Investigating the Productivity and Sustainability of Weathered Basement Aquifers in Tropical Africa Using Numerical Simulation and Global Sensitivity Analysis

Marco Bianchi,David M J Macdonald,Alan M Macdonald,Enoch B Asare

doi:10.1029/2020wr027746

Marco Bianchi, David M J Macdonald + Show 2 more

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https://doi.org/10.1029/2020wr027746

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Abstract

AbstractGroundwater stored in weathered basement aquifers (WBAs) is a strategic water resource. In this study, we investigate the productivity of WBAs and sustainability of groundwater abstractions using a novel process‐based stochastic modeling approach, which is applied to simulate abstractions in the Precambrian basement aquifer in Ghana. The statistical distribution of the generated synthetic yield data was found in very good agreement with observed yield data from the same Ghanaian aquifer. Further analysis provided robust insights regarding how different hydrogeological parameters of the WBA, and their interplay, control aquifer productivity and sustainability. Results indicate that 97% of the simulated abstractions could sustain the yield of a hand pump (6 L/min), approximately 30% could also sustain yields >60 L/min, while only 1% could sustain yields greater than 300 L/min. The model indicates that an aquifer transmissivity value of approximately 1.4 m2/day is required for a successful hand‐pumped borehole, while a higher yielding source (60 L/min) requires a transmissivity value of at least 9.5 m2/day. A global sensitivity analysis of 13 model input parameters shows that the thickness of the regolith and the maximum hydraulic conductivity developed at the base of the saprolite are the critical factors controlling success and sustainability for low yielding hand‐pumped boreholes. For higher yielding supplies, the net recharge, the depth to groundwater, and the aquifer extent become increasingly significant. Results from this work have important implications for the potential for increased development of groundwater from WBAs in tropical Africa.

Highlights

Crystalline rocks underlie approximately 34% of Africa's land surface, where approximately half of the rural population live (MacDonald & Calow, 2009)
Much of the rural population depend on groundwater from the weathered basement as their primary source of drinking water, abstracting groundwater mostly with shallow wells and boreholes equipped with hand pumps (WHO/UNICEF, 2020)
The simulated data were truncated with a cutoff at 6 L/min to be consistent with the way the reference data are reported by Carrier et al (2011)

Summary

Introduction

Crystalline (igneous and metamorphic) rocks underlie approximately 34% of Africa's land surface, where approximately half of the rural population live (MacDonald & Calow, 2009). Much of the rural population depend on groundwater from the weathered basement as their primary source of drinking water, abstracting groundwater mostly with shallow wells and boreholes equipped with hand pumps (WHO/UNICEF, 2020). As demand for a reliable source of fresh water rises, groundwater stored in basement aquifers is expected to become increasingly developed due to its widespread availability (Braune & Xu, 2010; MacDonald et al, 2012) and ability to buffer climatic variability (Calow et al, 2010; Cuthbert et al, 2019; Taylor et al, 2013). The predicted increase in abstraction is to address the lack of basic drinking water (WHO/UNICEF, 2020) and for an increase in productive uses, mainly irrigation (Agrawal & Jain, 2019; Altchenko & Villholth, 2015), to help achieve the Sustainable Development Goals (SDG) (Velis et al, 2017). Hydrogeological investigations of WBAs in different parts of the world have shown that groundwater flow is commonly compartmentalized

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