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Groundwater movement and water chemistry associated with weathering profiles of the African surface in parts of Malawi

Abstract This paper summarizes the main results of recent geomorphological research on the hydrogeology of weathering profiles on the African erosion surface in Malawi. Deep regolithic profiles have developed by protracted and aggressive weathering and differential leaching. In the advanced stages of weathering, congruent kaolinite ‘dissolution’ causes the saprolite to collapse, forming a thick residuum of the most resistant materials, dominated by silica in the form of quartz and iron as goethite. Aluminium has been extensively leached. Such profiles pertain to a mechanism of land surface formation dominated by the activity of infiltrating water rather than direct surface runoff and it results in terrain with a generally basined configuration. Low-lying areas are occupied by dambos. These are clay-filled bottomlands which are seasonally waterlogged and are located where lithology favours leaching and saprolite collapse. The contemporary fluviatile-like configuration of the dambos is attributed to post-incision modification of the ancient land surface. As a result, their area is reduced and they become inset into it, following geological controls which express integrated groundwater movement within the saprolite. The infill is shown to be essentially an alumino-silicate evaporite rather than of alluvial origin. Analysis of dambo and dambo-peripheral profiles supports this genetic model, identifying the continuity of the contemporary dambo infill with a ‘palaeodambo’ clay wedge below the surficial sands in peripheral situations. The very low permeability of the clay is held to be responsible for the marginal seepage zone, fed by shallow throughflow from topographically higher profiles and augmented by upward discharge of deep water. Elements lost from the interfluves were identified in evaporites which occur on the dambo floors in the dry season. These include aluminium. Analysis of water discharging into the dambos failed to identify this element. There is a need for reassessment of technique to determines element mobilization in tropical groundwater, where organic binding is implicated.