Using field assessment and numerical modelling tools to optimize a water abstraction system in the Shashane Sand River aquifer (Zimbabwe)

In the perspective of finding alternative water resources for rural communities to support current needs and future increased crop production, a portion of the Shashane River in the southern part of Zimbabwe is considered as a case study to characterize sand river alluvial aquifers and analyse their potential in sustaining such development schemes. Field assessment was coupled with modelling to achieve the objective of this research. Soil gradation analysis in combination with in-situ slug tests and geophysical assessment were performed to determine the hydraulic properties that define the geometry, storage and flow capacity of the aquifer. Discharge records as well as groundwater level data were used to provide a dynamic analysis on the recharge frequency and the potential water losses from the system. The findings were used to build a 3D transient numerical groundwater model in order to understand the groundwater flow and the natural variation of the storage. In addition, the model was used to estimate the recharge of the system as well as assess the impact of intensified abstractions on the storage and the different hydrologic components of the aquifer. The aquifer has an available storage capacity of 0.35Mm3 along the studied stretch of 5.6km. Such capacity is depleted naturally to its half due to evaporation. Less water is evaporated when abstractions are increased. The system has the potential to fulfil a total demand of 800 m3/day without failure which can irrigate a potential area of 3.5ha per km. Despite the increased abstractions, the recharge in the following wet season is almost certainly ensured by surface flow except for some extended dry years that occur once every 20 years. The increased abstractions lead to an increase in recharge which might reduce runoff in the following year to a certain extent and thus affect other users downstream, including dependent ecosystems, which requires further studies. The aquifer studied can support substantial socio-economic development plans as it was shown that if coupled with good management practices, it can lead to considerable livelihood improvement. The use of the model can be extended to test different development alternatives and serves as a tool to justify future development decisions