Chernet T, Travi Y, Valles V
Lab, Hydrogéologie, Université d'Avignon, France.
Water Res. 2001 Aug;35(12):2819-32. doi: 10.1016/s0043-1354(01)00002-1.
The natural waters of the Lakes Region in the Main Ethiopian Rift (MER) show serious problems of chemical quality. The high content of fluoride reaches 300 mgl(-1) and affects the health of the population who live in this area. Furthermore, the alkaline and sodic characteristics (8<pH<10 and 0.9 x 10(-4) < Na < 0.35 Ml(-1)) of these waters destroy the soil for agricultural use. This geochemical study is conducted using analytical data issued from literature (UNDP, 1973: Chernet, 1982) and several field sampling and laboratory analyses completed during the last 4 yr. Chemical analyses of 320 samples taken throughout the region have been analysed using the computer software AQUA for geostatistic, statistic, chemical equilibria and simulation of evaporative concentration process. The results show that the waters issuing from volcanic rocks are characterised by a positive alkalinity residual of calcite. When they concentrate due to the effect of climate, the precipitation of calcite causes a decrease in the chemical activity of calcium. This results in an increase in solubility of fluoride, previously controlled by equilibrium with CaF, and the element concentrates without being significantly affected by the precipitation of fluorite. As water concentrates, the low concentration of dissolved calcium emphasises the alkaline characteristics. As a consequence, the pH reaches very high values (9-10) which make the waters unsuitable for agriculture. The adverse elements. namely fluoride, sodium, and alkalinity accumulate in the lower zones of the basins. The acquisition of high fluoride content and the alkaline-sodic characteristics depends primarily on the unbalanced initial stage between the carbonate alkalinity and calcium [(HCO3) > 2(Ca + Mg)]. which results from the weathering and dissolution of the volcanic rocks. The predictive qualities of the model AQUA were tested in the context of this region. A simulation of the concentration of the waters by evaporation has shown chemical variations relatively similar to those obtained with field data which include the minor chemical elements such as the fluoride or the dissolved silica. For field data, enrichment is more variable and may represent localised conditions of mineralization. This model is then used to study a method of defluoridation by supplying these waters with calcium in the form of gypsum. This method also allows the reduction of the alkalinizing and sodifying characteristics of the waters without reaching the fluoride standard concentration accepted for drinking waters. For drinking purpose, waters of the Ethiopian rift should undergo another method of defluoridation.
埃塞俄比亚主裂谷（MER）湖区的天然水域存在严重的化学质量问题。氟含量高达300毫克/升，影响了该地区居民的健康。此外，这些水域的碱性和钠质特性（8＜pH＜10且0.9×10⁻⁴＜Na＜0.35摩尔/升）破坏了农业用土壤。本地球化学研究利用文献（联合国开发计划署，1973年；切尔内特，1982年）发布的分析数据以及过去4年完成的多次实地采样和实验室分析开展。使用计算机软件AQUA对该地区采集的320个样本进行了化学分析，以进行地质统计学、统计学、化学平衡和蒸发浓缩过程模拟。结果表明，源自火山岩的水体具有方解石正碱度残留特征。当它们因气候影响而浓缩时，方解石沉淀导致钙的化学活性降低。这导致先前由与CaF平衡控制的氟溶解度增加，并且该元素得以浓缩，而不受萤石沉淀的显著影响。随着水的浓缩，溶解钙的低浓度突出了碱性特征。结果，pH值达到非常高的值（9 - 10），使这些水域不适用于农业。有害元素，即氟、钠和碱度，在盆地的较低区域积累。高氟含量以及碱性 - 钠质特性的形成主要取决于碳酸盐碱度与钙之间的初始不平衡阶段[（HCO₃）＞2（Ca + Mg）]，这是由火山岩的风化和溶解导致的。在该地区的背景下对AQUA模型的预测质量进行了测试。通过蒸发对水体浓缩的模拟显示出与实地数据相对相似的化学变化，其中包括氟或溶解硅等微量化学元素。对于实地数据，富集变化更大，可能代表局部矿化条件。然后使用该模型研究一种通过以石膏形式向这些水体供应钙来进行除氟的方法。该方法还可以降低水体的碱化和钠化特性，而不会达到饮用水可接受的氟标准浓度。出于饮用目的，埃塞俄比亚裂谷的水体应采用另一种除氟方法。
