Groundwater flow modeling: a case study of the Lower Rusizi alluvial plain aquifer, North-Western Burundi

The study area, in northwestern Burundi, is an alluvial plain consisting of fine clayey sands
and coarse sands with mixed lithology. The aquifer of the lower Rusizi plain could be considered as
confined under a clay layer. A 2D horizontal groundwater flow model was developed under steady-state
conditions using the Modflow software. The study aims to determine the most productive
areas of this confined alluvial aquifer and the main aquifer inflow and outflow values together with
the recharge and river–aquifer interactions. The groundwater potential is dependent on the spatial
distribution of hydraulic conductivity and aquifer thickness values providing the local transmissivity
values. The calibrated model made it possible to assess the spatial distribution of the hydraulic
conductivity values at the regional scale, which ranged from 6 x 10-6 (contact between alluvial
plain and Precambrian basement) to 7.5 x 10-3 m/s (coastal barriers). The results also provided the
computed groundwater flow directions and an estimation of the groundwater levels in areas not
yet investigated by drilling. The results of the computed groundwater flow budget allowed us to
deduce that recharge and river–aquifer interaction constitute the main inflow while the downwards
boundaries (where piezometric heads could be prescribed) are the main zones where outflows
occur. The results of this model can be used in the planning of pumping test programs, locating
areas with high groundwater potential to plan water supply for different private and public users.
This predictive tool will contribute to the resolution of problems related to the use and integrated
management of the groundwater resource in this part of Burundi.