EVALUATION OF THE GROUNDWATER POTENTIAL OF THE MALALA ALLUVIAL AQUIFER, LOWER MZINGWANE RIVER, ZIMBABWE

Abstract

The study site lies in a semi arid climatic zone in the south eastern part of Zimbabwe. The overall climate, with a mean annual rainfall of 350 mm/annum is not generally suitable for farming, but groundwater from alluvial aquifers is a possible resource for supplementary irrigation. The main source of water for most purposes is surface water, but its scarcity causes problems for general livelihoods. The Mzingwane river is ephemeral and thus only flows for a limited period of time during the year. The local community as such has to rely on groundwater from alluvial aquifers for domestic purposes and food production. Alluvial aquifers have the advantage that when they are deeper than 1m, less water is lost to evaporation. However, the main challenge with such aquifers is that abstraction for large scale use is expensive and usually requires investment in motorized pumps which can be expensive for the local communities This study evaluated groundwater resources at a local scale by characterizing the Malala alluvial aquifer, which covers a stretch of 1000 m of the Mzingwane river and is on average 200 m wide. The aquifer is recharged, naturally, by flood events during the rainy season and, artificially, by managed dam releases from Zhovhe dam during the dry season. The Malala site was selected from geological mapping and resistivity studies. The site shows indications of deeper sand layers and hence would be expected to have a higher potential of storing more groundwater. Piezometers were installed in the river channel to monitor the water level fluctuations in the alluvial aquifer. Water samples were collected from Zhovhe dam, Mazunga area and Malala alluvial aquifer in order to analyse the major ion chemistry of the water at the aquifer and at the source of recharge. A piper diagram analysis showed that the water in the alluvial aquifer can be classified as sodium sulphate. The water is also of a low sodium hazard and can therefore be used for irrigation without posing much risk to the compaction of soils. Laboratory tests were carried out to characterize the Malala alluvial aquifer material for the porosity, hydraulic conductivity and specific yield of the aquifer. The porosity of the alluvial aquifer was calculated to be 39% with a hydraulic conductivity of 59.76 md-1 and a specific yield value of 5.4 %. The slope of the alluvial aquifer was measured as 0.38 %. Resistivity surveys showed that the alluvial aquifer has an average depth of 13.4 m. The alluvial aquifer is more enhanced on the upstream part of the dolerite dyke. The bedrock is metamorphic rock mainly tonalitic and granodioritic gneisses. A sieve analysis experiment showed that the alluvial aquifer is sand. Water level observations from the installed piezometers indicated that the water levels dropped on average by 0.75 m within 97 days after the observed dam release. The alluvial aquifer system can store approximately 1 035 000 m3 of water per km length of the river. 116 000 m3 of this water is readily available for abstraction and has a potential of irrigating at least 11.6 ha/annum. An increase in the number of timed releases can lead to an increase in the groundwater potential. 46.4 ha of land can be irrigated from the alluvial aquifer from at least four releases per annum which saturate the aquifer. The alluvial aquifer can thus store a significant amount of water and has a high groundwater potential to sustain both domestic and irrigation water supply through out the year.