Hydrochemical Characterisation of High-Fluoride Groundwater and Development of a Conceptual Groundwater Flow Model Using a Combined Hydrogeological and Hydrochemical Approach on an Active Volcano: Mount Meru, Northern Tanzania

George Bennett,Jill Van Reybrouck,Karen Fontijn,Kristine Walraevens,Mary Kisaka,Ceven Shemsanga,Ines Tomašek,Matthieu Kervyn

doi:10.3390/w13162159

Abstract

This study characterises high-fluoride groundwater in the aquifer system on the flanks of Mount Meru, focusing on parts of the flanks that were only partially or not at all covered by previous research. Additionally, we analyse the impact of rainwater recharge on groundwater chemistry by monitoring spring discharges during water sampling. The results show that the main groundwater type in the study area is NaHCO3 alkaline groundwater (average pH = 7.8). High F− values were recorded: in 175 groundwater samples, the concentrations range from 0.15 to 301 mg/L (mean: 21.89 mg/L, median: 9.67 mg/L), with 91% of the samples containing F− values above the WHO health-based guideline for drinking water (1.5 mg/L), whereas 39% of the samples have Na+ concentrations above the WHO taste-based guideline of 200 mg/L. The temporal variability in F− concentrations between different seasons is due to the impact of the local groundwater recharge. We recommend that a detailed ecohydrological study should be carried out for the low-fluoride springs from the high-altitude recharge areas on the eastern and northwestern flanks of Mount Meru inside Arusha National Park. These springs are extracted for drinking purposes. An ecohydrological study is required for the management of these springs and their potential enhanced exploitation to ensure the sustainability of this water extraction practice. Another strategy for obtaining safe drinking water could be to use a large-scale filtering system to remove F− from the groundwater.

Highlights

In Tanzania, the availability of water is constricted by the increasing water needs due to a rapid population growth and ongoing climate change that is responsible for repeated drought episodes and the drying up of surface water bodies [1]
This study found that the aquifer composed of fractured mafic volcanics, breccia, and tuffs shows low total dissolved solids (TDS) and F− values compared to the aquifer composed of lahars, which are susceptible to weathering and lead to high TDS and F− values
Na+ is the dominant cation in 99% (n = 180) of the analysed water samples (n = 181), whereas HCO3 − + CO3 2− is the dominant anion in all samples; Water 2021, 13, x FOR PEER REVIEWtherefore, the main water type for both surface water and groundwater in the 11 of 30 area is

Summary

Introduction

In Tanzania, the availability of water is constricted by the increasing water needs due to a rapid population growth and ongoing climate change that is responsible for repeated drought episodes and the drying up of surface water bodies [1]. Groundwater is the main source of domestic water supply for both urban and rural areas in Tanzania, which consumes about 60% of total groundwater use whereas irrigation, mining and industrial use, livestock, and dry land fishing consume 40% [1]. The main challenges for groundwater quantity are depletion of shallow aquifers due to over-pumping, climate change, conflicting land uses, and uncontrolled urban development [1,2,3]. High F− concentrations exceeding the WHO health-based guideline for drinking water (1.5 mg/L) [10] have been reported in both the surface and groundwater in Tanzania [2,3,4,5,6,7,11], Ethiopia [12,13], and Kenya [14,15,16]

Methods

Results

Conclusion