Hydrogeochemical study in the Main Ethiopian Rift: new insights to the source and enrichment mechanism of fluoride
The central Main Ethiopian Rift suffers a severe water quality problem, characterized by an anomalously high fluoride (F) content that causes an endemic fluorosis disease. The current study, conducted in the Ziway–Shala lakes basin, indicates that the F content exceeds the permissible limit for drinking prescribed by the World Health Organization (WHO; 1.5 mg/l) in many important wells (up to 20 mg/l), with even more extreme F concentration in hot springs and alkaline lakes (up to 97 and 384 mg/l respectively). The groundwater and surface water from the highlands, typically characterized by low total dissolved solids (TDS) and Ca (Mg)–HCO3 hydrochemical facies, do not show high F content. The subsequent interaction of these waters with the various rocks of the rift valley induces a general increase of the TDS, and a variation of the chemical signature towards Na–HCO3 compositions, with a parallel enrichment of F. The interacting matrixes are mainly rhyolites consisting of volcanic glass and only rare F-bearing accessory minerals (such as alkali amphibole). Comparing the abundance and the composition of the glassy groundmass with other mineral phases, it appears that the former stores most of the total F budget. This glassy material is extremely reactive, and its weathering products (i.e. fluvio/volcano-lacustrine sediments) further concentrate the fluoride. The interaction of these “weathered/reworked” volcanic products with water and carbon dioxide at high pH causes the release of fluoride into the interacting water. This mainly occurs by a process of base-exchange softening with the neo-formed clay minerals (i.e. Ca–Mg uptake by the aquifer matrix, with release of Na into the groundwater). This is plausibly the main enrichment mechanism that explains the high F content of the local groundwater, as evidenced by positive correlation between F, pH, and Na, and inverse correlation between F and Ca (Mg). Saturation indices (SI) have been calculated (using PHREEQC-2) for the different water groups, highlighting that the studied waters are undersaturated in fluorite. In these conditions, fluoride cannot precipitate as CaF2, and so mobilizes freely without forming other complexes. These results have important implications for the development of new exploitation strategies and accurate planning of new drilling sites.
KeywordsMain Ethiopian Rift Aqueous geochemistry Geoscience and health Leaching tests Water–rock/sediment interaction
Fluorine is the lightest element of the halogen groups, the most electronegative (Pauling 1960) and reactive of all chemical elements and is mobile under high-temperature conditions. The earth’s crust has an average abundance of 625 mg/kg (Edmunds and Smedley 2005). The concentration of fluorine in most basaltic rocks ranges from 0.01 to 0.1 wt% (Allmann and Koritnig 1974) whereas most granites and rhyolites show a range of 0.01 to 0.2 wt% (Brehler and Fuge 1974). The majority of fluoride on the Earth’s surface is derived from rock minerals whereas other sources such as air, seawater and anthropogenic activities constitute a relatively small proportion (Fuge 1988; Lahermo et al. 1991). Fluorine forms the main mineral fluorite (CaF2), and considering the similarity of ionic radius (F− = 1.23–1.36 Å, OH− = 1.37–1.40 Å; Evans Jr 1995; Brownslow 1996) and charge, it can partially replace OH− in the structure of hydrous minerals, i.e., amphiboles, micas, apatite and clay minerals (Handa 1975; Pickering 1985; Wenzel and Blum 1992; Subba Rao and Devadas 2003).
In natural waters fluorine is commonly a minor component, with contents usually lower than 1 mg/l. Even taking into consideration aquifers containing fluorine-bearing minerals, waters rarely contain more than 8 mg/l, due to the very low solubility of F-bearing minerals (Boyle and Chagnon 1995). Although CaF2 is one of the major sources of fluoride, its solubility in fresh water and its dissociation rate are very low (Nordstrom and Jenne 1977). Generally, groundwater fluoride is introduced through water–rock interaction in the aquifers (Edmunds and Smedley 1996; Nordstrom et al. 1989; Gizaw 1996; Saxena and Ahmed 2001; Saxena and Ahmed 2003; Carrillo-Rivera et al. 2002).
In particular, the natural waters of the East African Rift are characterized by a high fluoride concentration (Kilham and Hecky 1973; Chernet 1982; Calderoni et al. 1993; Gizaw 1996; Darling et al. 1996) that mostly exceeds 1.5 mg/l, i.e., the tolerance limit for drinking water (WHO 1984). Above this threshold, the high fluoride concentration causes dental fluorosis (above 1.5 mg/l), skeletal fluorosis (above 4 mg/l) and crippling fluorosis (above 10 mg/l; Dissanayaka 1991). The population in the study area is affected by these health problems (mottled teeth and skeletal fluorosis; Tekle-Haimanot et al. 1987; Kloos and Tekle-Haimanot 1999) that are clearly linked to the high fluoride concentration observed in the waters of the rift.
Though the health related impact of fluorine is well known, the hydrogeochemistry of fluorine, with respect to the controlling mineral phases, and the processes that induce its enrichment are not extensively studied in the Ethiopian rift system. Therefore, the purpose of this study is to investigate the problem of fluoride with respect to its sources and genesis.
In this framework we performed an integrated study of both waters and coexisting solid aquifer matrixes in the Ziway–Shala lakes basin of the Ethiopian rift valley, in order to unravel the water–rock/sediment interactions that ultimately lead to the peculiar geochemical features of the Ethiopian rift waters. Therefore, the hydrochemical investigation was coupled with the mineralogical/geochemical characterization of the lithologies outcropping in the area. Moreover, laboratory leaching tests were also carried out to evaluate the potential release of fluoride from the various rock/sediment types. These approaches serve to understand the lithologic sources and the enrichment mechanisms controlling the anomalous fluoride content in the water.
Geographic and geologic description of the area
Different climatic conditions characterize the highlands, the escarpment and the rift valley. Annual rainfall ranges from around 650 mm in the rift valley to 1,100 mm in the highlands (Ayenew 1998). Mean annual temperature is less than 15°C in the highlands and more than 20°C in the lowlands, and evaporation ranges from more than 2,500 mm on the rift floor to less than 1,000 mm in the highlands (Le Turdu et al. 1999).
The central sector of the MER is characterized by a chain of lakes (Ziway–Langano–Abijata–Shala), where the outcropping lithologies consist of volcanites (pyroclastic products of felsic composition and subordinate basaltic lava flows) and sediments (Benvenuti et al. 2002); the latter are characterized by fluvio-lacustrine and volcano-lacustrine facies and represent weathered/remobilized volcanic rocks and silicic tephra.
In particular, most of the rift floor is covered by silicic pyroclastic materials mainly consisting of peralkaline rhyolitic ignimbrites, interlayered with basalts and tuffs, associated with layered and unwelded pumices (Di Paola 1972). Ashes are frequently found inter-bedded with ignimbrites and pumice layers.
The western escarpment consists of basaltic lava flows, with inter-bedded ignimbritic horizons, overlain by massive rhyolites, tuffs and basalts (Di Paola 1972; Merla et al. 1979; Woldegabriel et al. 1990). The eastern plateau is characterized by shield volcanoes consisting of mainly trachytes with subordinate basalts and phonolites (Di Paola 1972). For the most part the rift floor is covered with lacustrine deposits and volcano–clastic and fluvial sediments.
Active faulting within the rift valley causes a geothermal circulation of hot fluids. This could be due to the persistence of magma at shallow depth, generating heat that induces geothermal and fumarolic activities (e.g. at Aluto-Langano; Fig. 1) and high temperature thermal fluids outpouring as springs bordering the lakes.
Sampling and analytical techniques
The sampling strategy, that is, the selection of the sampling sites (reported in Fig. 1), was planned taking into consideration the previous hydrogeological/geological/geochemical studies of the area. Field work was performed during dry seasons (January 2006 and 2007). Water samples were collected from hot springs, lakes, boreholes, rivers and stored in 100 ml polyethylene bottles after filtering using 0.45 μm membrane filters. pH, electrical conductivity, and temperature were measured in situ. Out of 48 new water samples, 11 are from hot springs, 2 are from deep geothermal wells, 22 are from groundwater wells, 5 are from lakes and 8 are from rivers.
Analyses of major cations (Na, K, Mg, Ca) were carried out at the Department of Earth Sciences of the University of Ferrara using AA spectrometry, anions (F, Cl, SO4) were analyzed by ion chromatography and spectral photometry. Titration techniques were used to analyze total alkalinity (CO3 + HCO3). Analytical precision and accuracy are estimated as better than 5% (10% at sub-ppm levels) for both anions and cations on the basis of repeated analysis of samples and standards. The reproducibility of the data has been also cross-checked in the external laboratories of the Technical-Industrial Institute (ITI) of Ferrara.
Rock samples were preliminary characterized studying thin-sections at the transmitted light microscope. Whole-rock analysis of selected samples were carried out by X-ray fluorescence (XRF) using a wavelength-dispersive automated ARL Advant’X spectrometer at the Department of Earth Sciences of the University of Ferrara. Accuracy and precision, based on the analysis of certified international standards, are estimated as better than 3% for Si, Ti, Fe, Ca and K, and 7% for Mg, Al, Mn, Na. In situ analyses of the constituent phases (minerals and volcanic glasses) were carried out with a Cameca SX 50 microprobe (CNR-IGG Institute of Padova) using natural silicates and oxides as standards.
Analytical results and discussion
Geochemistry of water in the Main Ethiopian Rift
The geochemical features of water in the studied area [Table 1, electronic supplementary material (ESM)] are extremely variable, ranging from low TDS (and low F) in rivers to very high TDS (and high F) in the rift groundwater wells, hot springs and lakes (Table 1, ESM). The average TDS in the groundwater wells and hot springs are 1,050 and 3,610 mg/l respectively. TDS is even more variable in lake waters, with values which vary from the fresh water Ziway lake (379 mg/l), brackish water Langano lake (1,377 mg/l) to the saline Shala, Abijata and Chitu lakes (11,563; 52,725 and 64,267 mg/l respectively). pH ranges from near neutral to alkaline. An extreme value up to pH 10 is recorded in the alkaline lake Chitu.
Groundwater from the highlands (Ayenew 2008), typically show Ca (Mg)–HCO3 hydrochemical facies similar to that of rivers and cold springs where the sum of Ca and Mg exceeds Na and K. On the contrary hot springs and most groundwater in the rift display a Na–HCO3 fingerprint, with Na and HCO3 proportions constituting more than 80% of all the ionic species in the solution. Often the high fluoride concentration is associated with Na–HCO3 type of waters.
Therefore, it can be concluded that there is a general compositional change from Ca–Mg/HCO3 to Na–HCO3 along the groundwater flow path from the highlands to the rift floor (Ayenew 2005). Fluoride concentration increases during this evolution displaying low values in Ca–Mg/HCO3 waters from the highlands and higher values in the Na–HCO3 waters from the rift. This enrichment has been associated with lithologic variability and geothermal manifestations, although the relative importance of the two has not yet been established.
Saturation indices (SI) of fluorite (CaF2) and calcite (CaCO3) were calculated to constrain the observed chemical evolution (Appelo and Postma 2005). Calculations were carried out using the PHREEQC-2 software (Parkhurst and Appelo 1999) with the standard PHREEQC, WATEQ4F database (Ball and Nordstrom 1991), and a database derived from MINTEQA2 (Allison et al. 1990). The saturation indices of fluorite obtained with the different databases show only 5–8% of variations, and the result showed that: river samples and groundwater samples from highlands have the more negative SI (average values of −2.06 and −1.92 respectively); geothermal wells have an average value of −1.12, and hot springs have an average value of −0.48; groundwater from rift floor have an average value of −0.69; lakes have an average value of −0.42. Therefore, although a slight increase of fluorite SI can be observed along the flow path, fluorite precipitation is unlikely.
The Calcite SI of all sample groups oscillates around zero, suggesting conditions close to equilibrium for this mineral phase. This in turns means that the observed hydrochemical evolution of groundwater from the highlands to the rift cannot be related to significant calcite precipitation.
Chemical and mineralogical composition of rocks
Bulk rock XRF analyses of the aquifer solid matrixes are reported in Table 2 (ESM). The results, reported in an alkali–silica classification diagram, indicate that the prevalent volcanic rocks are rhyolites (i.e. felsic magmas) and that the fluvio/volcano-lacustrine sediments represent the weathered (re-deposited) products of the above mentioned volcanic rocks.
The petrographic investigation of representative thin sections shows that they are characterized by a few crystals of quartz, alkali–feldspar, orthopyroxene and amphibole within a prevalent glassy groundmass. Potential F-bearing minerals such as fluorite and apatite have not been observed.
Microprobe analyses have been carried out on the main mineral phases and on the glassy matrix (Table 3, ESM). The results revealed that the concentration of F is as high as 180 ppm in the glassy groundmass and up to 260 ppm in accessory phases such as alkali amphibole (riebeckite composition). Therefore, considering the modal proportions of the investigated rocks (hydrous mineral phases are extremely rare); we assume that most of the fluorine budget is concentrated in the glassy matrix.
This glassy matrix is reactive and easily affected by weathering processes that induce the neo-formation of clay minerals that can potentially trap fluorine (as F can replace OH in phyllosilicates).
This implies that the weathering products of these volcanic rocks, i.e. the clay-rich fluvio/volcano-lacustrine sediments, are enriched in F with respect to the original mother rocks.
Coherently, the higher concentrations of F in groundwater have been recorded in those wells drilled on the fluvio/volcano-lacustrine sediments.
Comparing the relative concentration of major ions in the waters from the rift valley, Na is always higher than K, since Na is more abundant than K in the host rocks, and K-minerals in primary volcanic parageneses are more resistant to weathering than Na-minerals (i.e. plagioclase is more alterable than K-feldspar); moreover K is easily stabilized in neo-formation minerals (clay minerals). Mg is lower in concentration than Ca, probably due to the low abundance of Mg in the outcropping rocks. Depletion of Ca and Mg and enrichment in Na and K is observed along the groundwater flow paths, moving from the highlands to the rift axial zone. The high concentration of bicarbonate is not related to calcite dissolution (carbonates are not present in the studied area) and therefore it is induced by magmatic outgassing (CO2 can upraise along the many faults still active in the rift). CO2 also increases the water aggressivity, i.e. its capability to trigger water/rock interaction processes thus explaining the positive relationships between HCO3 and other parameters such as Na, Cl and F.
A plausible hypothesis is that during explosive volcanic eruption, volcanic gases such as H2O, CO2, SO2, HCl, H2S and HF may have been trapped within the tephra (Giggenback 1996). The adsorption process is effective on smaller particles having large surface area (Oskarsson 1980) such as ash particles (<2 mm). Since fluorine is highly soluble in water, it can be subsequently transferred in the water system if the volcanic deposits are leached by water (Gregory 1996) during water-rock interactions. In this hypothesis, the anomalous F concentration recorded in thermal water may not reflect a direct magmatic contribution (presence of juvenile F-rich fluids), but it probably simply means that hot water is more aggressive and capable to leach the aquifer matrix.
To verify the mentioned process we performed laboratory leaching-tests on representative lithotypes outcropping in the rift valley. The powdered samples were mixed with distilled water having a pH of about 5.5, at a ratio of 1:5 (10 g/50 ml; room temperature), and shaken for 12 months at a frequency of 100 rev/min. The experiment was carried out in closed system, i.e. utilizing closed plastic bottles which did not allow interaction with atmospheric gases (such as CO2).
The concentration of F in the real natural water can be higher than those recorded in the experimental leachates, as the resulting F concentration in experimental leachates is expected to increase with the increase of time. Higher temperature and introduction of CO2 have to be considered as additional factors favouring high fluoride in the natural waters of the rift.
These tests, that simulate the water–rock/sediment interaction processes, highlight the potential contribution (i.e. the leachability) of each investigated lithotype to the release of fluorine in the water system.
The variation of pH (the original distilled water had pH 5.5) towards alkaline values observed in the experiments, shows that the system consumes H+ (probably during hydrolysis reactions), and that the resulting alkaline conditions favours the substitution of OH− with exchangeable F− in fluoride-rich minerals (Guo et al. 2007).
The chemistry of the lake water appears even more complicated due to evaporation processes, which ultimately lead to very extreme compositions such as those of Lake Abijata. In this framework, further input of fluorine can be attributed to peculiar minerals included in the relative sediments, such as trona (Na2CO3–NaHCO3·2H2O) which usually contain trace amounts of fluoride.
Fluoride enrichment mechanism
Fluoride ion is a strong ligand, it could form a number of complexes with cations such as Al, Mg, Fe, (Nordstrom and Jenne 1977). However, the higher pH of the rift waters does not allow the formation of these complexes and the fluoride persists freely in solution.
Therefore, Na/HCO3 ratio higher than one, indicates that the incongruent dissolution of feldspars (important constituent minerals of volcanic rocks) by interaction with carbonic acid is not the only controlling hydrogeochemical process.
This hypothesis is supported by the spatial distribution of the different water types, with the occurrence of the Ca (Mg)–HCO3 hydrochemical facies in the highlands (and the escarpment) and the presence of Na–HCO3 water signature in the rift, where ion exchange with sediments occurs.
The anomalous enrichment of fluoride in waters from the Ethiopian rift valley is a geogenic anomaly related to the peculiar geological framework of the area. We demonstrated that such enrichment does not necessarily require the existence of uprising F-rich juvenile fluids. It appears that the F enrichment is coupled with a more general hydrochemical evolution, in which Ca–HCO3 waters, typical of the highlands, are transformed along the flow path into Na–HCO3 waters (typical of the rift area). This evolution, clearly related to water–rock/sediment interactions, is probably triggered/favoured by the high geothermal gradient and the high activity of CO2 that characterize the rift valley. In these interactions the matrixes are mainly rhyolites consisting of volcanic glass (usually more than 95% in proportion) and only rare F-bearing accessory minerals (such as alkali amphiboles). The abundance and the composition (analyzed by electron microprobe) of the glassy groundmass, compared to those of the other mineral phases indicate that the former plausibly stores most of the total F budget. This glassy material is extremely reactive, and its weathering products (i.e. the fluvio/volcano-lacustrine sediments) can further concentrate the fluoride.
Therefore, the interaction of these “reworked” volcanic products with water and carbon dioxide (juvenile?), progressively converted them into a “secondary” clay-bearing mineral assemblage, which, under high pH conditions, can release fluoride into the interacting water.
The resulting waters, typically characterized by high pH (~7–9.1) are also affected by a further process of base-exchange softening with the neo-formed clay minerals (i.e. Ca–Mg uptake by the aquifer matrix, with release of Na to the groundwater). These are plausibly the main enrichment mechanisms that explain the high F content of the local groundwater, as evidenced by a positive correlation between F, pH, and Na, and inverse correlation between F and Ca (Mg). Under these conditions, fluoride cannot precipitate as fluorite (CaF2), and so it mobilizes freely without forming other complexes.
Considering that water treatments are expensive and scarcely efficient, we suggest avoiding (as far as possible) the drilling of new wells and starting to plan new strategies for water exploitation within the highlands (where fluoride concentration is typically low). These aqueduct infrastructures would imply an initial investment, but would be cost-effective in the long-term.
This paper is based on the Ph.D. studies of the first author at the University of Ferrara, where the chemical analyses have been carried out with the assistance of Renzo Tassinari. The authors also thank Raul Carampin for the microprobe analyses at the IGG-CNR Institute of Padova.
- Allison JD, Brown DS, Novo-Gradac KJ (1990) MINTEQA2/PRODEFA2, A geochemical assessment model for environmental systems, version 3.0 user’s manual: Environmental Research Laboratory, Office of Research and Development. US Environmental Protection Agency, Athens, p 106Google Scholar
- Allmann R, Koritnig S (1974) Fluorine. In: Wedepohl, KH (ed) Handbook of geochemistry. Springer, Berlin, 9-A-9-OGoogle Scholar
- Ashely PP, Burley MJ (1994) Control on the occurrence of fluoride in groundwater in the rift valley of Ethiopia. In: Nash H, McCall GJH (eds) Groundwater quality. Chapman & Hall, London, pp 45–54Google Scholar
- Ayenew T (1998) The hydrogeological system of the Lake District Basin. Central Main Ethiopian Rift. PhD Thesis, Free University of Amsterdam, The NetherlandsGoogle Scholar
- Ayenew T (2005) Major ions composition of the groundwater and surface water systems and their geological and geochemical controls in the Ethiopian volcanic terrain. SINET Ethiopian J Sci 28(2):171–188Google Scholar
- Ball JW, Nordstrom DK (1991) WATEQ4F-User’s manual with revised thermodynamic data base and test cases for calculating speciation of major, trace and redox elements in natural waters: US geological survey open-file report 90–129, p 185Google Scholar
- Benvenuti M, Carnicelli S, Belluomini G, Dainelli N, Di Grazia S, Ferrari GA, Iasio C, Sagri M, Ventra D, Atnafu Balemwald, Kebede Seifu (2002) The Ziway–Shala basin (Main Ethiopian Rift, Ethiopia): a revision of basin evolution with special reference to the late quaternary. J Afr Earth Sci 35:247–269CrossRefGoogle Scholar
- Boyle DR, Chagnon M (1995) An incidence of skeletal fluorosis associated with groundwaters of the maritime carboniferous basin, Gaspe region, Quebec. Canada: Environ Geochem Health 17:5–12Google Scholar
- Brehler B, Fuge R (1974) Chlorine. In: Wedepohl KH (ed) Handbook of geochemistry. Springer-Verlag, Berlin, 17-A–17-OGoogle Scholar
- Brownslow AH (1996) Geochemistry. Prentice Hall, New Jersey, p 580Google Scholar
- Calderoni G, Masi U, Petrone V (1993) Chemical features of spring waters from the East African Rift. A reconnaissance study. In: Abbate E, Sagri M, Sassi PP (eds) Geology mineral resources of Somalia and surrounding regions. Istituto Agronomico Oltremare, Firenze, pp 699–710Google Scholar
- Chernet T (1982) Hydrogeology of the lakes region, Ethiopia (Lakes Ziway, Langano, Abitata, Shalla and Awassa). In: The provisional military government of socialist Ethiopia-ministry of mines and energy. Ethiopian Institute of Geological Surveys, Addis Ababa, p 97Google Scholar
- Dainelli N, Benvenuti M, Sagri M (2001) Geological map of the Ziway–Shala lakes basin (Ethiopia). 1:250000, Italian ministry for university and scientific and technological research (MURST)Google Scholar
- Deer WA, Howie RA, Zussman J (1992) An introduction to the rock-forming minerals, 2nd edn. Wiley, New York, p 720Google Scholar
- Edmunds M, Smedley P (2005) Fluoride in natural waters. In: Selinus O (ed) Essentials of medical geology: the impacts of natural environment on public health. Elsevier, Amsterdam, pp 301–329Google Scholar
- Evans HT Jr (1995) Ionic radii in crystals. CRS Handbook of chemistry and physics, 75th ed. CRC Press, p 1913–1995Google Scholar
- Gregory N (1996) Toxicity hazards arising from volcanic activity. Surveillance 23(2):14–15Google Scholar
- Le Turdu C, Tiercelin JJ, Gibert E, Travi Y, Lezzar KE, Richert JP, Massault M, Gasse F, Bonnefille R, Decobert M, Gensous B, Jeudy V, Tamrat E, Umer M, Martens K, Atnafu B (1999) The Ziway–Shala lake basin system, Main Ethiopian Rift: influence of volcanism, tectonics, and climatic forcing on basin formation and sedimentation. Palaeogeogr, Palaeoclimatol, Palaeoecol 150:135–177CrossRefGoogle Scholar
- Merla G, Abbate E, Azzaroli A, Bruni P, Canuti P, Fazzuoli M, Sagri M, Sacconi P (1979) A geological map of Ethiopia and Somalia, 1:2,000000, CNR, FirenzeGoogle Scholar
- Parkhurst DL, Appelo CAJ (1999) User’s guide to PHREEQC (Version 2) a computer program for speciation, batch-reaction, one-dimensional transport, and inverse geochemical calculations. US geological survey water-resources investigations report, 99-4259, p 310Google Scholar
- Pauling L (1960) The nature of the chemical bond and the structure of molecules and crystals: an introduction to modern structural chemistry. Cornell University Press, NY, p 644Google Scholar
- Saxena VK, Ahmed S (2003) Inferring the chemical parameters for the dissolution of fluoride in groundwater. Environ Geol 43:731–736Google Scholar
- Tekle-Haimanot R, Fekadu A, Bushra B (1987) Endemic fluorosis in the Ethiopian Rift Valley. Trop Geogr Med 39:209–217Google Scholar
- WHO (1984) Guidelines for drinking water quality. Drinking water quality control in a small community supplies, 3. WHO, GenevaGoogle Scholar