Indroduction to DRASTIC

Why are aquifer susceptibility (vulnerability) maps useful?

Aquifer vulnerability mapping is a spatially distributed measure of the susceptibility of an area to groundwater contamination. Aquifer vulnerability maps are beneficial to planners, developers, and governments who can utilize them for development planning, identifying potentially highly sensitive areas, and prioritizing and designing monitoring programs (Aller et al. 1987). Vulnerability mapping does not replace site specific investigations, but can act as a guide to determine the intensity of site investigation needed and to emphasize highly vulnerable areas.

What is DRASTIC?

The seven factors of DRASTIC

It is a method of determining if an area's groundwater is likely to be contaminated when a pollutant is placed on the surface. The DRASTIC method considers a generic contaminant (such as animal waste, land-fill, etc... ) introduced at the surface and moving downward due to precipitation at the speed of water (Aller et al 1987).

DRASTIC is an acronym for the seven factors that determine how likely an area's groundwater is to be contaminated:

  • Depth to water table,
  • Recharge,
  • Aquifer media,
  • Soil media,
  • Topography,
  • Impact of the vadose zone, and
  • hydraulic Conductivity of the aquifer.

For a given location each of the seven factors is rated and given a score from one(the lowest vulnerability) to ten(the highest vulnerability) ( Table 1).

The adjacent diagram shows the locations of the seven DRASTIC factors. (Created by Richard Franklin & Robert Turner of the GSC. Modified by Cyrille Medard de Chardon.)

Final Vulnerability

The final vulnerability is calculated by the weighted sum of each factor's rating (r):

V = 5D r+ 4R r+ 3A r+ 2S r+ 1T r+ 5I r+ 3C r

DRASTIC factor weights

The resulting vulnerability is a relative measure of the pollution potential of that area, ranging from 23 (low vulnerability) to 230 (high vulnerability).

Limitations of DRASTIC

DRASTIC has difficulty representing partially confined, leaky, or stacked aquifers and does not take into account recharge and discharge areas (Aller et al. 1997). If the dominant hydraulic gradient is upwards, as in a discharge area, then the DRASTIC method may over-predict vulnerability. Conversely, if the hydraulic gradient is downwards, contaminants may move into the groundwater system more easily than suggested by the DRASTIC rating. Aquifer vulnerability can only be mapped for areas larger than a 100 acres (~40 ha) using the DRASTIC method (Aller et al. 1997).

Despite its restrictions and limitations, DRASTIC provides a good relative measure of the vulnerability of an area to contamination for use in sustainable development and groundwater protection planning.

 

References

Aller, L., Bennett, T., Lehr, J., Petty, R. and G, Hackett (1987) DRASTIC: A Standardized System for Evaluating Ground Water Pollution Potential Using Hydrogeologic Settings. National Water Well Association, Dublin Ohio / EPA Ada, Oklahoma. EPA-600/2-87-035.

Van Stempvoort, D., Ewert, L., Wassernaar, L. (1992) AVI: A method for groundwater protection mapping in the prairie provinces of Canada. Prairie Provinces Water Board, Saskatoon, Saskatchewan.