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.
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.