Chokwé and Guija, two districts of the Gaza province, in Mozambique, are located at the downstream part of the Limpopo river basin. The latter is a shared basin between four countries in which the development of water infrastructures upstream has a high impact on the water quality and availability downstream. Furthermore, neither the quantity nor the quality of the Limpopo River’s water that is entering the territory of Mozambique is properly monitored. Moreover, the Limpopo River has been characterized to be no longer perineal which makes the farming conditions more difficult. In fact, the average annual rainfall and evapotranspiration are respectively of 610 and 1649 mm in the province of Chokwé and the irrigation scheme is able to cover the water demand of about 20%.
The need for an alternative water resource is necessary in order to support the existing system. It is in that perspective that this research is studying the use of sand rivers as a potential water resource to support particularly the small-scale farmers. For that to be done, several field measurement and laboratory analysis have been carried out, such as geophysical survey, groundwater level measurements, hydraulic conductivity measurement, water chemistry and stable isotopes analysis, sediment and porosity analysis.
The study shows that the annual flood event of the Limpopo River is the main process that recharges the sand river aquifer. However, the majority the flood’s water is not stored in the sand river system but it refills the sand river aquifer to its initial groundwater level, which will be followed by progressive losses up to the next flood event. The geophysics revealed a total depth of around 15 meters in Guija and 10 meters in Chokwe, in which 10 (±1) m and 6 (±1) m are saturated by the dry period. In addition to that, the effective porosity is around 20% which can store a lot of water given that the sand river in both sites is underplayed by a clayey formation. The field measurements indicate that the groundwater from the sand river is discharging into the river in the dry period. The groundwater has low salinity and sodium hazard compared to the river’s water, which makes it more suitable for irrigation purposes. The main processes that affect the water quality are evaporation, degassing, dissolution and dilution. The dilution takes place only during the flood events while the other processes will continuously affect the water quality of the whole system. The study shows that the groundwater contained in the sand river tends to have similar characteristics as the rainwater, and its characteristics remain the same at the end of the dry period.