Browsing by Author "Kisaka, Mary"

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    Geogenic groundwater pollution in volcanic rock aquifer systems on the eastern, western and northern flanks of Mount Meru, Tanzania–special reference to fluoride
    (Vrije Universiteit Brussel, 2019) Bennett, George; Rombaut, Stefanie; Van Reybrouck, Jill; Shemsanga, Ceven; Kisaka, Mary; Tomašek, Ines; Fontijn, Karen; Kervyn, Matthieu; Walraevens, Kristine
    No abstract
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    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
    (MPDI, 2021) Bennett, George; Reybrouck, Jill Van; Shemsanga, Ceven; Kisaka, Mary; Tomašek, Ines; Fontijn, Karen; Kervyn, Matthieu; Walraevens, Kristine
    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.
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    Hydrogeochemical characteristics of shallow groundwater in volcanic rock aquifer systems in the western and northern flanks of Mount Meru, Tanzania
    (Ghent University, 2018) Bennett, George; Walraevens, Kristine; Van Reybrouck, Jill; Segers, Laura; Shemsanga, Ceven; Kisaka, Mary; Kervyn, Matthieu; Fontijn, Karen
    .No abstract
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    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
    (Elsevier, 2022) Bennett, George; Reybrouck, Jill Van; Shemsanga, Ceven; Kisaka, Mary; Tomašek, Ines; Fontijn, Karen; Kervyn, Matthieu; Walraevens, Kristine
    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.
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    The late quaternary eruptive history of Meru volcano, northern Tanzania
    (Elsevier, 2021) Kisaka, Mary; Fontijn, Karen; Shemsanga, Ceven; Tomašek, Ines; Gaduputi, Sankaranna; Debaille, Vinciane; Delcamp, Audray; Kervyn, Matthieu
    Mt. Meru, in the northern Tanzania divergence zone within the East African Rift System, is a historically active volcano, with its last eruption in 1910 CE. The flank deposits of Meru are dominated by lava flows, debris avalanche deposits as well as major pyroclastic formations indicative of Plinian-style eruptions. The stratigraphy, spatial extent, and chronology of these pyroclastic deposits have, however, not been systematically studied. Here we report on the detailed reconstruction of the stratigraphy and eruptive dynamics of Late Quaternary Meru explosive eruptions, based on field investigations, geochronological and geochemical analyses. The findings indicate that Meru had at least three moderate-to-large-scale explosive eruptions over the past 40,000 years. The oldest Meru explosive event we recognize (MXP1) generated pyroclastic density currents (PDCs), while the second (MXP2) and the third, most intense explosive event (MXP3) generated both pyroclastic fallout and PDCs. Conventional radiocarbon dating of 6 palaeosols underlying MXP2 and MXP3, dated at ~34.1–38.5 ka cal BP and ~31.5–36.9 ka cal BP respectively, suggest these two eruptions may have followed each other relatively close in time. The compositional range of pumice lapilli of both MXP2 and MXP3 is limited to tephriphonolite-phonolite. Dispersal and thickness data of the better preserved and larger MXP3 deposits suggest a minimum bulk volume of 2.5 km3 of pumice fallout and 1.2 km3 of PDCs, respectively, which corresponds to a total erupted mass of at least 2.7 × 1012 kg and a magnitude of 5.4. Similar large-scale eruptions in the future would have a considerable impact on the nearby large urban population in Arusha city and its suburbs. This new information is, therefore, vital for the long-term volcanic hazard assessment in Northern Tanzania.
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    Naturally occurring potentially toxic elements in groundwater from the volcanic landscape around Mount Meru, Arusha, Tanzania and their potential health hazard
    (Elsevier, 2021) Tomašek, Ines; Mouri, Hassina; Dille, Antoine; Bennett, George; Bhattacharya, Prosun; Brion, Natacha; Elskens, Marc; Fontijn, Karen; Gao, Yue; Gevera, Patrick Kirita; Ijumulana, Julian; Kisaka, Mary; Leermakers, Martine; Shemsanga, Ceven; Walraevens, Kristine; Wragg, Joanna; Kervyn, Matthieu
    The population of the semi-arid areas of the countries in the East African Rift Valley (EARV) is faced with serious problems associated with the availability and the quality of the drinking water. In these areas, the drinking water supply largely relies on groundwater characterised by elevated fluoride concentration (> 1.5 mg/L), resulting from interactions with the surrounding alkaline volcanic rocks. This geochemical anomaly is often associated with the presence of other naturally occurring potentially toxic elements (PTEs), such as As, Mo, U, V, which are known to cause adverse effects on human health. This study reports on the occurrence of such PTEs in the groundwater on the populated flanks of Mt. Meru, an active volcano situated in the EARV. Our results show that the majority of analysed PTEs (Al, As, Ba, Cd, Cr, Cu, Fe, Mn, Ni, Se, Sr, Pb, and Zn) are within the acceptable limits for drinking purpose in samples collected from wells, springs and tap systems, suggesting that there is no immediate health risk associated with these PTEs. However, some of the samples were found to exceed the WHO tolerance limit for U (> 30 μg/L) and Mo (> 70 μg/L). The sample analysis also revealed that in some of the collected samples, the concentrations of total dissolved solids, Na+ and K+ exceed the permissible limits. The concerning levels of major parameters and PTEs were found to be associated with areas covered with debris avalanche deposits on the northeast flank, and volcanic ash and alluvial deposits on the southwest flanks of the volcano. The study highlights the need to extend the range of elements monitored in the regional groundwater and make a more routine measurement of PTEs to ensure drinking water safety and effective water management measures