuracy: The 1:500,000 scale map in this atlas is a generalized version of a series of more detailed maps developed for Georgia's eighteen Regional Development Centers (RDC's) and plotted at a scale of 1:100,000 for them. It is intended primarily for planning and educational purposes. The pollution susceptibility maps were developed by using a computer based geographic information system (GIS) to overlay different natural resource and demographic databases. Though published at a scale of 1:500,000, this derivative map is a compilation of data having scales ranging from 1:250,000 to 1:500,000. Because individual map layers are from different sources, a feature from one source may not coincide exactly with the same or related features from another source. Overlay accuracy appears to be about one mile or less in most cases. The one mile limit on accuracy was developed in consultation with the GIS staff of the United States Geological Survey. Areas of less than one square mile are not shown on the 1:500,000 scale map. They are present, however, in the more detailed digital databases. 2.5 Lineage 2.5.1a Source Information 2.5.1.1 Source Citation: Georgia Geologic Survey 2.5.1.2 Source Scale Denominator: 500,000 2.5.1.3 Type of Source Media: derivative database 2.5.1.5 Source Citation Abbreviation: GGS 2.5.1.6 Source Contribution: GGS performed data processing into Arc/Info of the data , documented the data to GGS standards, and published the data 2.5.2a Process Information: Procedures used to create or automate data: ( For a discussion of DRASTIC and DRASTIC methodology, and a discussion of ratings, please see Appendix B. This section discusses only GIS methodology utilized to create the database. ================================================================== Because of severe disk space limitations, the DRASTIC database was developed on a county by county basis. Before processing began the statewide databases geology, STATSGO soils, and slope were clipped by county. The following aml was used to automate: (Note: all aml's in this description input a file listing county names.) &ARGS FILE &ECHO &ON COMO CLIP.COMO &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] CLIP GEO>GEO500J JACK2>TEMPLATES>%COUNTY% ~ COUNTY>%COUNTY%>GEO POLY &END &ECHO &OFF COMO -END =================================================================== Three statewide databases, geology, slope, and soils, were used to derive the ratings for the seven DRASTIC parameters. Once ratings for all seven DRASTIC parameters had been agreed upon, the ratings had to be assigned to the databases. (Please see Appendix B for tables showing the ratings.) ================================================================== The geology database was used to develop ratings for aquifer media, impact of the vadose zone media, and hydraulic conductivity of the aquifer. Ratings were assigned to the lithologic unit using the following aml: &ARGS FILE COMO DRASTICGEO.COMO &SEVERITY &ERROR &IGNORE &WATCH DRASTICGEO.WATCH &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] &SYS A COUNTY>%COUNTY% ADDITEM GEO.PAT GEO.PAT DRASTIC-AQ 3 3 I ADDITEM GEO.PAT GEO.PAT DRASTIC-VADOSE 3 3 I ADDITEM GEO.PAT GEO.PAT DRASTIC-HC 3 3 I AE MAPE GEO EDITC GEO EDITF LABEL SELECT ALL /* RESELECT ACODE2 = 'Qs' OR ACODE2 = 'Mh' OR ACODE2 = 'Nm' OR ~ ACODE2 = 'Nu' OR ACODE2 = 'Pcd' OR ACODE2 = 'Ei' OR ACODE2 = ~ 'Ecm' OR ACODE2 = 'Es' OR ACODE2 = 'Etw' OR ACODE2 = 'Ecl' OR ~ ACODE2 = 'Kb' /* CALC DRASTIC-AQ = 15 CALC DRASTIC-VADOSE = 25 CALC DRASTIC-HC = 18 NSELECT /* RESELECT ACODE2 = 'Pu' OR ACODE2 = 'Pls' OR ACODE2 = 'Plg' OR ~ ACODE2 = 'Mh' OR ACODE2 = 'Mfs' OR ACODE2 = 'Mls' OR ACODE2 ~ = 'DMu' OR ACODE2 = 'Dfm' OR ACODE2 = 'Da' or ACODE2 = 'Srm' ~ OR ACODE2 = 'Omb' OR ACODE2 = 'Oa' OR ACODE2 = 'Or' OR ACODE2 ~ = 'Cc' OR ACODE2 = 'Ccs' OR ACODE2 = 'Cr' OR ACODE2 = 'Cch' /* CALC DRASTIC-AQ = 6 CALC DRASTIC-VADOSE = 10 CALC DRASTIC-HC = 6 NSELECT /* RESELECT DRASTIC-AQ = 0 /* CALC DRASTIC-AQ = 9 CALC DRASTIC-VADOSE = 15 CALC DRASTIC-HC = 9 /* SAVE GEO /* Q /* BUILD COUNTY>%COUNTY%>GEO POLY &END &WATCH &OFF COMO -END =================================================================== The slope database was used to develop ratings for topography and net recharge. The following aml was used to assign ratings: &ARGS FILE &echo &on &SEVERITY &ERROR &IGNORE &WATCH drasticslope.WATCH &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] ADDITEM /ARC3/EXPORT/SLOPE/%COUNTY%.PAT /ARC3/EXPORT/SLOPE/%COUNTY%.PAT DRASTIC- ADDITEM /ARC3/EXPORT/SLOPE/%COUNTY%.PAT /ARC3/EXPORT/SLOPE/%COUNTY%.PAT DRASTIC- AE MAPE /ARC3/EXPORT/SLOPE/%COUNTY% EDITC /ARC3/EXPORT/SLOPE/%COUNTY% EDITF LABEL SELECT ALL /* RESELECT GRID-CODE LE 6 CALC DRASTIC-TOPO = 10 CALC DRASTIC-RECH = 36 NSELECT /* RESELECT GRID-CODE GT 6 CALC DRASTIC-TOPO = 5 CALC DRASTIC-RECH = 32 /* SAVE /ARC3/EXPORT/SLOPE/%COUNTY% /* Q /* BUILD COUNTY>%COUNTY%>/ARC3/EXPORT/SLOPE/%COUNTY% POLY &END &echo &off &WATCH &OFF ===================================================================== The STATSGO soils database was used to develop ratings for soil. The following aml was used to assign ratings: &ARGS FILE COMO DRASTICSOIL.COMO &SEVERITY &ERROR &IGNORE &WATCH SOIL.WATCH &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] &SYS A COUNTY>%COUNTY% ADDITEM STATSGO.PAT STATSGO.PAT DRASTIC-SOIL 4 5 N 1 AE MAPE STATSGO EDITC STATSGO EDITF LABEL SELECT ALL RELATE RESTORE VICKI!SOIL.EXP /* CALC DRASTIC-SOIL = SOIL//SOIL-RATING * 2 /* SAVE STATSGO /* Q /* BUILD COUNTY>%COUNTY%>STATSGO POLY &END &WATCH &OFF COMO -END &RETURN ================================================================== A depth-to-water database was not available for the entire state of Georgia, so one was developed by overlaying the STATSGO soils database with the slope database. The following aml was used to automate processing: &ARGS FILE &echo &on &WATCH UNION.WATCH &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] UNION /ARC1/EXPORT/STATSGO/%COUNTY% /ARC3/EXPORT/SLOPE/%COUNTY% ~ /ARC3/EXPORT/SLOPE-SOIL/%COUNTY% 1 JOIN &END &echo &off &watch &off =================================================================== The resulting composite coverage was used to develop a statewide depth-to-water database. The following aml was used to assign ratings: &ARGS FILE &echo &on &watch DRASTICDW.watch &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] ADDITEM %county%.PAT %county%.PAT DRASTIC-DW 3 3 I AE MAPE %county% EDITC %county% EDITF LABEL SELECT ALL RELATE RESTORE /arc4/VICKI!SOIL.EXP RESELECT SOIL//WTDEPL LE 5 CALC DRASTIC-DW = 50 NSELECT /* RESELECT SOIL//WTDEPL GT 5 AND SOIL//WTDEPL LE 15 CALC DRASTIC-DW = 45 NSELECT /* SELECT ALL /* RES GRID-CODE GT 6 CALC DRASTIC-DW = 35 NSELECT /* SAVE %county% /* Q /* BUILD %COUNTY% POLY &END &echo &off &WATCH &OFF ================================================================= Once ratings had been assigned to all databases for all seven parameters, the 4 databases were overlaid using the UNION command. The following aml was used to automate processing. &ARGS FILE &echo &on &WATCH UNION.WATCH &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] UNION /ARC1/EXPORT/STATSGO/%COUNTY% /ARC3/EXPORT/SLOPE/%COUNTY% ~ /ARC3/EXPORT/DRASTIC/%COUNTY%1 1 JOIN UNION /ARC1/EXPORT/DRASTIC/%COUNTY%1 /ARC3/EXPORT/GEOLOGY/%COUNTY% ~ /ARC3/EXPORT/DRASTIC/%COUNTY%2 1 JOIN UNION /ARC1/EXPORT/DRASTIC/%COUNTY%2 /ARC3/EXPORT/SLOPE-SOIL/%COUNTY% ~ /ARC3/EXPORT/DRASTIC/%COUNTY%3 1 JOIN &END &echo &off &watch &off ================================================================= Once the derivative database was developed using the UNION command, the final DRASTIC score had to be calculated for each polygon. The following aml was used to automate processing: &ARGS FILE &echo &on &watch CALCDRASTIC.watch &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] ADDITEM %county%3.PAT %county%3.PAT DRASTIC 6 7 N 1 AE MAPE %county%3 EDITC %county%3 EDITF LABEL SELECT ALL /* CALC DRASTIC = DRASTIC-AQ + DRASTIC-HC + DRASTIC-VADOSE + ~ DRASTIC-SOIL + DRASTIC-TOPO + DRASTIC-RECH + DRASTIC-DW /* SAVE %county%3 /* Q /* BUILD %COUNTY%3 POLY &END &echo &off &WATCH &off &RETURN ================================================================== Following the calculation of the DRASTIC Index for individual hydrogeologic settings, the range of index scores in Georgia was divided into three parts (see Note 1, Appendix B). Those areas having a DRASTIC Index of less than 141 were considered to have a relatively low pollution susceptibility for Georgia and were mapped accordingly. Areas having an index between 141 and 181 were considered to have average pollution susceptibility and hydrogeologic settings with DRASTIC index values greater than 181 were considered to have relatively high pollution susceptibility. The following aml was used to automate processing: &ARGS FILE &echo &on &watch CALCDRASTIC.watch &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] ADDITEM %county%3.PAT %county%3.PAT SUSCEP 7 7 C AE MAPE %county%3 EDITC %county%3 EDITF LABEL SELECT ALL /* RES DRASTIC LT 141 CHANGE SUSCEP R LOW NSEL RES DRASTIC GT 181 CHANGE SUSCEP R HIGH NSEL RES DRASTIC GE 141 AND DRASTIC LE 181 CHANGE SUSCEP R AVERAGE RES DRASTIC = 888 CHANGE SUSCEP R WATER /* SAVE %county%3 /* Q /* BUILD %COUNTY%3 POLY &END &echo &off &WATCH &off &RETURN ================================================================= In order to increase available disk space, the derivative coverages were reduced to primary elements of three categories: high, average, or low susceptibility by eliminating adjacent polygons having the same susceptibility rating using the DISSOLVE command. The following aml was used to automate: &ARGS FILE COMO DISSOLVE.COMO &SEVERITY &ERROR &IGNORE &S FILE := [OPEN %FILE% STATUS -R] &S COUNTY := [READ %FILE% READSTAT] &DO &WHILE %READSTAT% NE 102 &S COUNTY := [READ %FILE% READSTAT] DISSOLVE COUNTY>%COUNTY%>DRASTIC3 COUNTY>%COUNTY%>DRASTIC3.DIS SUSCEP POLY &END COMO -END ================================================================== County based coverages were joined into a statewide coverage using the MAPJOIN command: MAPJOIN DRASTIC4 (list of county-based coverages) ================================================================== For publication purposes, areas less than 1 sq. mile were eliminated using the ELIMINATE command: ELIMINATE DRASTIC4 DRASTIC5 KEEPEDGE POLY RES AREA LT 2589.988 ================================================================= DATABASES USED Elassal, A. and Carvso, V. 1984. Digital Elevation Models. U.S. Geological Survey Circular 895-B, 40 pp. _____. Geologic Map of Georgia, 1976, Georgia Geologic Survey, one sheet. Scale 1:500,000. _____. Geologic Map of the Tertiary and Quaternary Formations of Georgia, 1948, Georgia Geologic Survey, one sheet. Scale 1:250,000. _____. Major Highways and Roads, 1974, National Cartographic Information Center. Scale 1:2,000,000. _____. Slope Map of Georgia Developed from Analysis of TIN Processed Digital Elevation Model Data (60 meter intervals). U.S. Geological Survey. Scale 1:250,000. _____. Soils Map of Georgia Developed from Soil Conservation Service County Soils Series Maps (STATSGO). U.S. Department of Agriculture Soil Conservation Service. Scale 1:250,000. _____. Rivers and Lakes, State of Georgia Hydrology Base Map. U.S. Geological Survey. Scale 1:500,000. 3.0 Spatial Data Organization Information: 3.1 Indirect Spatial Reference Method: dms 3.2 Direct Spatial Reference Method: polygon 3.3 Point and Vector Object Information 3.3.1 SDTS Terms Description: 3.3.1.1 Object Type: polygon 3.3.1.2 Object Count: 2239 3.3.1.1 Object Type: Universe Polygon 3.3.1.2 Object Count: 1 4.0 Spatial Reference Information: 4.1 Horizontal Coordinate System Definition: Albers Conic Equal Area 4.1.2.1.2 Map Projection Parameters 4.1.2.1.2.1a Standard Parallel 29 30 00 4.1.2.1.2.1b Standard Parallel 45 30 00 4.1.2.1.2.2 Longitude of Central Meridian -83 30 00 4.1.2.1.2.3 Latitude of Projection Origin 23 00 00 4.1.2.1.2.4 False Easting 0 4.1.2.1.2.5 False Northing 0 4.1.2.4.1 Planar Coordinate Encoding Method coordinate pair 4.1.2.4.2 Coordinate Representation 4.1.2.4.2.1 Abscissa Resolution .1 4.1.2.4.2.2 Ordinate Resolution .1 4.1.2.4.2.4Planar Distance Units meters 4.1.4 Geodetic Model 4.1.4.1 Horizontal Datum Name: North American Datum of 1983 (NAD83) 4.1.4.2Ellipsoid Name: GRS 80 4.1.4.3 Semi-major Axis: 6378206.4 4.1.4.4 Denominator of Flattening Ratio: 294.98 5.0 Entity and Attribute Information: 5.1 Detailed Description 5.1.1 Entity Type 5.1.1.1 Entity Type Label: drastic2.pat 5.1.1.2 Entity Type Definition: Polygon attribute table 5.1.2 Attribute 5.1.2.1 Attribute Label suscep 5.1.2.2 Attribute Definition: describes the relative susceptibility of the surficial aquifer to ground-water pollution. 5.1.2.4 Attribute Domain Values: 7 Character alphanumeric high, average, low, water 6.0 Distribution Information: 6.1 Distributor: See Point of Contact, 1.9 6.2 Resource Description: DOCUMENTATION REPORT 96-? 6.3 Distribution Liability: Users must assume responsibility to evaluate the usability of this data for their purposes. 6.4 Standard Ordering Process: Contact the Georgia Geologic Survey, see 1.9 6.5 Custom Order Process: Contact the Georgia Geologic Survey, see 1.9 7.0 Metadata Reference Information: 7.1 Metadata Date: 1992 7.2 Metadata Review Date: 12/20//95, by Alan Sandercock, GGS 7.2 Metadata Review Date: 01/25/96 by Roger Carter, GGS 7.4 Metadata Contact: 10.1.1 Contact Person: Victoria Trent 10.1.2 Contact Organization: Georgia Geologic Survey 10.3 Contact Position: Senior Geologist 10.4 Contact Address 10.4.2 Address: 19 Martin Luther King, Jr. Dr. SW 10.4.3 City: Atlanta 10.4.4 State: Georgia 10.4.5 Postal Code: 30334 10.4.6 Country: USA 10.5 Contact Voice Telephone: (404) 656-3214 10.7 Contact Facsimile Telephone: (404) 657-8379 10.8 Contact Electronic Mail Address: vptrent@mail.dnr.state.ga.us 10.9 Hours of Service: 8:00 a.m. - 4:30 p.m. EST 7.5 Metadata Standard Name: FGDC Content Standards for Digital Geospatial Metadata 7.6 Metadata Standard Version: 6/8/94 APPENDIX B APPENDIX B. DRASTIC MAPPING TO DETERMINE THE VULNERABILITY OF GROUND WATER TO POLLUTION Victoria P. Trent, Georgia Geologic Survey, 19 MLK Jr. Dr. SW, Room 400, Atlanta, GA 30334. ABSTRACT: The Georgia Geologic Survey has developed a 1:500,000 scale map which shows the relative susceptibility of the shallow water table aquifer in Georgia to pollution from manmade surface sources. Relative susceptibility was derived by generally following the DRASTIC method developed by the United States Environmental Protection Agency. DRASTIC is a methodology that allows the pollution potential of any hydrogeologic setting to be systematically evaluated. Areas within the state of Georgia are classified as having a relatively lower, average, or higher susceptibility to pollution. DRASTIC mapping is intended to provide a standardized technical basis for environmental decision making using existing data. The pollution susceptibility map was developed using a computer based geographic information system to overlay different natural resource and demographic data bases. KEY WORDS: ground water, DRASTIC, pollution susceptibility, environmental planning, Georgia INTRODUCTION The Georgia Geologic Survey has developed a 1:500,000 scale map which shows the relative susceptibility of the shallow water table aquifer in Georgia to pollution from the man-made surface sources. Relative susceptibility was derived by generally following the DRASTIC method developed by the United States Environmental Protection Agency (EPA) (Aller and others, 1987). Areas within the state of Georgia are classified as having a relatively lower, average, or higher susceptibility to pollution (see Note No. 1). All of Georgia, in some form or fashion, is susceptible to ground-water pollution. Some areas, however, are more susceptible than other areas. DRASTIC mapping demonstrates that the Fall Line Hills region of the Upper Coastal Plain, the Dougherty Plain and the limestone valleys of northwest Georgia are the general areas in Georgia that are most susceptible to ground-water pollution. The massive crystalline rocks, with their overlying clayey residuum, of the Piedmont/Blue Ridge region are the least susceptible to pollution. There have been a number of attempts to develop systems for assessing the susceptibility of shallow unconfined ground water to pollution. The most well known is DRASTIC, which is a qualitative guideline system for ground-water vulnerability assessments (LeGrand and Rosen, in press). According to LeGrand and Rosen: "Drastic has been found to have some statistical and descriptive properties that are very advantageous, but a problem is that to many users it is not clear what the final product, or DRASTIC-index, really means. Another problem is that such assessments tend to be regarded as monumental simplistic truths, which DRASTIC indices are certainly not, since they by necessity are associated with several kinds of uncertainties, due to system build up and geological heterogeneity's. DRASTIC has been widely used by a number of states in the USA for planning purposes. We believe that the main reason for this is not that DRASTIC necessarily is that excellent a system, but that there is really no other standard technique available for vulnerability mapping in the United States." The 1:500,000 scale map in this atlas is a generalized version of a series of 1:100,000 scale maps developed for Georgia's eighteen Regional Development Centers (RDC's) and is intended primarily for planning and educational purposes. The pollution susceptibility maps were developed by using a computer based geographic information system (GIS) to overlay different natural resource and demographic data bases. Though published at a scale of 1:500,000, this derivative map is a compilation of data having scales ranging from 1:100,000 to 1:500,000. Because individual map layers are from different sources, a feature from one source may not coincide exactly with the same or related features from another source. Overlay accuracy appears to be about one mile or less in most cases (see Note 2). THE USE OF EPA'S "DRASTIC" METHODOLOGY FOR ENVIRONMENTAL MAPPING DRASTIC mapping is intended to provide standardized technical basis for environmental decision making using existing data. This map will assist planners, managers and administrators in evaluating the relative vulnerability of shallow aquifers in Georgia to ground-water pollution. The map does not address the pollution susceptibility of an area with man-made modification, nor is the map appropriate for making site specific recommendations. DRASTIC is a methodology that allows the pollution potential of any hydrogeologic setting to be systematically evaluated with existing information anywhere in the United States (Aller and others, 1987). The system involves two major themes: The designation of mappable units, termed hydrogeologic settings, and the superposition of a relative ranking system (see Note 3). The term DRASTIC is an acronym derived from the seven hydrogeologic parameters deemed most influential to pollution susceptibility. They are depth to water(D), net recharge(R), aquifer media(A), soil media(S), topography(T), impact of the vadose zone(I), and hydraulic conductivity(C) of the aquifer. Each factor is incorporated into a relative rating scheme that uses a combination of weights and ratings to produce a numerical value called the DRASTIC Index. The higher an area scores on the index, the more vulnerable or more susceptible the area is believed to be to ground-water pollution. However, as Aller and others (1987) point out, DRASTIC is neither designed for, nor is it intended to replace on-site inspections, or site specific hydrogeologic investigations. DRASTIC mapping alone should not be used to specifically site any type of facility or practice. Rather, DRASTIC is intended to provide a basis for comparative evaluation of areas with respect to the potential for pollution of ground water. DESCRIPTION OF DRASTIC PARAMETERS (from Aller and others, 1987) Depth to water in an unconfined aquifer is defined as the depth from ground surface to the water table. Depth to water is important primarily because it determines the depth of earth materials through which a pollutant must travel before reaching the aquifer. Depth to water also is important because the longer the travel time for a pollutant to reach the water table, the greater the opportunity for oxidation of the pollutant or sorption onto soil particles. Net recharge is defined as the total amount of water precipitated to the ground surface which infiltrates to reach the aquifer. Recharge is important because it can transport a pollutant vertically into the water table and horizontally within the aquifer. Recharge is also an important factor for dispersion and dilution of a pollutant in the vadose zone. Aquifer media refers to the earth materials which serve as an aquifer. An aquifer is defined as a subsurface lithologic unit which will yield sufficient quantities of water for use. Ground water in aquifers is contained within the pore spaces, fractures, and solution cavities of the rock. The aquifer referred to in this atlas is the unconfined surficial aquifer, where the water table represents the uppermost elevation where the earth materials are filled (saturated) with water. The type of aquifer media affects the ground-water flow system, which in turn affects the amount of time for attenuation of pollutants. Soil media refers to the uppermost portion of the vadose zone characterized by significant biological activity. For purposes of this study, soil is considered the upper weathered zone of the earth which averages a depth of six feet or less from the ground surface. Soil has a significant impact on the amount of recharge which can infiltrate to the ground and hence the ability of the pollutant to move vertically into the vadose zone. Topography refers to the slope and slope variability of the land surface. Topography helps control the likelihood that a pollutant will run off over the surface or will remain on the surface in one area long enough to infiltrate. Topography also influences soil development and indicator of ground-water flow direction. The vadose zone is defined as that zone above the water table which is unsaturated or discontinuously saturated. The type of vadose zone media determines the attenuation characteristics of pollutants below the soil horizon and above the water table. Hydraulic conductivity refers to the ability of aquifer materials to transmit water, which in turn controls the rate at which ground water will flow under a given hydraulic gradient. The rate at which ground water flows controls the rate at which a pollutant moves away from the point at which it entered the aquifer. METHODS Overview The seven DRASTIC parameters influencing the relative pollution susceptibility of the shallow ground water in Georgia were derived from three existing statewide natural resource databases: slope (1:250,000), soils (1:250,000), and geology (1:500,000). All databases underwent quality assurance and quality control procedures before use. A fourth database, average depth to the water table, was derived from the slope and soils data bases. Ratings for aquifer media, impact of the vadose zone media, and hydraulic conductivity of the aquifer were derived from the geology data base. Soil ratings were derived from soil-type descriptions assigned by the United States Soil Conservation Service. Net recharge and topography ratings were derived from the slope data base. All mapping was done utilizing the overlay process of a geographic information system using ARC/INFO software. Development of the seven DRASTIC parameters and numerical values used for mapping the pollution susceptibility of Georgia is described in the following subsections. Depth to Water Depth to the water table was estimated by using a combination of the slope and soils data bases. Following the DRASTIC methodology, described by Aller and others, (1987), depth to water was assigned a weight of 5. Areas having a slope of greater than 6% were assumed to have a depth to water of 15 feet or more and were assigned the recommended DRASTIC rating of 7 (see Note 4 for discussion of the use of 6% slope as the measure of depth to water). For areas having slopes equal to or less than 6%, depth to water was estimated further by identifying which of such areas had soils associated with a shallow water table and which areas did not. This meant that if an area had a slope of 6% or less and soil associations characterized by "shallow water table", then the depth to water was assumed to be less than or equal to 5 feet, and were assigned the recommended DRASTIC rating of 10. On the other hand, if an area had a slope of 6% or less and a soil association not characterized by a "shallow water table" then the depth to water was assumed to be 5 - 15 feet and was assigned the recommended DRASTIC rating of 9. The ratings, in turn, were multiplied by a weight of five, resulting in DRASTIC numbers of 35, 45, and 50 respectively. Depth to Water DRASTIC DRASTIC Number Rating (weight 5) 0'-5' 10 50 >5'-15' 9 45 >15' 7 35 Recharge The length, shape, and steepness of slope determine the rate of runoff of precipitation. Runoff generally is more rapid on steep slopes than on areas where the ground surface is level or nearly level. In these latter areas, more time is available for precipitation to penetrate into and percolate through the vadose zone down to the water table. Aller and others (1987) estimated a typical net recharge of 4-7 inches for the residual soils overlying crystalline rocks of the Piedmont/Blue Ridge. For the Valley and Ridge and Cumberland Plateau, they reported a typical net recharge of 2-4 inches per year for folded sedimentary rocks of moderate slope (e.g., >6%) and a typical net recharge of 10+ inches per year for low slope valley limestones. All of Aller and others (1987) shallow slope (<6%) Coastal Plain settings had a net recharge of 10+ inches. Based upon Aller and others (1987) frequent use of the 6% slope to differentiate areas of higher and lesser recharge as well as their frequent use of 10+ inches per year as a measure of higher and lower net recharge, those portions of Georgia having a slope of less than or equal to 6% were assumed to have a net recharge of 10+ inches per year and were assigned the recommended DRASTIC rating of 9. Areas having a slope of greater than 6% were assumed to have a net recharge of less than 10 inches per year and were assigned a net recharge rating of 8. These ratings when multiplied by the recommended DRASTIC weight of 4, result in DRASTIC numbers of 36 and 32 respectively, for net recharge. Actual net recharge in Georgia probably averages about 6 inches per year (Carter and Stiles, 1983). This means that by following Aller and others (1987) recommendations, actual net recharge is slightly overestimated presenting a slight overestimation of actual pollution susceptibility. Aquifer Media, Impact of the Vadose Zone, and Hydraulic Conductivity In the DRASTIC methodology, lithologic outcroppings are used as surrogates for aquifers. This means that any standard geologic map which depicts the distribution of lithologic units can be used as an "aquifer media" map. According to Aller and others (1987), the vadose zone encompasses those materials between the base of the soil profile and the water table. For unconfined aquifer systems, these would be the same as the "aquifer media". In other words, the DRASTIC methodology allows any standard geologic map, which depicts the distribution of lithologic units, to be used as a measure of the impact of the vadose zone. Ranges of hydraulic conductivity have been established for a number of lithologic units (Freeze and Cherry, 1979). Thus, by knowing the lithology of the aquifer media, an estimate can be made of hydraulic conductivity. All geologic units mapped at a scale of 1:500,000 on the Geologic Map of Georgia (1976) and the limestone units mapped on the Tertiary and Quaternary Formations of Georgia (1948) map were assigned a numerical rating for aquifer media, impact of the vadose zone, and hydraulic conductivity of the shallow aquifer. The DRASTIC ratings ranged from 1-10, according to the pollution potential of the lithologic unit. Following the DRASTIC methodology, ratings assigned to aquifer media and to the hydraulic conductivity of the aquifer were assigned a weight of 3, and the impact of the vadose zone media ratings were assigned a weight of 5. The numerical rating was multiplied by the weight factor to obtain a DRASTIC number for the particular lithologic unit, as follows: Aquifer Impact of Geologic DRASTIC Media Vadose Zone Unit Rating (weight 3) (weight 5) Metamorphic/igneous rock 2 6 10 Weathered metamorphic/igneous 3 9 15 Bedded sedimentary 5 15 25 Massive sandstone 6 18 30 Sand/gravel 8 24 40 Karstic limestone 10 30 50 Geologic Unit Hydraulic DRASTIC Conductivity Number (gpd/ft2) (weight 3) bedded sedimentary 1 - 100 3 metamorphic/igneous 101 - 300 6 weathered metamorphic/igneous 301 - 700 9 massive sandstone 701 - 1000 18 sand/gravel 1001 - 2000 24 karstic limestone 2000+ 30 Soil Media Soil media DRASTIC ratings ranging from 1 to 10 were assigned to the soils data base according to soil types. The soils parameter was assigned a weight of 2 in conformity to DRASTIC methodology. The ratings were multiplied by the weight of 2, resulting in a DRASTIC soil index of 2 - 20, as follows: Soil Media DRASTIC Soil Number Rating (weight 2) Thin or absent 10 20 Gravel 10 20 Sand 9 18 Peat 8 16 Shrinking Aggregated Clay 7 14 Sandy loam 6 12 Loam 5 10 Silty loam 4 8 Clay loam 3 6 Muck 2 4 Nonshrinking/Nonaggregated clay 1 2 Topography As used in DRASTIC, "topography" means slope. Areas with a slope of less than or equal to 6% were assigned a topography DRASTIC rating of 10 (see Note 5 for use of 6% slope for assessing topography). Areas with a slope of greater than 6% were assigned a topography DRASTIC rating of 5. These ratings were multiplied by the weight of 5, resulting in DRASTIC numbers of 10 and 5, respectively, for topography. Interpretation Once the DRASTIC ratings were assigned for the seven parameters, the pollution susceptibility for each hydrogeologic setting was estimated by calculating the DRASTIC Index. The equation for calculating the DRASTIC Index is: DrDw+RrRw+ArAw+SrSw+TrTw+IrIw+CrCw = DRASTIC Index where: r = rating w = weight. Following the calculation of the DRASTIC Index for individual hydrogeologic settings, the range of index scores in Georgia was divided into three parts (see Note 1). Those areas having a DRASTIC Index of less than 141 were considered to have a relatively low pollution susceptibility for Georgia and were mapped accordingly. Areas having an index between 141 and 181 were considered to have average pollution susceptibility and hydrogeologic settings with DRASTIC index values greater than 181 were considered to have relatively high pollution susceptibility. The subdivision of the range of index values found in Georgia into three pollution susceptibility categories is relative, and for that reason, is meaningful only within the state of Georgia. In other words, a hydrogeologic setting with a DRASTIC Index of 135 might be considered to have relatively low pollution susceptibility in Georgia, but might be one of the highest values measured in another state. Alternatively, a setting with an index value of 182 is considered relatively susceptible to pollution in Georgia, but 182 may be considered a low or medium value in another state where pollution susceptible hydrogeologic settings are widespread. RESULTS Hydrogeologic settings with higher DRASTIC index values are most common where karst topography is developed in limestones. Karst topography is characterized by sinkholes, swallow holes, and underground caverns and is caused by the dissolution of underlying beds of limestone or dolomite. Karst areas are widely recognized by hydrogeologists as being extremely prone to ground-water pollution. Areas of karst are common on southwestern Georgia and in the valleys of northwestern Georgia. Areas where highly porous sandy soils occur, such as the Fall Line Hills, also are generally highly susceptible to pollution. On the other hand, areas in Georgia where the shallow aquifers are clayey or where slope is greater than 6%, such as the Piedmont, have relatively low pollution susceptibility. The shallow ground water underlying most of Georgia's Coastal Plain is highly to moderately susceptible to pollution due to limestone karst, sandy soils, and the relatively flat lying topography which are common in the region. In contrast, areas in the Piedmont and Blue Ridge physiographic provinces of northern Georgia have susceptibility indices that are lower than those in the Coastal Plain. This is due to the clayey saprolite which typically overlies the igneous and metamorphic rocks which characterize the region, and the greater slope of the land. The relatively low pollution ratings in the Piedmont and Blue Ridge, however, do not mean that the region is not susceptible to ground-water pollution. The region is susceptible to pollution, just relatively less so than most areas in the Coastal Plain. The Valley and Ridge physiographic province of northwestern Georgia has pollution susceptibility ratings ranging from higher to lower, depending on the geology and slope of the area Most of the higher susceptibility areas are in valleys underlain by karstic limestone aquifers. RECHARGE AREAS PROTECTION The Georgia Planning Act of 1989 requires that land-use ordinances, which protect ground water, be passed and enforced by local governments having jurisdiction over significant ground-water recharge areas shown on Hydrologic Atlas 18 (Davis and others, 1989). Within significant recharge areas, the relative degree of protection required is further defined on the basis of whether the area has a high pollution susceptibility. For example, the Rules for Recharge Area Protection require that local governments enforce a 150% larger lot size requirement for septic systems that are within a significant groundwater recharge area as well as being in an area of higher pollution susceptibility. Larger lot sizes may be recommended, but not necessarily required, if the area has a higher pollution susceptibility but is not within a significant recharge area. Funding was partially provided by the United States Environmental Protection Agency. ACKNOWLEDGEMENTS This map was developed by EPD with the assistance of the United States Geological Survey, Water Resources Division, Georgia District. REFERENCES Aller, L., Bennett, T., Lehr, J.H., Petty, R.J., and Hackett, G. 1987. DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings: U.S. Environmental Protection Agency, (EPA-600/2-87-035),455 pp. Carter, Robert F. and Stiles, Harold R., 1983. Average Annual Rainfall and Runoff in Georgia, 1941 - 1970: Georgia Geologic Survey Hydrologic Atlas 9, 1 sheet. Freeze, R.A. and Cherry, J.A., 1979. Groundwater, Prentice-Hall, 604 pp. Georgia Department of Natural Resources, 1975. Resource Assessment Program: Soils, 70 pp. LeGrand, Harry E. and Rosen, Lars (in press) Common Sense in Ground-Water Protection and Management in the United States; Ground Water. Davis, K.R., Donahue, J.C., Hutcheson, R.H., and Waldrop, D.L. 1989. Most Significant Ground Water Recharge Areas of Georgia. Georgia Geologic Survey Hydrologic Atlas 18, one sheet. _____,1976,Geologic Map of Georgia: Georgia Geologic Survey, one sheet. DATABASES USED Elassal, A. and Carvso, V. 1984. Digital Elevation Models. U.S. Geological Survey Circular 895-B, 40 pp. _____. Geologic Map of Georgia, 1976, Georgia Geologic Survey, one sheet. _____. Geologic Map of the Tertiary and Quaternary Formations of Georgia, 1948, Georgia Geologic Survey, one sheet. _____. Major Highways and Roads, 1974, National Cartographic Information Center. Scale 1:2,000,000. _____. Slope Map of Georgia Developed from Analysis of TIN Processed Digital Elevation Model Data (60 meter intervals). U.S. Geological Survey. Scale 1:250,000. _____. Soils Map of Georgia Developed from Soil Conservation Service County Soils Series Maps (STATSGO). U.S. Department of Agriculture Soil Conservation Service. Scale 1:250,000. _____. Rivers and Lakes, State of Georgia Hydrology Base Map. U.S. Geological Survey. Scale 1:500,000. Note 1: In 1986, the Research Triangle Institute (Nees and Salmons, 1987) informally developed a DRASTIC rating for every county in the United States as part of EPA's National Survey of Pesticides in Drinking Water Wells. In Georgia, these rating values range from 128 to 219. Approximately one third of Georgia counties had an assigned DRASTIC rating of more than 181, approximately one third had a DRASTIC rating of 141 to 181, and approximately one third had a DRASTIC rating of less than 141. Based on this precedent, ratings below 141 in Georgia were classified as "lower", ratings of 141-181 were classified as "average", and ratings above 181 were classified as "higher". Note 2: The one mile limit on accuracy was developed in consultation with the GIS staff of the United States Geological Survey. Areas of less than one square mile are not shown on the 1:500,000 scale map. They are present, however, in the 1:100,000 digital datbases. Note 3: The hydrogeologic settings (from Aller and others, 1987) generally correspond to Georgia physiographic provinces as follows: (1) Setting 6: Nonglaciated Central Region, which corresponds to the Cumberland Plateau and Ridge and Valley. (2) Setting 8: Piedmont and Blue Ridge. (3) Setting 10: Atlantic and Gulf Coastal Plain, which generally correspond to the Fall Line Hills of the Upper Coastal Plain. (4) Setting 11: Southeast Coastal Plain, which generally corresponds to the Dougherty Plain and the Lower Coastal Plain. Within each of these settings, there are subsettings. For example, for the Atlantic and Gulf Coastal Plain, there are five subsettings (e.g., regional aquifers, unconsolidated and semi-consolidated shallow surficial aquifers, river alluvium without overbank deposits, and swamps). For each subsetting, general hydrogeologic features and ranges are delimited and DRASTIC ratings are recommended. Note 4: In unconfined aquifers, the water table generally is a subdued replica of the surface topography. Typically the depth to water is greatest at hill tops and hill slopes and is least at valleys, flood plains, and other relatively flat low lying areas. In 1978, the Georgia Department of Natural Resources (DNR) published an assessment of all the U.S. Soil Conservation Service's 43 soil associations in Georgia. DNR's assessment indicated that soils characterized as having a shallow water table rarely occurred on slopes in excess of 6%. Soils characterized as having either a deep or a shallow water table, however, occur on slopes of less than 6%. By identifying which areas had both slopes of less than 6% (from the slope data base) as well as soils characterized by shallow water table (from the soils data base) the depth to water could be further estimated. For the four Georgia hydrogeologic settings (Aller and others, 1987, pg.21), seven depth to water ranges are recommended. Of these seven, 0-5 feet, 5-15 feet, and greater than 15 feet are most appropriate ranges for Georgia (e.g., monitoring well data from solid waste landfills indicate that the 0-5 feet, 5-15 feet, and greater than 15 feet are typical depth to water ranges throughout most of Georgia (McLemore, W.H., 1992, personal communication). Aller and others (1987) as well as U.S. Soil Conservation Service also use the 6% slope as a common break generally separating upland areas with a deeper water table from lowland areas with a shallow water table. Note 5: Aller and others (1987) present a graph (their figure 7) of ranges of DRASTIC ratings. The curve for their graph is S-shaped with the inflection point generally corresponding to a slope of 6% and a rating of 5. Therefore, the maximum rating for a slope of 6% or less would be 5 and the maximum rating for a slope of greater than 6% would be 10. These values were selected for use. PART C - THE DIGITAL FILES CONTAINING THE GIS DATABASE: GROUND-WATER POLLUTION SUSCEPTIBILITY MAP OF GEORGIA SUMMARY STATEMENT / DESCRIPTION OF THE CONTENTS OF PART C The digital files are written on the enclosed150 megabyte data cartridge. The disk is in DG/UX version 5.4R3.10 tar (tape archive) format. The disk contains: 1. READ.ME - an ascii file which is a digital copy of this summary. 2. DOCUMENT.ASC - an ascii file which is a digital copy of this publication. 3. DOCUMENT.MIF - a digital copy of this publication, in Framemaker format.