Influence of Basalt Weathering on Shallow Groundwater Quality in Semi-Arid Cawoods-Mazunga, Zimbabwe: Petrographic Study

W. Moyce, M. Meck, R. Owen, and D. Love (Zimbabwe)

Keywords

Alluvial aquifer, weathering, groundwater quality, semiarid climate

Abstract

The weathering of basalt results in the release of metal ions Ca+2, Mg+2, Fe+2M and Na+ and result in formation of clay minerals and ferric oxides. The calcrete formation from weathering of the basalt flushes the Ca, Mg and HCO3 from the system during the dry season and Mg is stored in the sepiolite. Sodium and chloride levels are a threat to alluvial flood plain aquifers making the water saline and less suitable for irrigation and the water –rock interactions, influences the water quality. The calcrete formation will liberate salts to the surface and increases the Na/Ca ratios. This can threaten the Cawoods Mazunga citrus plantation which are sensitive to salts. The Mg is flushed out of the system and hence does not cause any Mg hazards during the dry season but the irrigation can liberate it from the sepiolite causing magnesium hazards. During severe droughts the river might be recharged by the groundwater which is saline and eventually salinisation of the river. The calcrete can act either as a semi-permeable layer minimising groundwater recharge and thus increasing run-off aiding high erosion of the topsoil. It can also aid in high groundwater recharge raising the water table increasing the threat of land salinisation.

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