DRASTIC

DRASTIC

DRASTIC is an acronym for a systematic method used to evaluate the potential for groundwater contamination. It stands for Depth to water, Recharge, Aquifer media, Soil media, Topography, Impact of the vadose zone, and Conductivity (hydraulic) of the aquifer. This method is widely used in hydrogeology and environmental science to assess the vulnerability of groundwater to pollution.

Overview

The DRASTIC method was developed by the United States Environmental Protection Agency (EPA) in the 1980s. It provides a standardized approach to evaluate the intrinsic vulnerability of groundwater to contamination based on seven hydrogeological factors. Each factor is assigned a weight and a rating, which are then combined to produce a DRASTIC index.

Components of DRASTIC

Depth to Water

The depth to water is the distance from the ground surface to the water table. It is a critical factor because it determines the time available for a contaminant to be attenuated before reaching the aquifer. Shallower depths indicate higher vulnerability.

Recharge

Recharge refers to the amount of water per unit area that infiltrates the ground and reaches the aquifer. Higher recharge rates can increase the potential for contaminants to reach the groundwater.

Aquifer Media

Aquifer media describes the geological material that makes up the aquifer. Different materials, such as sand, gravel, or clay, have varying abilities to filter and attenuate contaminants.

Soil Media

Soil media refers to the uppermost layer of the earth where plants grow. The type of soil affects the movement of water and contaminants. For example, sandy soils allow for rapid infiltration, while clayey soils may impede it.

Topography

Topography is the slope of the land surface. Steeper slopes can lead to rapid runoff, reducing the potential for infiltration and contamination.

Impact of the Vadose Zone

The vadose zone is the area between the land surface and the water table. Its characteristics influence the movement of contaminants. A thick vadose zone with low permeability can protect the aquifer from contamination.

Conductivity (Hydraulic)

Hydraulic conductivity is a measure of the aquifer's ability to transmit water. High conductivity indicates that water and contaminants can move quickly through the aquifer.

Calculation of the DRASTIC Index

The DRASTIC index is calculated using the formula:

DRASTIC Index = DrDw + RrRw + ArAw + SrSw + TrTw + IrIw + CrCw

Where:

• D, R, A, S, T, I, C are the ratings for each factor.
• Dw, Rw, Aw, Sw, Tw, Iw, Cw are the weights assigned to each factor.

The ratings are based on site-specific data, while the weights are standardized values reflecting the relative importance of each factor.

Applications

DRASTIC is used by environmental scientists, planners, and policymakers to identify areas at risk of groundwater contamination. It helps in land-use planning, environmental impact assessments, and the development of groundwater protection strategies.

Limitations

While DRASTIC is a valuable tool, it has limitations. It assumes uniformity within each factor across the study area and does not account for specific contaminant properties or human activities. Therefore, it should be used in conjunction with other methods and local knowledge.

See Also

• Groundwater
• Hydrogeology
• Environmental protection

References

• Aller, L., Bennett, T., Lehr, J. H., Petty, R. J., & Hackett, G. (1987). DRASTIC: A standardized system for evaluating groundwater pollution potential using hydrogeologic settings. U.S. Environmental Protection Agency.