An Overview of Aquifer Physiognomies and the δ18O and δ2H Distribution in the South African Groundwaters

Tamiru A Abiye,Haile Mengistu,Molla B Demlie

doi:10.3390/hydrology8020068

Abstract

A comprehensive assessment of the stable isotope distribution in the groundwater systems of South Africa was conducted in relation to the diversity in the aquifer lithology and corresponding hydraulic characteristics. The stable isotopes of oxygen (18O) and hydrogen (2H) in groundwater show distinct spatial variation owing to the recharge source and possibly mixing effect in the aquifers with the existing water, where aquifers are characterized by diverse hydraulic conductivity and transmissivity values. When the shallow aquifer that receives direct recharge from rainfall shows a similar isotopic signature, it implies less mixing effect, while in the case of deep groundwater interaction between recharging water and the resident water intensifies, which could change the isotope signature. As aquifer depth increases the effect of mixing tends to be minimal. In most cases, the isotopic composition of recharging water shows depletion in the interior areas and western arid zones which is attributed to the depleted isotopic composition of the moisture source. The variations in the stable isotope composition of groundwater in the region are primarily controlled by the isotope composition of the rainfall, which shows variable isotope composition as it was observed from the local meteoric water lines, in addition to the evaporation, recharge and mixing effects.

Highlights

Environmental isotopes are routinely used worldwide in the study of surface water and groundwater systems, as they provide unique information on the transport and interconnectivity of the resources and their reservoirs [1,2]
Limited areas of the South African landmass are characterized by primary porosity aquifers that are limited to the Kalahari Group sediments, Cenozoic coastal sediments such as the Maputaland Group, Algoa Group, and some basaltic areas in the Karoo sequence
Aquifers in South Africa have diverse hydraulic characteristics where hydraulic conductivity plays a crucial role in facilitating groundwater recharge from rainfall, delaying recharge in the vadose zone and favoring evaporation on the surface, while transmissivity allows the mixing of recharged water during circulation within aquifers

Summary

Introduction

Environmental isotopes are routinely used worldwide in the study of surface water and groundwater systems, as they provide unique information on the transport and interconnectivity of the resources and their reservoirs [1,2]. Since precipitation is the predominant source of water both for surface water and groundwater reservoirs, monitoring of the isotopic composition of rainfall helps to understand the source for moisture, condensation, and moisture transport processes in the atmosphere, on land, and in the subsurface up to the time of groundwater recharge. Groundwater recharge from rainfall and surface water sources could take place either directly or indirectly, which is an important process for the renewability of the scarce water resource in arid and semi-arid regions including southern Africa. The presence of large water bodies that can generate sufficient vapor for rainfall has direct implications for the variation of stable isotopes of atmospheric water, surface water, and groundwater. The arid and semi-arid climatic conditions of South Africa are known to create

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Results

Conclusion