Hydrogeological investigations in the upper Ouémé catchment in Benin, West Africa

The scope of this PhD thesis was the hydrogeological conceptualisation of the Upper Ouémé river catchment in Benin. The study area exceeds 14,500 km**2 and is underlain by a crystalline basement. At this setting the typical sequence of aquifers - a regolith aquifer at the top and a fractured bedrock aquifer at the bottom - is encountered, which is found in basement areas all over Africa and elsewhere in the world. The chosen regional approach revealed important information about the hydrochemistry and hydrogeology of this catchment.
Based on the regional conceptual model a numerical groundwater flow model was designed. The numerical model was used to estimate the impact of climate change on the regional groundwater resources.
This study was realised within the framework of the German interdisciplinary research project IMPETUS (English translation: "Integrated approach to the efficient management of scarce water resources in West Africa"), which is jointly managed by the German universities of Bonn and Cologne. Since the year 2000 the Upper Ouémé catchment was the principal target for investigations into the relevant processes of the regional water cycle. A first study from 2000 to 2003 (Fass, 2004, http://nbn-resolving.de/urn:nbn:de:hbz:5n-03849) focused on the hydrogeology of a small local catchment (~30 km**2).
In the course of this thesis five field campaigns were underdone from the year 2004 to 2006. In the beginning of 2004 a groundwater monitoring net was installed based on 12 automatic data loggers. Manual piezometric measurements and the sampling of groundwater and surface water were realised for each campaign throughout the whole study area. Water samples were analysed for major ions, for a choice of heavy metals and for their composition by deuterium, oxygen-18 and tritium. The numerical model was performed with FEFLOW.
The hydraulic and hydrochemical characteristics were described for the regolith aquifer and the bedrock aquifer. The regolith aquifer plays the role of the groundwater stock with low conductivity while the fractures of the bedrock may conduct water relatively fast towards extraction points. Flow in fractures of the bedrock depends on the connectivity of the fracture network which might be of local to subregional importance.
Stable isotopes in combination with hydrochemistry proved that recharge occurs on catchment scale and exclusively by precipitation. Influx of groundwater from distant areas along dominant structures like the Kandi fault or from the Atacora mountain chain is excluded. The analysis of tritium in groundwater from different depths revealed the interesting fact of the strongly rising groundwater ages. Bedrock groundwater may possibly be much older than 50 years.
Equilibrium phases of the silicate weathering products kaolinite and montmorillonite showed that the deeper part of the regolith aquifer and the bedrock aquifer feature either stagnant or less mobile groundwater while the shallow aquifer level is influenced by seasonal groundwater table fluctuations. The hydrochemical data characterised this zone by the progressive change of the hydrochemical facies of recently infiltrated rainwater on its flow path into deeper parts of the aquifers. Surprisingly it was found out that seasonal influences on groundwater hydrochemistry are minor, mainly because they affect only the groundwater levels close to the surface.
The transfer of the hydrogeological features of the Upper Ouémé catchment into a regional numerical model demanded a strong simplification. Groundwater tables are a reprint of the general surface morphology. Pumping or other types of groundwater extraction would have only very local impact on the available groundwater resources.
It was possible to integrate IMPETUS scenario data into the groundwater model. As a result it was shown that the impact of climate change on the groundwater resources until the year 2025 under the given conditions will be negligible due to the little share of precipitation needed for recharge and the low water needs for domestic use.
Reason for concern is the groundwater quality on water points in the vicinity of settlements because of contamination by human activities as shown for the village of Dogué. Nitrate concentrations achieved in many places already alerting levels. Health risks from fluoride or heavy metals were excluded for the Upper Ouémé area.

本博士论文的研究范畴为贝宁境内上韦梅河（Upper Ouémé river）流域的水文地质概念建模（hydrogeological conceptualisation）。研究区面积超过14500 km²，基底为结晶基底（crystalline basement）。在该类地质背景下，普遍发育典型的含水层序列：上部为风化壳含水层（regolith aquifer），下部为裂隙基岩含水层（fractured bedrock aquifer），该序列在非洲乃至全球其他基岩分布区域均有出现。本次采用的区域尺度研究方法，为该流域的水化学（hydrochemistry）与水文地质（hydrogeology）特征提供了重要研究成果。
基于该区域概念模型，本研究构建了地下水流数值模型（numerical groundwater flow model），并借助该模型评估了气候变化对区域地下水资源的影响。
本研究依托德国跨学科研究项目**IMPETUS（英文全称为"Integrated approach to the efficient management of scarce water resources in West Africa"，即“西非稀缺水资源高效管理综合方法”）**开展，该项目由德国波恩大学与科隆大学联合管理。自2000年起，上韦梅河流域便成为区域水循环相关过程研究的核心靶区。2000至2003年的首项研究（Fass, 2004, http://nbn-resolving.de/urn:nbn:de:hbz:5n-03849）聚焦于一处小型局部流域（约30 km²）的水文地质特征。
在本论文研究期间，研究团队于2004至2006年间开展了五次野外考察。2004年初，研究团队基于12台自动数据记录仪（data loggers）搭建了地下水监测网。每次野外考察期间，研究人员均在全研究区内开展手动测压水位测量（piezometric measurements），并采集地下水与地表水样本。本次分析的水样指标包括主要离子、选定重金属，以及氘（deuterium）、氧-18（oxygen-18）与氚（tritium）的同位素组成。本研究的数值模型采用FEFLOW软件完成构建与运算。
研究人员分别阐述了风化壳含水层与基岩含水层的水力与水化学特征。风化壳含水层为低导水性的地下储水介质，而基岩裂隙则可相对快速地将地下水输送至取水点。基岩裂隙内的水流取决于裂隙网络（fracture network）的连通性，该连通性可呈现局地至次区域尺度的影响范围。
稳定同位素（stable isotopes）结合水化学分析结果证实，流域尺度的地下水补给仅由大气降水完成。排除了沿坎迪断层（Kandi fault）等优势构造来自远端区域的地下水补给，以及来自阿塔科拉山脉（Atacora mountain chain）的地下水补给的可能性。对不同深度地下水的氚含量分析发现了一个有趣的现象：地下水年龄（groundwater ages）呈现显著递增趋势，基岩地下水的年龄可能远超50年。
硅酸盐风化（silicate weathering）产物高岭石（kaolinite）与蒙脱石（montmorillonite）的平衡相分析结果表明，风化壳含水层深部与基岩含水层内的地下水多处于停滞或低流动性状态，而浅层含水层则受地下水位季节性波动（seasonal groundwater table fluctuations）的影响。水化学数据显示，随着降雨入流水在含水层中向深部运移，其水化学相（hydrochemical facies）逐渐发生演化。值得注意的是，季节性因素对地下水水化学的影响相对微弱，这主要是因为其仅能影响近地表浅层的地下水位。
将上韦梅河流域的水文地质特征转化为区域数值模型时，需要进行大幅简化。研究区内地下水位形态大体复刻了地表宏观地貌形态。抽水或其他类型的地下水开采仅会对局部区域的地下水资源产生影响。
研究团队成功将IMPETUS项目的情景数据（scenario data）整合至地下水流模型中。结果显示，在现有条件下，至2025年气候变化对地下水资源的影响可忽略不计——这是因为补给所需的降水占比极低，且居民生活需水量较低。
值得关注的问题是居民点周边取水点的地下水水质，该区域因人类活动遭受污染，多格（Dogué）村的地下水污染情况即为典型例证。多地的硝酸盐（nitrate）浓度已达到警戒水平。而氟化物（fluoride）与重金属引发的健康风险在上韦梅河流域则可排除。