Groundwater Flow Modeling: A Case Study of the Lower Rusizi Alluvial Plain Aquifer, North-Western Burundi

Pierre Claver Ngenzebuhoro,Ilias Kacimi,Louis Nahimana,Philippe Orban,Alain Dassargues,Tarik Bahaj

doi:10.3390/w13233376

Pierre Claver Ngenzebuhoro, Ilias Kacimi + Show 4 more

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https://doi.org/10.3390/w13233376

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Abstract

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.

Highlights

The aquifer of the lower Rusizi plain is one of the largest and the most important aquifers in Burundi, providing groundwater for domestic, agricultural, and industrial uses.A mathematical model is an interesting tool to integrate the hydrogeological processes that control the distribution and availability of groundwater in this aquifer for optimal management of this resource for present and future needs
The spatial distribution of the hydraulic conductivity values estimated by the model confirmed the heterogeneity of the aquifer in the lower Rusizi Plain
Compared to the rest of the Lower Rusizi plain, from the northeast to the southeast, corresponding to the limit between the alluvial plain and the Precambrian basement, the hydraulic conductivity values are generally low and range around 9 × 10−6 m/s with an exception observed in polygon K50 (Figure 8) where the hydraulic conductivity value was estimated at 5 × 10−4 m/s

Summary

Introduction

The aquifer of the lower Rusizi plain is one of the largest and the most important aquifers in Burundi, providing groundwater for domestic, agricultural, and industrial uses.A mathematical model is an interesting tool to integrate the hydrogeological processes that control the distribution and availability of groundwater in this aquifer for optimal management of this resource for present and future needs. Various studies, including a recent drilling campaign in the lower Rusizi plain, have been conducted in this area to characterize the local hydrogeological conditions of the lower Rusizi plain aquifer in the past [1,2,3]. All these studies have made it possible to collect different types of data and prepare them for the development of a reliable conceptual model to prepare the numerical model. The aquifer’s geological formations, (2) to the estimation of recharge, and (3) to the spatial distribution of hydraulic conductivity values in the study area. The main objective of this work is to determine the most productive zones in groundwater, to evaluate the water exchanges between the aquifer and different rivers, on the one hand, and between the aquifer and Lake Tanganyika on the other hand

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