The development of crystalline basement aquifers in a tropical environment

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Abstract
Chemical weathering is the dominant process in the development of a weathering profile on crystalline basement rocks. In the saturated lower part of the weathering profile, groundwater is the principal chemical reagent and the rate of groundwater flow determines the rate of weathering reactions. The search for successful borehole sites in this environment also depends upon finding locations where the rate of groundwater flow is at an optimum. This search could therefore be facilitated by an appreciation of the part which groundwater flow plays in the chemical weathering process. The principal factors which control the quantity of groundwater flow through a unit cross sectional area of the weathering system are the availability of recharge, the permeability of the weathered material and the hydraulic gradient between the recharge and discharge areas. Of these three, the importance of recharge is clear as this determines the input to the groundwater system. The permeability of the weathered material is a function of the extent of chemical weathering and is therefore intrinsically linked to the history of groundwater flow through the system. The remaining independent variable is the hydraulic gradient. This is normally developed in what is generally a relatively thin weathered layer which closely follows the surface morphology. The hydraulic gradient is therefore a function of the average surface slope over the groundwater flow path and therefore related to the surface morphology. A plot of recharge against surface morphology therefore becomes a valid projection upon which the various groundwater provinces can be plotted and provides a useful means of classifying the various occurrences of groundwater within the weathered profile.