INFLUENCE OF BASALT WEATHERING ON SHALLOW GROUNDWATER QUALITY IN SEMI-ARID CAWOODS-MAZUNGA, ZIMBABWE: PETROGRAPHIC STUDY

摘要

The weathering of basalt results in the release of metalrnions - Ca~(+2), Mg~(+2), Fe~(+2) and Na~+ and result in formation ofrnclay minerals and ferric oxides. The calcrete formationrnfrom weathering of the basalt flushes the Ca, Mg andrnHCO3 from the system during the dry season and Mg isrnstored in the sepiolite. Sodium and chloride levels are arnthreat to alluvial flood plain aquifers making the waterrnsaline and less suitable for irrigation and the water –rockrninteractions, influences the water quality. The calcreternformation will liberate salts to the surface and increasesrnthe Na/Ca ratios. This can threaten the Cawoods Mazungarncitrus plantation which are sensitive to salts. The Mg isrnflushed out of the system and hence does not cause anyrnMg hazards during the dry season but the irrigation canrnliberate it from the sepiolite causing magnesium hazards.rnDuring severe droughts the river might be recharged byrnthe groundwater which is saline and eventuallyrnsalinisation of the river. The calcrete can act either as arnsemi-permeable layer minimising groundwater rechargernand thus increasing run-off aiding high erosion of therntopsoil. It can also aid in high groundwater rechargernraising the water table increasing the threat of landrnsalinisation.