Aquifer vulnerability assessment of the Dahomey Basin using the RTt method

This paper presents the theory and application of rainfall and travel time (RTt) vulnerability method. The method is applied on a shallow aquifer in the Dahomey Basin of south-western Nigeria with the results being compared to other established vulnerability assessment methods. The method follows the source pathway receptor concept of aquifer vulnerability assessment where rainfall represents the driving force for the downward percolation of contaminant to the water table while travel time represents ease of contaminant movement to groundwater. The travel time is derived from the depth to water table and infiltration rate in the vadose zone. Aquifer vulnerability maps were produced by interpolation techniques using the direct weight method. RTt vulnerability data were rated into five classes and assigned equal values ranging from 12 to 100. These classes and ranges include: 12–29 very low, 29–47 low, 47–65 moderate, 65–83 high and 83–100 very high vulnerability. The resultant RTt vulnerability map was compared with the established DRASTIC, PI and aquifer vulnerability index (AVI) vulnerability assessment methods. The AVI shows high vulnerability throughout the study areas while DRASTIC ranged between low, moderate and high vulnerability similar to the RTt method. The low to moderate vulnerability was areas receiving low rainfall and higher vadose thickness. RTt vulnerability comparison with these methods and its validation show the advantages of RTt method in assessing aquifer vulnerability with a few critical parameters, particularly for the assessment of shallow resource aquifers in data-lacking areas.