IHE Delft Institute for Water Education, Department of Water Science and Engineering, 2611 AX Delft, the Netherlands.
IHE Delft Institute for Water Education, Department of Water Science and Engineering, 2611 AX Delft, the Netherlands.
Sci Total Environ. 2019 Apr 15;661:723-736. doi: 10.1016/j.scitotenv.2018.12.343. Epub 2018 Dec 27.
In this study hydrochemical, isotopic and multivariate statistical tools are combined with a recharge analysis and existing geophysical data to improve understanding of major factors controlling freshwater occurrence and the origins of high salinities in the multi-layered coastal aquifer system of the Great Maputo area in Mozambique. Access to freshwater in this semi-arid area is limited by an inefficient public supply network, scarce surface waters, long droughts and an increasing population growth. Groundwater has a large potential to enhance water security, but its exploitation is threatened by both coastal and inland salinization mechanisms that are poorly understood. A GIS approach is utilized to classify potential recharge zones based on hydrogeological properties and land use/cover, whereas potential recharge rates are estimated through a root zone water balance method. In combination with water stable isotope data results reveal that extreme rainfall events provide the most relevant contributions to recharge, and interception and evaporation play an important role in the low recharge areas. Hierarchical clustering of hydrochemical and isotopic data allows the classification of six water groups, varying from fresh to brackish/salt waters. Corresponding scatter plots and PHREEQC modelling show evaporation and mixing with seawater (up to 5%) as major processes affecting salinity in the area. The co-occurrence of high alkalinity and Cl concentrations, in combination with piezometric and geo-electrical data, suggests that: 1) inland brackish/salt groundwater is caused by mixing with seawater trapped within clay layers; and 2) brackish/salt surface waters result from seepage of brackish groundwater into rivers and wetlands, followed by evaporation, hence increasing salinity and δO values. Mixing with small fractions of trapped seawater as main salinity source, rather than halite dissolution, is further corroborated by Br/Cl ratios of brackish/salt water samples near the ocean ratio. Cation exchange upon salinization is mainly observed in the semi-confined aquifer, while freshening takes place in the phreatic aquifer, particularly in areas presenting high recharge rates.
在这项研究中，水文化学、同位素和多元统计工具与补给分析和现有地球物理数据相结合，以提高对控制莫桑比克大马普托地区多层沿海含水层系统淡水存在和高盐度成因的主要因素的理解。在这个半干旱地区，由于公共供水网络效率低下、地表水稀缺、长期干旱和人口增长，淡水供应有限。地下水具有增强水安全的巨大潜力，但由于沿海和内陆盐化机制尚未得到充分理解，其开采受到威胁。GIS 方法用于根据水文地质特性和土地利用/覆盖情况对潜在补给区进行分类，而潜在补给速率则通过根区水量平衡法进行估算。与水稳定同位素数据相结合的结果表明，极端降雨事件对补给的贡献最大，截留和蒸发在低补给区发挥着重要作用。水文化学和同位素数据的层次聚类允许将水分为六组，从淡水到微咸水/盐水。相应的散点图和 PHREEQC 模型表明，蒸发和与海水混合（高达 5%）是影响该地区盐度的主要过程。高碱度和 Cl 浓度的共存，结合测压和地球电学数据，表明：1）内陆微咸水/盐水地下水是由与被困在粘土层中的海水混合引起的；2）微咸地表水是由微咸地下水渗入河流和湿地引起的，随后蒸发，从而增加了盐分和 δO 值。与近海水样的 Br/Cl 比值相比，与小部分被困海水混合作为主要盐分来源，而不是石盐溶解，进一步证实了这一点。咸化过程中的阳离子交换主要发生在半承压含水层中，而淡化作用发生在潜水含水层中，特别是在高补给率的地区。
