United States Environmental Protection Agency Robert S. Kerr Environmental Research Laboratory Ada OK 74820 Research and Development EPA/600/S2-87/035 Sept. 1987 Project Summary DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings Linda Aller, Truman Bennett, Jay H. Lehr, Rebecca Petty, and Glen Hackett A methodology is described that will allow the pollution potential of any hydrogeologic setting to be systemat- ically evaluated anywhere in the United States. The system has two major portions: the designation of mappable units, termed hydrogeologic settings, and the superposition of a relative rating system called DRASTIC. Hydrogeologic settings form the basis of the system and incorporate the' major hydrogeologic factors which affect and control ground-water move- ment including depth to water, net recharge, aquifer media, soil media, topography, impact of the vadose zone media and hydraulic conductivity of the aquifer. These factors, which form the acronym DRASTIC, are incorporated into a relative ranking scheme that uses a combination of weights and ratings to produce a numerical value called the DRASTIC Index. Hydrogeologic settings are combined with DRASTIC Indexes to create units which can be graphically displayed on a map. The application of the system to 10 hydrogeologically variable coun- ties resulted in maps with symbols and colors which illustrate areas of ground- water contamination vulnerability. The system optimizes the use of existing data to rank areas with respect to pollution potential to help direct inves- tigations and resource expenditures and to prioritize protection, monitoring and clean-up efforts. The full report was submitted in partial fulfillment of Contract No. CR- 810715-01 by the National Water Well Association under sponsorship of the U.S. Environmental Protection Agency. This report covers a period from October 1983 to March 1987, and work was completed as of April 1987. This Project Summary was devel- oped by EPA's Robert S. Kerr Environ- mental Research Laboratory, Ada, OK, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report ordering information at back). Introduction This research project was designed to create a methodology that will permit the ground-water pollution potential of any hydrogeologic setting to be systemati- cally evaluated anywhere in the United States. The methodology has been incorporated into a standardized system that can be readily displayed on maps using existing information. The concepts inherent to the system were developed assuming a contaminant with the mobil- ity of water, introduced at the surface and flushed into the ground water by precipitation. The methodology is designed to evaluate ground-water pollution potential from a regional perspective and should be applied to areas 100 acres or larger in size. ------- Results The system that has been developed has two major parts, the designation of mappable units, termed hydrogeologic settings; and the superposition of a relative ranking system, called DRASTIC, which helps the user evaluate the relative ground-water pollution potential of any hydrogeologic setting. The standardization system for evalu- ating ground-water pollution potential has been developed within the frame- work of an existing classification system of ground-water regions of the United States (Figure 1). These regions include: 1. Western Mountain Ranges 2. Alluvial Basins 3. Columbia Lava Plateau 4. Colorado Plateau and Wyoming Basin 5. High Plains 6. Nonglaciated Central Region 7. Glaciated Central Region 8. Piedmont and Blue Ridge 9. Northeast and Superior Uplands 10. Atlantic and Gulf Coastal Plain 11. Southeast Coastal Plain 12. Alluvial Valleys 13. Hawaiian Islands 14. Alaska 15. Puerto Rico and Virgin Islands For the purpose of the present system Region 12 (Alluvial Valleys) has been incorporated into each of the other regions and Region 15 (Puerto Rico and Virgin Islands) has been omitted. Because pollution potential cannot be determined on a regional scale smaller "hydrogeologic settings" were deve- loped within each region. A hydrogeo- logic setting is a composite description of all the major geologic and hydrologic factors which affect and control ground- water movement into, through and out of an area. It is defined as a mappab unit with common hydrogeologic cha acteristics, and as a consequenci common vulnerability to contaminatioi These hydrogeologic settings form tti basis of the system and permit furth< delineation of the factors that affei pollution potential (Figure 2). Inherent in each hydrogeologic settin are the physical characteristics th< affect the pollution potential of grour water. After evaluating a number i factors, the most important mappab factors that control ground-water polli tion potential were determined to be: D—Depth to Water Table R— (Net) Recharge A—Aquifer Media S—Soil Media T—Topography (Slope) I—Impact of Vadose Zone C—Conductivity (Hydraulic) of th Aquifer 2. Alluvial Basins 9. Northeast and , Superior Uplands 7. Glaciated Central region 6. Nonglaciated Central region 9. Northeast and Superior Uplands 1. Western Mountain Ranges 16. Nonglaciated Central regio v 4. Colorado Plateau and Wyoming Basin O^^yV 6. Nonglaciated Central .region 6. Nonglaciated Central'region . Coastal Pl^ 0 500 Af//es 0 flOO Kilometers Figure 1. Ground-water regions of the United States (After Heath. 1984). 2 ------- Hawaii (12C) Volcanic Uplands This hydrogeologic setting is characterized by moderately rolling topography, at medium elevations, and rich, dark, soils developed from the basaltic bedrock. The soils are permeable, rainfall is high, and recharge is high. Bedrock is composed primarily of alternating extrusive basaltic lava flows and interlayered weathered zones formed between flows. Ground water occurs at moderate to deep depths, and aquifer yield is controlled by fracture zones, vesicular zones (both primarily cooling features) and the inter-flow weathered zones. Hydraulic conductivity is high. As with other settings in Hawaii, heavy pumping stresses often result in salt-water intrusion. This is a reflection of the fact that each island is surrounded by and underlain by salt water, with the fresh water occurring in a lenticular body that floats on the salt water. Ground water yield is therefore limited quite specifically to the amount of water recharged annually Figure 2. Format of hydrogeological setting. The DRASTIC factors represent meas- urable parameters for which data are generally available from a variety of sources without detailed reconnais- sance. Sources of this information are listed in Table 1. Each DRASTIC factor has been eval- uated with respect to each other to determine the relative importance of each factor. Each DRASTIC factor has been assigned a relative weight ranging from 1 to 5 (Table 2), with the most significant factors having a weight of 5 and the least significant a weight of 1. These weights are constant and may not be changed. A special case for the DRASTIC Index was developed for agricultural areas where the application of pesticides are a concern. Pesticide DRASTIC was created to address the important pro- cesses which affect the fate and trans- port of pesticides into the subsurface. The weights assigned for each factor have been modified to reflect the poten- tial impacts of pesticide application on ground water (Table 3). Each DRASTIC factor has been divided into either ranges or significant media types which have an impact on pollution potential. Each range or media has then been evaluated with respect to each other to determine the relative signifi- cance of each range or media with respect to pollution potential. These ranges and media have been assigned a rating of from 1 to 10, with the most significant range or media having a rating of 10, and the least significant a rating of 1 (Tables 4-10). Once the DRASTIC Index has been computed, it is possible to identify areas which are more likely to be susceptible to ground-water contamination relative to one another. The higher the DRASTIC Index, the greater the ground-water pollution potential. A DRASTIC Index calculated for one setting may then be compared to values obtained in other settings m the same region or in a different region. Hydrogeologic settings are combined with DRASTIC Indexes to create mappa- ble units which present a "picture" of the geologic and hydrogeologic condi- tions of an area. To fully demonstrate and test the system, DRASTIC was applied to 10 counties across the United States representing a diversity of hydrogeologic conditions including: 1. Cumberland County, Maine; 2. Finney County, Kansas; 3. Gillespie County, Texas; 4. Greenville County, South Carolina; 5. Lake County, Florida; 6. Minidoka County, Idaho; 7. New Castle County, Delaware; 8. Pierce County, Washington; 9. Portage County, Wisconsin; and 10. Yolo County, California. These counties were chosen to repre- sent both rural and urban areas and to exemplify both an abundance and scar- city of available hydrogeologic data. Pollution potential maps showing hydro- geologic settings and DRASTIC Indexes were manually drawn for each county. To assist in map readability, each dem- onstration map was color-coded using the DRASTIC Index. A national color code scheme was developed based on a simplified statistical evaluation of fre- quency of Index occurrence. The colors of the spectrum were chosen to show the levels of relative vulnerability to pollution. The warm colors indicate areas with potentially greater problems and cool colors indicate areas with lower susceptibility to ground-water pollution. Figure 3 illustrates the superposition of the national color code on a portion of the pollution potential map for Yolo County, California. Various screens have been chosen to simulate the color variations on the map. Conclusion The system presents a simple and easy-to-use approach to assess the ground-water pollution potential of any area. Although the final system appears simplistic, the system actually includes many complex concepts and relation- ships. Before an attempt is made to make full use of this system, the user must develop an appreciation for the complex- ity of evaluating ground-water pollution potential. It is not necessary to under- stand every concept in detail, but the ------- Table 1. Sources of Hydrogeologic Information Depth to Source Water U.S. Geological Survey X State Geological Surveys X State Department of Natural/ Water Resources X U.S. Department of Agriculture-Soil Conservation Service State Department of Environmental Protection X Clean Water Act "208" and other Regional Planning A uthorities X County and Regional Water Supply Agencies and Companies (private water suppliers) X Private Consulting Firms Ihydrogeologic, engineering) X Related Industry Studies (mining, well drilling, quarrying, etc.) X Professional Associations (Geological Society of America, National Water Well Association. American Geophysical Union) X Local Colleges and Universities (Departments of Geology. Earth Sciences, Civil Engineering) X Other Federal/ State Agencies (Army Corps of Engineers. National Oceanic and Atmospheric Administration X greater the depth of understanding, the more useful the system becomes. DRASTIC produces mappable results that provide a basis for the comparative ownli la t inn nf aroflQ with rfl*»nflPt to thfi c vdi ua LIUI i ui a i GOO win i i c9|joL>i \.\j n iw potential for ground-water pollution. DRASTIC should not be used for site specific investigations, but is best applied on a regional basis to areas greater than 100 acres in size. DRASTIC is designed to assist individuals with resource allocation and prioritization of many types of ground-water related activities as well as to provide a practical educa- tional tool. Net Aquifer Soil Recharge Media Media X X X X X X X X X X X X X X X X X X X X X Impact of the Hydraulic Conductivi Topography Vadose Media of the Aquifer X X X X X X X X X X X X X X X X X X X X X X Table2. Assigned Weights for DRASTIC Table 3. Assigned Weights for Pesticide Features Feature Weight Depth to Water 5 Net Recharge 4 Aquifer Media 3 Soil Media 2 Topography 1 Impact of the Vadose Zone A4~w:« K DRASTIC Features Feature Depth to Water Net Recharge Aquifer Media Soil Media Topography Impact of the Vadose Zone A4<*w;». Pesticide Weight 5 4 3 5 3 A Hydraulic Conductivity of the Aquifer Hydraulic Conductivity of the Aquifer ------- Table 4. Ranges and Ratings for Depth to Water Depth to Water {reel/ Range Rating 0-5 10 5-15 9 15-30 7 30-50 5 50-75 3 75-100 2 700+ 7 Weight: 5 Pesticide Weight: 5 Table 5. Ranges and Ratings for Net Recharge Net Recharge (Inches) Range Rating 0-2 1 2-4 3 4-7 6 7-10 8 70+ 9 Weight: 4 Pesticide Weight: 4 Table 6. Ranges and Ratings for Aquifer Media Aquifer Media Range Massive Shale Metamorphic/ Igneous Weathered Metamorphic/ Igneous Glacial Till Bedded Sandstone, Limestone and Shale Sequences Massive Sandstone Massive Limestone Sand and Gravel Basalt Karst Limestone Weight: 3 Table 7. Ranges and Ratings for Soil Table Media Soil Media Range Rating Thin P" Absent in Gravel 10 Sand 9 Peat 8 Shrinking and/or Aggregated Clay 7 Sandy Loam 6 Loam 5 Silty Loam 4 Clay Loam 3 Muck 2 Nonshrinking and Nonaggregated Clay 1 Rating Typical Rating 1-3 2 2-5 3 3-5 4 4-6 5 5-9 6 4-9 6 4-9 6 4-9 8 2-10 9 9-10 10 Pesticide Weight: 3 8. Ranges and Ratings for Topography Topography (Percent Slope) Range Rating 0-2 10 2-6 9 6-12 5 12-18 3 18+ 1 Weight: 1 Pesticide Weight: 3 Weight: 2 Pesticide Weight: 5 ------- Table 9. Ranges and Ratings for Impact of the Vadose Zone Media Impact of the Vadose Zone Media Range Confining Layer Silt/Clay Shale Limestone Sandstone Bedded Limestone. Sandstone. Shale Sand and Gravel with Significant Silt and Clay Metamorphic/ Igneous Sand and Gravel Basalt Karst Limestone Weight: 5 Rating 1 2-6 2-5 2-7 4-8 4-8 4-8 2-8 6-9 2-10 8-10 Pesticide Weight: 4 Table 10. Ranges and Ratings for Hydraulic Conductivity T . . _ . Hydraulic Conductivity Typical Rating W«2' Range Rating 3 1-100 1 3 100-300 2 6 300-700 4 6 700-1000 6 6 1 000-2000 8 6 20OO+ 10 4 Weight: 3 Pesticide Weight: 2 8 9 10 ------- Sea/e //i I f/gure 3. Pollution potential map for a portion of Yolo County. California, showing the superposition of the national color code. ------- Linda Alter, Truman Bennett. Jay H. Lehr. Rebecca Petty, and Glen Hackett are with National Water Well Association. Dublin, OH 43017. Jerry Thornhill is the EPA Project Officer (see below) The complete report, entitled "DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings," (Order No. PB 87-213 914/AS; Cost: $48.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Robert S. Kerr Environmental Research Laboratory U.S. Environmental Protection Agency Ada, OK 74820 United States Environmental Protection Agency Center for Environmental Research Information Cincinnati OH 45268 Official Business Penalty for Private Use $300 EPA/600/S2-87/035 ------- |