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 × 10⁻⁶ (contact between alluvial plain and Precambrian basement) to 7.5 × 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.

The present study area is an alluvial plain consisting of fine clayey sand and coarse sand with mixed lithology. In September 2018, a water sampling campaign was carried out in 28 wells spread all over the plain. The interpretation of the results from the physicochemical analysis allows determination of the hydrogeochemical behaviour and quality of the groundwater sampled in this plain. The spatial distribution of physical and chemical parameters reveals a clear demarcation between the east and the west of the study area. Water samples from wells located in the western part of the plain show a high mineralisation with high electrical conductivity and total dissolved solids. Water samples from the southern and eastern parts show intermediate to low mineralisation. Graphical representations of chemical analysis in the Piper diagrams reveal that the groundwater in the study area can be classified into five families. The evaluation of water quality shows that concentration values of some ions like Na⁺, Ca²⁺, Mg²⁺, Cl⁻, HCO₃⁻ and SO₄²⁻ are generally high in the western part of the plain along the Rusizi River with very high values particularly observed in the Rusizi delta.