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Episodes 2022; 45(1): 73-86

Published online March 1, 2022

https://doi.org/10.18814/epiiugs/2021/021006

Copyright © International Union of Geological Sciences.

Water-rock interaction, formation and circulation mechanism of highly bicarbonate groundwater in the northwestern geothermal prospects of Rwanda

by Innocent Ndikubwimana1, Xumei Mao1*, Jean Damascene Niyonsenga2, Dongbo Zhu1, and Schadrack Mwizerwa3

1School of Environmental Studies, China University of Geosciences, Wuhan 430074, China
2Faculty of Environmental Studies, Department Environmental Information System, University of Lay Adventists Kigali, Kigali, Rwanda
3Faculty of Engineering, China University of Geosciences, Wuhan 430074, China

Correspondence to:E-mail: maoxumei@cug.edu.cn

Received: January 18, 2021; Revised: May 3, 2021; Accepted: May 3, 2021

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Five thermal springs, twelve non-thermal springs, and two lake water samples from the northwestern part of Rwanda were studied to assess their chemical characteristics and infer the formation mechanism of the thermal waters. Multicomponent mineral equilibrium (MME) geothermometer calculations at Gisenyi prospects with the highest in situ measured temperature (73.1°C) showed the reservoir temperature of 90±6°C. The MME temperature estimates agreed well with Silica-based, K-Mg and Mg-Li geothermometers while the other cation geothermometers (Na-K, Na-K-Ca, Na-K-Ca-Mg, and Na-Li) results are unreliable. Most of the non-thermal springs are Ca-Mg-HCO3 water-type while the thermal spring waters were majorly Na-HCO3. The δD composition varied from -16.6 to -5.9‰ and from -11.8 to -5.0‰, while the δ18O ranged from -4.17 to -3.5‰ and -4.32 to -2.7‰, for thermal and non-thermal springs, respectively. All isotopic ratios scattered around the meteoric water lines, thus indicating their similar meteoric origin. In addition, there was no observable δ18O positive shift speculating less extent of water-rock interactions while geogenic CO2 ingress into the waters has been ascertained by both isotopic and chemical component ratios. We proposed a circulation mechanism of the thermal waters for the study area.