A ground water pollution potential mapping program for Ohio was initiated in 1986 under the direction of the Ohio Department of Natural Resources, Division of Water. The DRASTIC mapping system, developed by Aller, et al., 1987, was chosen because it allows the pollution potential of an aquifer to be evaluated systematically using existing information. Vulnerability to contamination is a combination of hydrogeologic factors, anthropogenic influences, and sources of contamination for a given area. The DRASTIC system focuses on the hydrogeologic factors that influence ground water movement. Click here for a link to order the DRASTIC Manual by Aller, et al.

The system consists of two major elements:

1. The designation of mappable units, termed hydrogeologic settings, and

2. The superposition of a relative rating system to determine the pollution potential.

 

Hydrogeologic Settings

    To facilitate the designation of mappable units, the DRASTIC system used the framework of an existing classification system developed by Heath (1984), which divides the United States into 15 ground water regions based on the factors in a ground water system that affect occurrence and availability. Of the 15 regions, two fall within Ohio: 6- Nonglaciated Central Region and 7-Glaciated Central Region. Within each major hydrogeologic region, smaller units representing specific hydrogeologic settings are identified. Hydrogeologic settings form the basis of the system and represent a composite description of the major geologic and hydrogeologic factors that control ground water movement into, through, and out of an area. A hydrogeologic setting represents a mappable unit with common hydrogeologic characteristics and, as a consequence, common vulnerability to contamination (Aller et al., 1987). The figure below is an example of a typical hydrogeologic setting in the glaciated region of Ohio.

Example of a DRASTIC hydrogeologic setting. This setting is 7D Buried Valley. Hydrogeologic settings describe areas of common geologic and ground water characteristics that directly affect the vulnerability of an aquifer to contamination from the surface.

Relative Rating System

    Hydrogeologic settings form the basis of the system and incorporate the major hydrogeologic factors that affect and control ground water movement and occurrence.

These seven factors are:

D epth to Water
R echarge (Net Recharge)
A quifer Media
S oil Media
T opography (% slope)
I mpact of the Vadose Zone Media
C onductivity (Hydraulic) of the Aquifer)

    DRASTIC uses a numerical rating and weighting system that is combined with the seven DRASTIC factors listed above to calculate a ground water pollution potential index or relative measure of vulnerability to contamination. The DRASTIC factors are weighted from 1 to 5 according to their relative importance to each other with regard to contamination potential. Each factor is then divided into ranges or media types and assigned a rating from 1 to 10 based on their significance to pollution potential. (Higher numbers means higher pollution potential) The following table is an example of the rating and weighting system.

    Hydrogeologic settings are combined with the pollution potential indexes to create units that can be graphically displayed on a map (see below description).

SETTING 7D6		GENERAL
FEATURE	RANGE	RATING		WEIGHT		NUMBER
Depth to Water	30-50 feet	7	X	5	=	35
Net Recharge	4-7 inches	6	X	4	=	24
Aquifer Media	Sand & Gravel	6	X	3	=	18
Soil Media	Silt Loam	4	X	2	=	8
Topography	0-2% slope	10	X	1	=	10
Impact of Vadose Zone	Sand&Gravel/Silt&Clay	6	X	5	=	30
Hydraulic Conductivity	300-700 gpd/ft 2	4	X	3	=	12
DRASTIC INDEX						137

    Once a DRASTIC index has been calculated, it is possible to identify areas that are more likely to be susceptible to ground water contamination relative to other areas. The higher the DRASTIC index, the greater the vulnerability to contamination. The index generated provides only a relative evaluation tool and is not designed to produce absolute answers or to represent units of vulnerability.

    The maps are color-coded using ranges depicted on the map legend. The color codes used are part of a national color-coding scheme developed to assist the user in gaining a general insight into the vulnerability of the ground water in the area. The color codes were chosen to represent the colors of the spectrum, with warm colors (red, orange, and yellow) representing areas of higher vulnerability (higher pollution potential indexes), and cool colors (greens, blues, and violet) representing areas of lower vulnerability to contamination.

 

Hydrogeologic settings are combined with the pollution potential indexes to create units that can be graphically displayed on a map. (See below description)

    Here the first number (7) refers to the major hydrogeologic region (Glaciated Central Region) and the upper case letter (D) refers to a buried valley hydrogeologic setting. The following number (6) references a certain set of DRASTIC parameters that are unique to this setting and are described in the corresponding setting chart shown above. The number on the bottom (137) is the calculated pollution potential index for this unique setting. The charts for each setting provide a reference to show how the pollution potential index was derived.

    A report accompanies each map that includes general geologic and hydrogeologic information and describes the mapping process used to determine the ground water pollution potential of areas within a county.

Applications of Pollution Potential Maps

    The pollution potential mapping program offers a wide variety of applications in many counties.The ground water pollution potential maps are prepared to assist planners, managers, and state and local officials in evaluating the relative vulnerability of areas to ground water contamination from various sources of pollution. This information can be used to help direct resources and land use activities to appropriate areas, or to assist in protection, monitoring, and clean-up efforts.

    An important application of the pollution potential maps for many areas will be assisting in county land use planning and resource expenditures related to solid waste disposal. A county may use the map to help identify areas that are suitable for disposal activities. Once these areas have been identified, a county can collect more site-specific information and combine this with other local factors to determine site suitability.

    Pollution potential maps may be applied successfully where non-point source contamination is a concern. Non-point source contamination occurs where land use activities over large areas impact water quality. Maps providing information on relative vulnerability can be used to guide the selection and implementation of appropriate best management practices in different areas. Best management practices should be chosen based upon consideration of the chemical and physical processes that occur from the practice, and the effect these processes may have in areas of moderate to high vulnerability to contamination. For example, the use of agricultural best management practices that limit the infiltration of nitrates, or promote denitrification above the water table, would be beneficial to implement in areas of relatively high vulnerability to contamination.

    A pollution potential map can assist in developing ground water protection strategies. By identifying areas more vulnerable to contamination, officials can direct resources to areas where special attention or protection efforts might be warranted. This information can be utilized effectively at the local level for integration into land use decisions and as an educational tool to promote public awareness of ground water resources. Pollution potential maps may be used to prioritize ground water monitoring and/or contamination clean-up efforts. Areas that are identified as being vulnerable to contamination may benefit from increased ground water monitoring for pollutants or from additional efforts to clean up an aquifer.

    Individuals in the county who are familiar with specific land use and management problems will recognize other beneficial uses of the pollution potential maps. Planning commissions and zoning boards can use these maps to help make informed decisions about the development of areas within their jurisdiction. Developers proposing projects within ground water sensitive areas may be required to show how ground water will be protected.

    Regardless of the application, emphasis must be placed on the fact that the system is not designed to replace a site-specific investigation. The strength of the system lies in its ability to make a "first-cut approximation" by identifying areas that are vulnerable to contamination. Any potential applications of the system should also recognize the assumptions inherent in the system.

 

Sample Preprinted Ground Water Pollution Potential Map.

Enlarged area from above sample map.

 

Click here for a map of available counties.
To order available maps or for more information on mapping contact:

The Ohio Department of Natural Resources
Division of Water
Water Resources Section
2045 Morse Road, Bldg. B
Columbus, Ohio 43229-6693

Phone: (614) 265-6740
Fax: (614) 265-6767
E-mail: cleve.brown@dnr.state.oh.us

COST:      $10.00/Report (Plus postage and handling, See chart below)

Payment Methods:

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more than $50.00 and less than or equal to $100.00	add $ 8.50
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You may also want to see:

State Aquifer Mapping Project (1998 to 2000).

Ground Water Resources Maps