The role of superficial geology in controlling groundwater recharge in the weathered crystalline basement of semi-arid Tanzania

E Zarate,D Hobley,A.M Macdonald,R.T Swift,J Chambers,J.J Kashaigili,E Mutayoba,R.G Taylor,M.O Cuthbert

doi:10.1016/j.ejrh.2021.100833

Abstract

Study regionLittle Kinyasungwe River Catchment, central semi-arid Tanzania. Study focusThe structure and hydraulic properties of superficial geology can play a crucial role in controlling groundwater recharge in drylands. However, the pathways by which groundwater recharge occurs and their sensitivity to environmental change remain poorly resolved. Geophysical surveys using Electrical Resistivity Tomography (ERT) were conducted in the study region and used to delineate shallow subsurface stratigraphy in conjunction with borehole logs. Based on these results, a series of local-scale conceptual hydrogeological models was produced and collated to generate a 3D conceptual model of groundwater recharge to the wellfield. New hydrological insights for the regionWe propose that configurations of superficial geology control groundwater recharge in dryland settings as follows: 1) superficial sand deposits act as collectors and stores that slowly feed recharge into zones of active faulting; 2) these fault zones provide permeable pathways enabling greater recharge to occur; 3) ‘windows’ within layers of smectitic clay that underlie ephemeral streams may provide pathways for focused recharge via transmission losses; and 4) overbank flooding during high intensity precipitation events increases the probability of activating such permeable pathways. These conceptual models provide a physical basis to improve numerical models of groundwater recharge in drylands, and a conceptual framework to evaluate strategies (e.g., Managed Aquifer Recharge) to artificially enhance the availability of groundwater resources in these regions.

Highlights

Drylands cover 45 % of the earth’s surface (Pravalie, 2016) and support a population of around 2 billion people, 90 % of whom live in developing countries (UN, 2017)
It is likely that the structure and hydraulic properties of su perficial geology play a crucial role in controlling surface-groundwater interactions in drylands (Scanlon et al, 2006; Wheater et al, 2010)
We present a series of local- and basin-scale conceptual models that identify multiple potential recharge pathways, illustrating how su perficial geology may control focused recharge in the Little Kinyasungwe River Catchment

Summary

Introduction

Drylands (subhumid to hyper-arid regions) cover 45 % of the earth’s surface (Pravalie, 2016) and support a population of around 2 billion people, 90 % of whom live in developing countries (UN, 2017). Characterized by large atmospheric water demands and temperature contrasts (Cherlet et al, 2018), surface water is often seasonally or perennially absent in drylands (Wheater et al, 2008) and groundwater is often the principal and most reliable source of water in these regions (MacDonald et al, 2012; Shanafield and Cook, 2014). Heterogeneities in the hydraulic properties and structure of superficial deposits have been hypothesised to influence the subsurface dynamics of groundwater recharge to deeper aquifers (Acworth et al, 2020; Rau et al, 2017)

Objectives

Methods

Results

Discussion

Conclusion