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Hydrology around active volcanoes is strongly controlled by the interaction between groundwater, and the fluids, dissolved elements and heat associated with magmatic intrusion. The chemical and mechanical processes associated with magmatic unrest can result in observable changes in the hydrothermal system. Consequently, observations of chemical and...
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... One supporting factor is that the Rontu watershed has good permeability to recharge rainfall into the groundwater since the lithology is volcanic and alluvium. This result is similar to that presented by Jasim et al. [39], where areas with volcanic hydrogeological soil conditions have good capability to support urban development. ...
Citation: Husna, A.; Akmalia, R.; Rohmat, F.I.W.; Rohmat, F.I.W.; Rohmat, D.; Wijayasari, W.; Alvando, P.V.; Wijaya, A. Groundwater Sustainability Assessment against the Population Growth Modelling in Bima City, Indonesia. Water 2023, 15, 4262.
Abstract: Most of Indonesia's population lives in areas with volcanic-alluvium geological characteristics. Based on the national hydrogeological map of the Indonesian Geological Agency, areas with volcanic-alluvium geological conditions have high groundwater potential and potential for groundwater damage. This study aims to test the resilience of groundwater areas with volcanic-alluvial characteristics to population growth. The MODFLOW groundwater model was built based on the site's volcanic and alluvial geological conditions. This groundwater model was tested against pumping scenarios based on population water demand in 2011-2020 and then predicted population growth until 2030. The result shows that groundwater resilience in volcanic-alluvium locations has different characteristics based on lithology and population density characteristics. Urban areas that are mostly located in alluvium areas tend to have a linear groundwater decline pattern but have the sharpest groundwater decline gradient. In contrast, suburban areas in the alluvium-to-volcanic transition area initially experience exponential groundwater decline but change to linear, while rural areas located in volcanic areas that become the main development target have exponential groundwater decline characteristics. To counteract the continuous depletion of groundwater, researchers conducted a scenario for optimizing surface water use. Based on the results of the scenario, a 60% reduction in groundwater use is sufficient to stop continuous groundwater depletion. The results of this study can be used as a recommendation for long-term water resources management targets for volcanic and alluvium areas that are being targeted for development.
... To 161 achieve this goal, we systematically increased the matrix permeability (k m ) from 10 -16 162 m 2 to 10 -9 m 2 . The wide range of k m spanning about seven orders of magnitude is 163 reasonable given the diverse rock types in nature as reported in a previous study 164 (Jasim et al., 2019). In total, we have 12 scenarios with varying k m , possibly covering 165 dual porosity and dual permeability fracture-matrix systems. ...
... Furthermore, volcanic eruptions in these volcanic islands can strongly affect the island's hydrology (Herd et al., 2005;López-Saavedra and Martí, 2023), ultimately leading to potential biological and health hazards due to water and food supply contamination and worsening of hygienic conditions (Aubaud et al., 2013;Wiemken et al., 1981). The interplay between hydrothermal and hydrological processes at restless volcanoes is so intricate and influential that monitoring the associated activity is a fundamental tool for detecting and predicting volcanic unrest (Jasim et al., 2019). Valuable insights into volcanic activity can be obtained by monitoring variations in the chemical composition and temperature of groundwater, surface water, and steam at a volcano (Barberi et al., 1992). ...
... The permeability decreases with consolidation and increases with increasing porosity. The data was taken from a compilation provided byJasim et al. (2018). ...
Resource scarcity and climate change require a revision of the global energy production. Although the majority of energy production is still based on fossil fuels, a growing percentage comes from renewable energy resources. However, the increase in geothermal applications is small and energy produced by geothermal plants accounts for only a small part of the global energy demand. The importance of geothermal energy varies together with the given geological potential. In Iceland, with its favorable conditions, geothermal plants cover a high percentage of the local energy demand. The improvement of geological conditions through enhanced geothermal systems would enable a wide application of this renewable energy source in other countries as well. However, the current stimulation techniques for enhanced geothermal systems involve significant risks such as induced seismicity.
This thesis is part of the Ecofrac project, which aims to develop an environmental friendly, reservoir-wide chemical stimulation technique that eliminates the significant risks of conventional reservoir stimulation. This work focused on selecting the
environmental friendly acids and investigating favorable conditions for the stimulation. Laboratory experiments were conducted with the specially developed RESECO setup. The setup uses epoxy resin to prevent surrounding fluid flow during experi-
ments and proved suitable for a variety of stone types and pore geometries. Citric acid and also glycine showed high potential for reservoir stimulation for both carbonate and silicate rocks. However, the success of the stimulation depended heavily on
the stimulation conditions. Unfavorable conditions such as low acid concentration or low temperature could lead to severe fines migration. This process can be caused by the mobilization of existing fines or the generation of new fines due to mineral dis-
solution. Fines migration is a major threat to reservoir-wide chemical stimulation, as the fines cannot be removed as done during stimulation in the wellbore region. The success of reservoir-wide chemical stimulation depends on the dissolution mode. The reaction must be fast to prevent fines generation and long-lasting for high penetration depths into the reservoir. The reaction rate and the resulting dissolution mode are influenced by environmental conditions such as reservoir temperature or
fluid velocity, as well as acid concentration. Transport-limited dissolution modes with high Damköhler number are favorable for reservoir stimulation. As the environmental conditions vary from reservoir to reservoir, specific experiments have to be conducted on a case-by-case basis to identify ideal acid concentrations for
transport-limited dissolution modes. Validated with sufficient experimental results, numerical models like the one presented in this work can be used to determine ideal conditions and upscale the results to the reservoir scale.
... Volcanic eruptions can have significant impacts on groundwater resources, including changes in water quality and quantity, changes in the permeability of the subsurface, and contamination of water supplies (Taran and Kalacheva, 2020). Hydrology around active volcanoes is strongly controlled by the interaction between groundwater and the hydrothermal fluids, dissolved elements, and heat associated with magmatic intrusion (Jasim et al., 2019). The interplay between hydrological and volcanic systems plays a crucial role in volcanic disturbances (Albano et al., 2002). ...
... It is widely recognised that the variation of groundwater levels combined with more precise deformation data can provide valuable insights for eruption forecasting (Ingebritsen et al., 2021;Jasim et al., 2019;Nur et al., 2021;Poland and Anderson, 2020). The specific mechanisms causing these changes include strain, ground surface uplift or subsidence, recharge boiling, and alterations in aquifer permeability (Albano et al., 2002). ...
... This is because the amount of magma-contacted groundwater spatially changes, depending on the pathway of magma. In general, a volcano is permeable, and thus the infiltration and percolation of rainwater or snowmelt water easily occur at any time; though, in some cases, part of the water could be stored in the specified permeable layer producing groundwater flow [1]. For example, the eruption of Usu Volcano, Japan, in August 1977 occurred on the summit area as a magmatic eruption and ejected a large amount of pyroclastic materials [2]. ...
... ∆V/∆t = (P − E)A 0 + R in + G in − G out (1) where V is the water volume (m 3 ), A 0 is the water surface area (m 2 ), P is the precipitation (m/s) to the lake surface, E is the evaporation (m/s) at the lake surface, R in is the river inflow (m 3 /s), G in and G out are the groundwater inflow and outflow (m 3 /s), respectively, and t is time. The left side of Equation (1) indicates the water volume change per budget period ∆t. ...
... The discharge of the three inflowing streams (Figure 1b) was measured on non-rainfall days to evaluate river inflow R in in Equation (1). The stream water was also manually sampled to analyze the inorganic chemistry, related to C Rin in Equation (5). ...
Exploring how the hydrological and thermal conditions of a volcanic lake change in response to volcanic activity is important to identify the signs of a volcanic eruption. A water cycle system and a geothermal process in a crater lake, Okama, in the active Zao Volcano, Japan, were explored by estimating the hydrological and chemical budgets of the lake, and analyzing the time series of lake water temperature, respectively. In 2021, the lake level consistently increased by snowmelt plus rainfall in May–June, and then stayed nearly constant in the rainfall season of July–September. The hydrological budget estimated during the increasing lake level indicated that the net groundwater inflow is at any time positive. This suggests that the groundwater inflow to the lake is controlled by the water percolation into volcanic debris from the melting of snow that remained in the catchment. Solving the simultaneous equation from the hydrological and chemical budgets evaluated the groundwater inflow, Gin, at 0.012–0.040 m3/s, and the groundwater outflow, Gout, at 0.012–0.027 m3/s in May–September 2021. By adding the 2020 values of Gin and Gout evaluated at the relatively high lake level, it was found that Gin and Gout exhibit highly negative and positive correlations (R2 = 0.661 and 0.848; p < 0.01) with the lake level, respectively. In the completely ice-covered season of 15 December 2021–28 February 2022, the lake water temperature increased between the bottom and 15 m above the bottom at the deepest point, which reflects the geothermal heat input at the bottom. The heat storage change during the increasing water temperature was evaluated at a range of −0.4–5.5 W/m2 as the 10-day moving average heat flux. By accumulating the daily heat storage change for the calculated period, the water temperature averaged over the heated layer increased from 1.08 to 1.56 °C. The small temperature increase reflects a stagnant state of volcanic activity in the Zao Volcano. The present study could be useful to investigate how an active volcano responds to water percolation and geothermal heat.
... Geosonar scans so that the boundaries between layers are recorded in sufficient detail and can be read directly with the depth and porosity values. Rock type analysis was based on rock porosity values based on the results of studies conducted by Petrov et al. (2005), Giberti et al. (2006), Hemmings et al. (2015 and Jasim et al. (2019). An example of geosonar measurement results and their interpretation are presented in Figure 2. Aquifer characteristics were also determined based on geological and geomorphological conditions through analysis of Geological Maps, Hydrogeological Maps and Sounding Data. ...
... ,Petrov et al. (2005),Giberti et al. (2006),Hemmings et al. (2015) andJasim et al. (2019). ...
Article history: Like other natural resources, groundwater is also being exploited at an increasing rate, especially for domestic requirements. Groundwater is preferred as a domestic water source because of its continuous availability and relatively good quality. Unfortunately, not all places have sufficient groundwater availability of good quality. The purpose of this study was to analyze the characteristics of the aquifer in the study area and evaluate its groundwater potential for domestic needs. Aquifer characteristics were determined based on geological and geomorphological conditions, while groundwater potential was calculated using a static approach. The results showed that the characteristics of the aquifers in Kediri Regency are various. In the eastern and central parts of the study area, the characteristics of the aquifer can be in the form of unconfined aquifers with high productivity. In the western part, most of them have non-aquifer material, so it is difficult to find groundwater. Groundwater generally fills joints and diaclase formed in andesitic lava with low discharge. Although the conditions of the aquifer are various, in general, the potential for groundwater in Kediri Regency can still support its requirements because the potential for groundwater in Kediri Regency is 71,121,313,394 m 3 , while domestic requirement is 52,348,490 m 3 /year.
... Because the activity of hydrothermal fluids may be an indicator of magma stability, long-term continuous gravity observations can be a tool for monitoring the threats of volcanic eruptions in the TVG. The same idea for monitoring volcanic activity has also been raised in recent works, such as Jasim et al. (2018) and Hsieh and Ingebritsen (2019). ...
The Tatun Volcanic Group (TVG) is an active volcanic system that poses potential volcanic hazards to northern Taiwan. Groundwater migration can be sensitive to volcanic activities and can induce temporal gravity changes. Here we conducted four gravity measurement campaigns in 2012 with two CG5 relative gravimeters at a total of 31 gravity monitoring sites to probe the spatiotemporal groundwater variability in the TVG area. With careful adjustments to the gravity measurements, the standard point gravity error was approximately 8 μGal on average. The observed gravity changes were compared with the groundwater-induced gravity variations derived from two hydrological models: the continuous flow model that consisted of one permeable layer (Model A) and a dynamic groundwater flow model that used the hydrological solver MODFLOW (Model B). The results show that only gravity observations in the central TVG match the simulated gravity pattern, i.e., the hydrology in the central TVG follows free groundwater flows within the interconnected and porous strata. On the other hand, inconsistencies between simulated and observed gravity changes range from + 40 to – 80 μGal. The discrepancies were located at the eastern and western areas in the TVG and showed opposite variations over time. This pattern of temporal variability cannot be attributed to the use of shallow aquifer models that merely reflect the surface recharges and discharges. We infer that the pattern stems from deep flows, such as hydrothermal fluids alternately replenishing aquifers to the east and west of the TVG, i.e., deep fluid conduits may have developed to the east and west of the TVG and bypassed the central area.
... The model was developed based on hydrogeochemical processes, groundwater mineralisation, spring settings, and groundwater level in the shallow wells. The model suggests the influence of the volcanic gases on the groundwater mineralisation of the two hydrothermal springs (S42 and S43) and the two springs from the discharge area (S18 and STP6), which are flowing from the deepest flow line (Sawyer et al., 2008;Jasim et al., 2018;Fischer et al., 2019). ...
... anhydrite (CaSO4), thenardite (Na2SO4), and mirabilite (Na2SO4.10H2O) (Bakundukize et al., 2018;, and the admixture of volcanic gases (which are containing lots of CO2, SO2, HCl, HF) as Mount Meru is an active volcano (Sawyer et al., 2008;Jasim et al., 2018;Fischer et al., 2019). The significant moderate positive correlations of F⁻ with Na + and K + indicate that the progressive increase of F⁻ goes parallel with the increase in alkaline elements (suggesting progressive rockwater interaction), whereas the significant weak negative correlations of F⁻ with Ca 2+ and Mg 2+ indicate that the progressive increase of F⁻ goes parallel with the decrease in alkaline earth elements along the groundwater flow paths, through precipitation of carbonate minerals: ...
... The common natural sources for forming F⁻-rich groundwater include dissolution of natural F⁻-bearing minerals (such as amphibole, biotite, fluorapatite and fluorite) (Ghiglieri et al., 2012;Luo et al., 2018) and magmatic degassing related to volcanic activity (Sawyer et al., 2008;Jasim et al., 2018). Calcite precipitation, cation exchange, salinization and evaporation are important hydrogeochemical processes that allow for increasing F⁻ concentrations by reducing the Ca 2+ concentrations in groundwater (Nanyaro et al., 1984;Coetsiers et al., 2008;Luo et al., 2018). ...
In the Arusha volcanic region in northern Tanzania, within the eastern branch of the East African Rift System, water shortage is common and much of the surface water and groundwater contain fluoride (F⁻) concentration above the WHO limit (1.5 mg/L) recommended for drinking water. The groundwater is the main source of drinking water in the area. Prolonged intake of this high F⁻ water has caused dental and skeletal fluorosis among the local population. Limited studies particularly concerning the aquifer structure, groundwater potential, groundwater flow systems, and localities of low F⁻ groundwaters have been conducted in the area. In this study, primary data (field collected) and secondary data were used to study the aquifer characteristics, hydrochemistry, hydrogeochemistry, groundwater recharge, groundwater flow systems, and hydrothermal reservoir in the study area. The delineation of the aquifer structure and estimation of its hydraulic properties have been conducted with the aim of investigating the groundwater potential for different geological formations, to provide comprehensive knowledge for proper groundwater utilisation and management. The groundwater level monitoring was done for uncovering the spatiotemporal variation of groundwater levels in the aquifer and analysing the groundwater level response to rainfalls. Moreover, the characterisation of high F⁻ groundwater in the aquifer system has been done to understand the temporal and spatial changes in the groundwater chemistry, develop a conceptual groundwater flow model, and evaluate the hydrogeochemical processes responsible for temporal and spatial changes in the groundwater chemistry with the aim of identifying localities of low and high F⁻ groundwaters for the purpose to come up with guidelines to provide groundwater that can be used for drinking water supply without health impacts on the population. Lastly, the estimation of the temperature and circulation depth of the hydrothermal reservoir below the ash cone of Mount Meru was done to explore a possibility of finding low-temperature geothermal energy resources on the flanks of Mount Meru and develop a conceptual model for the hydrothermal reservoir in the area. The delineation of the aquifer structure using litho-hydrostratigraphical cross-sections and estimation of the hydraulic parameters using single well pumping tests show that the aquifer system on the flanks of Mount Meru is a sloping aquifer with sloping beds. On the far east of the eastern flank, the aquifer is composed of debris avalanche deposits, while on the north-eastern and west flanks the aquifer is composed of weathered fractured lava, whereas on the south-western flank, the aquifer is composed of different layers: pyroclastics on the top, weathered fractured lava, weathered pyroclastics, and again weathered fractured lava at the bottom. The aquifer is semi-confined on the north-eastern flank, by overlying debris avalanche deposits acting as an aquitard, while unconfined elsewhere. The transmissivity of the aquifer on the north-eastern flank is substantially increasing with increasing depth, while on the south-western flank, the transmissivity of the aquifer is variable, both at the shallow depth (exploited by hand-dug wells) and at larger depth (exploited by boreholes); indicating aquifer heterogeneity. On the north-eastern flank, the topmost part of the aquifer, exploited by hand-dug wells, has a low transmissivity (T=1.3 m2/d) and potential for smaller withdrawals for local water supply with limited consumption, while the upper part of the aquifer, captured by boreholes, has an intermediate transmissivity (T=35 m2/d) and potential for local water supply, whereas the deeper part of the aquifer has a high transmissivity (T=788 m2/d) with potential of somewhat regional importance. On the western flank, the aquifer has a very low transmissivity (T= 0.4 m2/d) and potential for local water supply with limited consumption. On the south-western flank, on average, the topmost part of the aquifer, exploited by hand-dug wells, has very low to intermediate transmissivity (range of T: 0.3–19 m2/d), leading to variable potential for smaller withdrawals for local water supply (private consumption), whereas the deeper part of the aquifer, captured by boreholes, has low to intermediate transmissivity (range of T: 9–43 m2/d) and potential for local water supply. For the analysis of the groundwater level response to rainfall in the shallow aquifer system on the flanks of Mount Meru, a conceptual model of groundwater flow has been defined, related to sloped aquifers, localised recharge, and propagation of groundwater waves (or pulses). The model has been used to explain the piezometric time series in the area. Groundwater flow on and around Mount Meru is occurring in shallow aquifers on the slopes of the mountain. Because of the large topographic differences and steepness of the slopes as well as the depositional mechanism, aquifer beds and layer bottoms show significant tilt and groundwater flow is not only controlled by piezometric gradients but also by gravity effects. The conceptual model was verified in the time series from monitored wells on all sides of the mountain and can explain the water level variations that are observed. Thus, the estimation of groundwater recharge using the water-table fluctuation (WTF) method or using models that utilise Darcy’s flow equation is unfit for the estimation of the diffuse recharge in the area. On the south-western flank of Mount Meru, the water level variations are very smooth, indicating a high hydraulic diffusivity in this aquifer. The geomorphology of the landscape in the study area plays a great role in controlling the groundwater flow paths. The general groundwater flow system on each flank is involving a multidirectional flow from the higher elevation areas, including the parasitic cones, towards the lower areas. The characterisation of 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, and the analysis of the impact of rainwater recharge on groundwater chemistry by monitoring spring discharges during water sampling 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. The investigation of the localities of low and high F⁻ groundwaters using the conceptual groundwater flow model and hydrogeochemical system analysis in the aquifer system in the study area for the purpose to come up with guidelines to provide groundwater that can be used for drinking water supply without health impacts on the population, shows that the groundwater chemistry of F⁻-rich NaHCO3 alkaline groundwater in the area is controlled by dissolution of weathering aluminosilicate minerals (especially Na-K-feldspars), dissolution of F⁻-bearing minerals, the precipitation of carbonate minerals as secondary products and the dissolution of magmatic gases. Evaporative concentration of solutes, precipitation and redissolution of evaporitic salts may locally play a role, especially on the north-eastern flank of Mount Meru. The low F⁻ groundwaters which can be used for drinking water supply without health impacts under the WHO limit (1.5 mg/L) are the low-fluoride springs from the high-altitude recharge areas on the eastern and north-western flanks of Mount Meru inside Arusha National Park, whereas on the western flank the groundwater meets the Tanzanian limit (4.0 mg/L). On the south-western flank, the shallow aquifer composed of alluvium deposits at lower elevations, shows F⁻ values that meet the Tanzanian limit. One of the three investigated deep boreholes on this flank also meets the Tanzanian limit, this suggests a possibility of finding more localities of relatively low F⁻ groundwaters in the deep aquifer. Yet, in general, the deposits at lower elevations (the debris avalanche deposits, mantling ash, alluvial fan deposits and lake deposits) are found to contain high to very high F⁻ values, whereas the deposits at high elevations (pyroclastics and lavas) contain groundwater of low F⁻ values. Thus, the internal texture and grain size of geological formations (causing variable weatherability), the burial depth of these formations (less weathering at depth) and the water residence times are the factors determining the groundwater mineralisation and F⁻ concentrations in the area. Deposits that are more weathered (e.g., debris avalanche deposits, mantling ash, alluvial fan deposits and lake deposits) have higher F⁻ values, whereas deposits at larger burial depths (e.g., fractured weathered lava) are less weathered and have low F⁻ values. Groundwater with a longer residence time (i.e., mature water) has a high F⁻ value, whereas groundwater with a shorter residence time (i.e., young water) has a low F⁻ value. The study identified that the deep hydrothermal system has influence on the high F⁻ groundwaters on the eastern and north-eastern flanks of Mount Meru. The estimation of the temperature and circulation depth of the hydrothermal reservoir below the ash cone of Mount Meru by using classical solute geothermometry analysis of the two hydrothermal springs located at the foot of the ash cone shows that, using silica geothermometers, the estimated reservoir temperatures range from 50–90 °C (average temperature: 75 °C), indicating a low-temperature hydrothermal reservoir. We would expect higher temperature on an active volcano, such as Mount Meru, the low temperature can be attributed to a great extent of cold groundwater mixing with the hydrothermal waters. The estimated circulation depth of the hydrothermal springs ranges from 0.6–1.4 km deep; the circulation depth may be shallower, since the faster velocity along the fractures will result in shallower depth of circulation (and water-rock interaction). Therefore, there is a possibility of finding low-temperature geothermal energy resources on the eastern flank of Mount Meru since the deep hydrothermal system has influence on the groundwater chemistry on the eastern and north-eastern flanks of Mount Meru. A preliminary conceptual model for the hydrothermal reservoir in Mount Meru is presented in this thesis. We recommend further working towards proving the existence of a potential groundwater reservoir below the ash cone, as well as a detailed analysis of the hydrothermal system in the area.
... The common natural sources for forming F − -rich groundwater include dissolution of natural F − -bearing minerals (such as amphibole, biotite, fluorapatite and fluorite) (Ghiglieri et al., 2012;Luo et al., 2018) and magmatic degassing related to volcanic activity (Sawyer et al., 2008;Jasim et al., 2018). Calcite precipitation, cation exchange, salinization and evaporation are important hydrogeochemical processes that allow for increasing F − concentrations by reducing the Ca 2+ concentrations in groundwater (Nanyaro et al., 1984;Coetsiers et al., 2008;Luo et al., 2018). ...
... However, no measurements were done to know what gases were exsolving from the magma. The two hydrothermal springs located just at the foot of the Ash cone show high values of alkalinity, SO 4 2− , Cl − and F − compared to the surrounding springs (Bennett et al., 2021), this suggests the influence of the admixture of volcanic gases: CO 2 , SO 2 , HCl and HF from depth (Sawyer et al., 2008;Jasim et al., 2018). This same activity can be expected to also affect much of the other groundwater, be it in a much weaker measure. ...
This study investigates the localities of low and high F⁻ groundwaters in the aquifer system on the flanks of Mount Meru to come up with guidelines to provide groundwater that can be used for drinking water supply without health impacts on the population. Our study focuses on parts of the flanks which were only partially or not at all covered by previous research. Results show that the groundwater chemistry of F⁻-rich NaHCO3 alkaline groundwater in the area is controlled by dissolution of weathering aluminosilicate minerals, dissolution of F⁻-bearing minerals, the precipitation of carbonate minerals as secondary products and the dissolution of magmatic gases. The low F⁻ groundwaters which can be used for drinking water supply without health impacts under the WHO limit (1.5 mg/L) are the low-fluoride springs from the high altitude recharge areas on the eastern and north-western flanks of Mount Meru inside Arusha National Park, whereas on the western flank the groundwater meets the Tanzanian limit (4.0 mg/L). On the south-western flank, the shallow aquifer composed of alluvium deposits at lower elevations, shows F⁻ values that meet the Tanzanian limit. One of the three investigated deep boreholes on this flank also meets the Tanzanian limit, suggesting a possibility of finding relatively low F⁻ groundwaters in the deep aquifer. Yet, in general, the deposits at lower elevations are found to contain high to very high F⁻ values, whereas the deposits at high elevations contain groundwater of low F⁻ values. Thus, the internal texture and grain size of geological formations, the burial depth of these formations and the water residence times are the factors determining the groundwater mineralisation and F⁻ concentrations in the area. The study identified that the deep hydrothermal system has influence on the high F⁻ groundwaters on the eastern and north-eastern flanks of Mount Meru.
