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

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.

Identification of low fluoride areas using conceptual groundwater flow model and hydrogeochemical system analysis in the aquifer system on the flanks of an active volcano: Mount Meru, Northern Tanzania

“…In the Arusha volcanic region in northern Tanzania, around Mount Meru, several studies with focus on F⁻ concentration in surface water (Kilham and Hecky, 1973;Nanyaro et al, 1984;Kitalika et al 2018) and groundwater (Ghiglieri et al, 2012;Chacha et al, 2018;Makoba and Muzuka, 2019;Bennett et al, 2021) have been conducted. The studies found that the general groundwater chemistry is F⁻-rich NaHCO3 alkaline water.…”

“…The studies found that the general groundwater chemistry is F⁻-rich NaHCO3 alkaline water. The main findings of these studies have been briefly summarised in the study of Bennett et al (2021), which described the hydrochemical characterisation of high-fluoride groundwater on the eastern, northern, western and south-western flanks of Mount Meru, the subject of a parallel to this paper. The studies that focused on the groundwater chemistry in the area found that rock mineral dissolution, water residence times (Ghiglieri et al, 2012;Chacha et al, 2018;Makoba and Muzuka, 2019;Ijumulana et al, 2020), exchange processes, calcite precipitation (Ghiglieri et al, 2012) and climatic conditions (temperature and rainfall), attributed to altitude and geographic positions (windward and leeward sides of Mount Meru) (Makoba and Muzuka, 2019) are controlling the groundwater chemistry in the area.…”

“…This concentration is above both WHO limit (1.5 mg/L) (WHO, 2017) and Tanzanian limit (4.0 mg/L) (Tanzania Bureau of Standards, 2008). Also, the study by Bennett et al (2021) found that 59% of the inventoried usable groundwater points are used to provide drinking water to the local community despite their high F⁻ values, which exceed both WHO and Tanzanian limits, as most local people do not have any alternative for drinking water. Other than high F⁻ values, the study by Tomašek et al (2022) found elevated levels of multiple potentially toxic elements (i.e., Molybdenum and Uranium) in some of the analysed sources, pose another health concern to the local communities.…”

Groundwater Resources Management: Reconciling Demand, High Quality Resources and Sustainability

“…The potentiality of quality analysis to support groundwater management was highlighted by Benett et al [11]. They collect hydrochemical and hydrogeological data to study the high fluorite (F − ) groundwater concentration in an active volcano area of Tanzania.…”

“…The first theme concerns modelling efforts supporting the sustainable management of aquifer systems [8,9]. The second theme is focused on groundwater vulnerability assessments against external pollution, focusing on groundwater quality management [10][11][12][13]. The third theme explores the potentiality of geophysics tools supporting groundwater management in the case of coastal aquifers [14][15][16].…”