Growing lake with growing problems: integrated hydrogeological investigation on Lake Beseka, Ethiopia
Growing lake with growing problems: integrated hydrogeological investigation on Lake Beseka, Ethiopia
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湖泊不断扩大，问题也越来越多：埃塞俄比亚贝塞卡湖综合水文地质调查
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发表时间：
2009
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通讯作者：
U. Bonn
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作者：
U. Bonn
埃塞俄比亚裂谷中复杂的构造和火山过程导致了火山构造凹陷的形成，这些构造凹陷成为许多裂谷湖泊的所在地。贝塞卡湖是梅尔北部靠近阿法尔三角的裂谷湖泊之一。该湖位于瓦瓦什国家公园的北部，在鸟类和野生动物的生态中扮演着重要的角色。自20世纪60年代末至70年代初以来，贝塞卡湖一直在以惊人的速度扩张。这种增长对周围的物理、水文和基础设施环境产生了不利影响。这项研究的主要目的是了解贝塞卡湖与周围地下水系统的水力相互作用，并确定和量化地下水在该湖水文中的作用。应用水化学、同位素水文学、补给量估算(地下水位波动、地球模拟和氯离子质量平衡)和地下水模拟(MODFLOW)等综合方法对研究区水文地质特征进行了研究。将地下水水位图与水体的水化学和同位素组成相结合，对贝塞卡湖流域的水流系统进行了分析。采用层次多元素聚类分析方法对研究区水化学水样进行了分类。该湖以Na-HCO3-Cl型水为特征，具有与流域西部流出的地下水系统相似的水化学特征。流域西部地下水系统和西缘温泉以Na-HCO3型水为特征，其平均同位素组成分别为-2.8‰(δO)和-10.7‰(δH)，与输入信号的同位素浓度相当。输入信号的同位素值分别为-3‰(δO)和-9.3‰(δH)，取自亚的斯亚贝巴降水的当地大气水线和沿贝塞卡湖蒸发水域标绘的当地蒸发线的交点。水化学和同位素证据表明，地下水从分水岭西部流出，以温泉的形式排入湖中。这一地下水流入构成了流入贝塞卡湖的主要水量，并构成其水量收支的组成部分。这一事实得到了地下水模拟结果的很好支持，该结果估计，流入贝塞卡湖的总水量的51%来自地下水渗入该湖。该模型计算的年渗漏量为33.8Mm。补给估算表明，地下水系统的补给来自于流域局部降水的入渗。然而，据估计，这种补给每年只有17.4毫米。因此，季节性地补充湖泊分水岭内含水层系统的地下水补给不足以解释贝塞卡湖的扩张。该模型估计，每年有30.45Mm的地下水通过地表边界横向流向湖泊流域的含水层系统。这种横向流入可能与山脉的补给有关。贝塞卡湖的扩张很可能与这些地下水的数量变化有关，这些地下水首先流向湖泊分水岭，然后流向贝塞卡湖。由于研究区的地质背景，构造引起的地下水动态水力坡度的改变可能导致了源源不断流入湖泊的温泉流量的增加。
The complex tectonic and volcanic processes in the Ethiopian rift valley have resulted in the formation of volcano-tectonic structural depressions that became sites for many rift valley lakes. Lake Beseka is one of the rift valley lakes in the northern section of the MER near to the Afar triangle. The lake plays an important role in the ecology of birds and wildlife, as it is located in the northern part of the Awash National Park. Lake Beseka has been expanding at an astounding rate since the late 1960s and early 1970s. This growth has had a detrimental effect on the surrounding physical, hydrological and infra-structural environment. This study is conducted with primary objective of understanding the hydraulic interaction of Lake Beseka with the surrounding groundwater system, and identifying and quantifying the role of groundwater in the hydrology of the lake. Integrated approaches of hydrochemistry, isotope hydrology, recharge estimation (water table fluctuation, EARTH modeling and chloride mass balance), and groundwater modeling (MODFLOW) are applied for hydrogeological characterization of the study area. The flow system in the watershed of Lake Beseka is analyzed by coupling a groundwater level map with the hydrochemical and isotopic composition of the water bodies. Hierarchical multi-element cluster analysis (HCA) is used to classify hydrochemical water samples of the study area into different groups. The lake is characterized by a Na-HCO3-Cl type of water, and has a hydrochemical signature similar to that of the groundwater system flowing from the western part of the watershed. The groundwater system in the western part of the watershed and hot springs that emerge at the western edge of the lake are characterized by a Na-HCO3 type of water with an average isotopic composition of -2.8 ‰ in δO and -10.7 ‰ in δH, which is comparable to the isotopic concentration of the input signal. The input signal has an isotopic value of -3 ‰ in δO and -9.3 ‰ in δH, and is derived from the intersection point of the local meteoric water line of Addis Ababa rainfall and the local evaporation line, which plots along evaporated waters of Lake Beseka. Hydrochemical and isotopic evidence indicates that groundwater flows from the western part of the watershed and discharges to the lake in the form of hot springs. This groundwater inflow constitutes the major water inflow to Lake Beseka and forms an integral part of its water budget. This fact is well supported by the groundwater modeling results, which estimate that 51% of the total water inflow to Lake Beseka comes from groundwater seepage to the lake. This seepage is computed by the model to be 33.8Mm annually. It is evident from the recharge estimation that recharge of the groundwater system comes from infiltration of local precipitation in the watershed. However, this recharge is estimated to be only 17.4 Mm annually. Thus, the groundwater recharge that seasonally replenishes the aquifer system within the lake watershed is not significant enough to explain the expansion of Lake Beseka. The model estimated that 30.45Mm of groundwater laterally flows to the aquifer system of the lake watershed annually across the surface boundaries. This lateral inflow might be related to recharge from the mountains. It is most likely that the expansion of Lake Beseka is related to changes in the amount of this groundwater, which first flows to the lake watershed and then to Lake Beseka. Owing to the geologic setting of the study area, tectonically induced modification of hydraulic gradient of the groundwater regime in the region might have resulted in an increase in the discharge of the hot springs that continuously flow to the lake.