Numerical modelling and hydrochemical characterisation of a fresh-water lens in the Belgian coastal plain

The distribution of fresh and salt water in coastal aquifers is influenced by many processes. The influence of aquifer heterogeneity and human interference such as land reclamation is illustrated in the Belgian coastal plain where, around A.D. 1200, the reclamation of a tidally influenced environment was completed. The aquifer, which was filled with salt water, was thereafter freshened. The areal distribution of peat, clay, silt and sand influences the general flow and distribution of fresh and salt water along with the drainage pattern and results in the development of fresh-water lenses. The water quality in and around the fresh-water lenses below an inverted tidal channel ridge is surveyed. The hydrochemical evolution of the fresh water lens is reconstructed, pointing to cation exchange, solution of calcite and the oxidation of organic material as the major chemical reactions. The formation and evolution of the fresh water lens is modelled using a two-dimensional density-dependent solute transport model and the sensitivity of drainage and conductivities are studied. Drainage level mainly influences the depth of the fresh-water lens, whereas the time of formation is mainly influenced by conductivity. Electronic Publication     

Evidence of dynamics reversal in tropical estuaries, geomorphological and sedimentological consequences (Salum and Casamance Rivers, Senegal)

South Dakar Senegambian estuaries are subject to an unusual hydrodynamical regime caused by weak or absent run-off. In the Salum delta, each distributary lacks fresh water during most of the year. Only the tidal flows are responsible for geomorphological and sedimentological effects. The current distribution shows a net discharge upstream due to the extensive evaporation and evapotranspiration in mangrove swamps and tidal flats. Consequently the salinity is always higher towards the river than near the sea. A high salinity bottom layer suggests the occurrence of a supersaline wedge of reverse sense to the salt wedge of a normal estuary. Such an inverse pattern is similarly displayed by sedimentological features (double upstream turned spits) and by the external location of the turbidity maximum. A coherent reverse estuary model is suggested from our field observations.     

Sustainable groundwater extraction in coastal areas: a Belgian example

Water extractions in coastal areas have to deal with salt water intrusion and lowering of hydraulic heads in valuable ecosystems. Therefore, sustainable management of fresh water resources in these areas is crucial. This is illustrated here with two water extractions in the western Belgian coastal plain which extract groundwater from a phreatic dune aquifer. One water extraction faced problems with salt water intrusion, while lowering of hydraulic heads was an issue for both. To remedy the salt water intrusion, it was found that decreasing the extraction rate was the only solution. To offset this and to increase hydraulic heads around both extractions, it was decided to artificially recharge the aquifer of the second extraction with tertiary treated wastewater. By taking these interventions, the combined production capacity of the water extractions was increased with 56% whereas 27% less water was extracted from the dune aquifer itself. Extraction history and the effects of interventions are illustrated for both water extractions with water quality data and fresh water head observations. A more detailed insight in groundwater flow and fresh–salt water distribution in the aquifer is provided by simulating the evolution of the water extractions with a 3D density dependent groundwater flow model.     

Groundwater scenario in Lakshadweep Islands

There are 36 islands in Lakshadweep archipelago, of which 10 are inhabited. The study focuses on the hydrogeological conditions prevailing in nine of these inhabited islands. Groundwater is the major source of fresh water and the well density is high in the range of 296 to 620 per km². The aspect ratios of these islands indicate that the shape of the islands have significant role on the stability of fresh water lens. The changes in water level do not reflect the real changes in the water resources as it is greatly influenced by tides. Moreover, the Ghyben-Herzberg relation in its ideal form does not hold good in these tiny oceanic islands. Hence, the rainfall recharge in the islands was assessed based on daily water budgeting. A relation between water surplus and annual rainfall could be established. This approach of recharge estimation could be applied for similar oceanic islands.     

Hydrochemical properties of transition zone between fresh groundwater and seawater in karst environment of the Adriatic islands,Croatia

Eastern coast of the Adriatic Sea consists of karstified carbonates. It belongs to the well-known Dinaric karst region. The coast is extremely indented and there are 718 islands with numerous rock crags and reefs. Some of the inhabited islands use their own water resources for public water supply, or plan to do it in the future. Since karst rocks are extremely permeable, the seawater intrudes into underground water resources, thereby forming the wedge. A wide transition zone occurs between this seawater wedge and fresh water aquifers. Consequently, island groundwater reserves turn brackish to a certain extent. In this study, 77 water samples were collected from a wide variety of water resources. Comprehensive statistical and mathematical multivariate analysis of these data was performed. Simple statistical approach showed several useful correlations among some parameters, and more complex multivariate techniques extracted three factors in connection with three natural processes: (1) mixing with the seawater, (2) carbonate dissolution and (3) human influence (pollution) and nitrogen transformation processes. The results of this study demonstrate that in situ measurement of electrical conductivity is adequate for the very rough field estimation of numerous parameters.     

华北平原典型区水体蒸发氢氧同位素分馏特征

为研究华北平原衡水地区水体蒸发氢氧同位素分馏特征,采集不同盐度的深层地下淡水(TDS 为0.61g/L)和浅层地下咸水(TDS为7.97g/L),现场开展室外器皿蒸发实验,获得了当地气象条件下氢氧同位素分馏参数.实验结果显示,淡水及咸水剩余表层水δ18O与剩余水比率f呈指数关系,与瑞利分馏模拟结果一致,δD和δ18O蒸发线斜率分别为4.78和4.69.整个蒸发过程中,淡水及咸水氢氧同位素值增量ΔδD分别为Δδ18O的4.82倍和4.76倍;剩余表层水相对于初始水δD和δ18O的变化量与累积蒸发量之比,淡水分别为2.68‰/cm和0.56‰/cm,咸水分别为2.78‰/cm和0.61‰/cm;而在不同的蒸发时段,剩余表层水δD和δ18O的变化量与蒸发量无明显相关性.受水分子扩散的影响,蒸发皿中氢氧同位素分馏在垂线上分层微弱.由于水体盐度较低,在当地气候条件下进行自由蒸发时,氢氧同位素分馏的盐效应可以忽略.     

Distribution,characteristics and influencing factors of fresh groundwater resources in the Loess Plateau,China

The fresh groundwater in the Loess Plateau serves as a major source of water required for the production and livelihood of local residents and is greatly significant for regional economic and social development and ecological protection. This paper analyzes the hydrogeological conditions and groundwater characteristics in the Loess Plateau, expatiates on the types and distribution characteristics of the fresh groundwater in the plateau, and analyzes the influencing factors and mechanisms in the formation of the fresh groundwater in the plateau as a priority. Based on this, it summarizes the impacts of human activities and climatic change on the regional fresh groundwater. The groundwater in Loess Plateau features uneven temporal-spatial distribution, with the distribution space of the fresh groundwater closely relating to precipitation. The groundwater shows a distinct zoning pattern of hydrochemical types. It is fresh water in shallow parts and is salt water in deep parts overall, while the fresh water of exploration value is distributed only in a small range. The storage space and migration pathways of fresh groundwater in the loess area feature dual voids, vertical multilayers, variable structure, poor renewability, complex recharge processes, and distinct spatial differences. In general, the total dissolved solids (TDS) of the same type of groundwater tends to gradually increase from recharge areas to discharge areas. Conditions favorable for the formation of fresh groundwater in loess tablelands include the low content of soluble salts in strata, weak evaporation, and special hydrodynamic conditions. Owing to climate change and human activities, the resource quantity of regional fresh water tends to decrease overall, and the groundwater dynamic field and the recharge-discharge relationships between groundwater and surface water have changed in local areas. Human activities have a small impact on the water quality but slightly affect the water quantity of the groundwater in loess.© 2021 China Geology Editorial Office.     

滨海岩溶地区海咸水入侵动力学系统研究

我国滨海岩溶地区岩溶系统的形成具有其独特的动力学过程。其中，既有自然环境条件下形成的岩溶特征，又有人类活动的地质效应——海咸水入侵的影响和改造。目前，海咸水入侵对滨海岩溶系统的影响和改造正在加剧，并将逐渐成为主导因素。防治滨海岩溶区海咸水入侵的根本，就是要查明咸-淡水过渡带中碳酸盐岩溶解、沉淀的动力机制。其中，混合溶蚀作用是关键。开展咸-淡水过渡带碳酸盐岩溶解、沉淀动力学的野外现场定位观测和室内动态模拟实验研究，并在查明咸-淡水过渡带碳酸盐岩溶解、沉淀动力机理的基础上，建立滨海岩溶地区碳酸盐岩溶解、沉淀系统动力学模型是十分必要和迫切的。研究各种因素综合影响下，滨海碳酸盐岩的混合溶蚀作用，将有利于查明滨海岩溶形成机理，进而为解决滨海岩溶地区海咸水入侵问题开辟一条新的途径。     

水位波动下包气带透镜体影响LNAPL迁移的数值模拟研究

轻质非水相液体（LNAPLs）在土壤包气带中具有多相态特征,非均质性、地下水位波动等因素将显著增加包气带内LNAPLs污染的复杂程度。已有研究多关注于揭示包气带内自由相LNAPLs的污染过程,少有更为深入地探究水位波动时非均质结构对LNAPLs迁移及各相态分布规律的影响。基于TOUGH2程序构建包气带多相流数值模型,以揭示透镜体结构与地下水位波动共同作用下LNAPLs迁移过程及相态分布。研究结果表明:（1）含水率变化是LNAPLs迁移分布的主要控制因素,包气带内受透镜体介质岩性、水位波动影响所呈现的含水率变化直接控制LNAPLs迁移规律及分布特征;（2）水位恒定时,细砂透镜体使LNAPL呈“蓄积穿透-横向扩展-绕流”迁移,粗砂透镜体则是LNAPL垂向迁移的“优势通道”,水位波动引起的细砂透镜体含水率变化使“绕流”显著增强,粗砂透镜体则进一步呈现“优势空间”作用;（3）水位恒定时,细砂透镜体模型中LNAPL滞留于透镜体内部,粗砂模型中LNAPL则集中于透镜体下方,水位波动下透镜体附近LNAPL分布范围扩展,两模型LNAPL分布面积较水位恒定时分别增大51%、63%;（4）两模型中LNAPL挥发通量均呈“先减小-后增大”规律,并受LNAPL-气体接触条件及LNAPL分布状况共同作用,水位波动打破三相平衡状态,主要表现为水位抬升阶段LNAPL挥发增强,此时两模型中平均挥发量较水位恒定时增大124%～126%。研究为非均质石油污染场地中的LNAPL污染过程认识提供了科学的理论依据。     

台风期间波浪和局地风对磨刀门水道盐淡水混合的影响机制

珠江磨刀门水道地处亚热带季风气候区,直面南海,容易受到热带风暴的袭击。选取台风"纳沙",采用SCHISM模型建立磨刀门水道三维水流盐度数值模型,通过数值试验对比,结合势能异常分析法,探究了台风期间波浪和局地风对磨刀门水道混合与层化的影响。结果显示:由于近岸及河道内水深较浅,波高整体较小,波浪对水体层化过程影响不大,对流速分布有一定的调整作用,使其分布更为均匀,并在一定程度上加强水体掺混,利于外海高浓度盐水向河口和海岸扩散。而台风期间强劲的局地风对垂向水体状态以及势能异常变化率各项均有显著作用,影响着盐淡水的层化混合过程。     

钱塘江河口段盐水入侵的时空变化及预测模型

杭州城市供水85%取自钱塘江河口段,取水水质在枯水大潮期都不同程度地受到盐水入侵的威胁,分析钱塘江河口盐水入侵时空变化及研制二维数值预测模型对保障城市供水安全十分必要。根据钱塘江河口段实测水文氯度资料,分析了强潮作用下盐水入侵的时空变化特征;据此构建考虑斜压作用的二维水流、盐度输移的耦合数学模型,计算格式采用守恒性较好的有限体积法;在模型验证的基础上,数值分析了径流和潮汐对钱塘江河口段盐水入侵的影响,结果表明河口段的盐水入侵明显地受径流和潮汐的影响,据此可通过增大上游新安江水库的下泄流量抑制盐水入侵上溯以减小取水口氯度及超标时间,确保用水安全。     

Evaluating the salinity distribution of a shallow coastal aquifer by vertical multielectrode profiling (Denmark)

A monitoring system, including five groups of piezometers and five vertical multielectrode profiling probes (VMEP), has been installed in an aquifer beneath a coastal dune in Denmark. In order to assess the salinity distribution within the aquifer, geoelectrical data were gathered in March, June and September 2008, by measuring a dipole-dipole and gradient array using multielectrode profiling. Interpretation of the processed resistivity data was performed by regularized inversion using a one-dimensional, horizontally layered model of formation resistivity. The standard deviation on estimated layer log-resistivity was 0.01–0.03. By estimating two parameters of a power function, observed fluid conductivities derived from samples of porewater were related to corresponding estimated formation resistivities. The conductivity profiles correlate with a winter situation in March with high sea level, active recharge and significant wave activity, causing increased hydraulic heads, a thicker freshwater lens and salt water overlying freshwater close to the sea. In June, the thickness of the freshwater lens is reduced due to less recharge and prevailing offshore winds, imposing density-stable conditions and a sharper transition between fresh and brackish water. During the autumn, aquifer recharge is enhanced and hydraulic heads increase, resulting in a thicker freshwater lens.     

中国沿海地区海水入侵现状与分析

首先介绍了我国存在着严重海水入侵灾害的三大河三角洲——长江三角洲、珠江三角洲、老黄河三角洲(莱州湾地区)海水入侵现状。长江口和珠江口海水入侵主要受河流入海径流量和河口潮汐影响,莱州湾地区海水入侵的产生则主要是人类过量开采地下淡水所致。地下淡水和咸水之间存在着一个动态平衡的混合界面,淡水储存一旦减少,平衡状态被打破,该界面便会向陆移动。最终产生海水入侵。海水入侵的发生受着诸多因素制约,在中国沿海地区。地下淡水的储备与人类的用水需求之间存在着矛盾,人类过量开采地下淡水是产生海水入侵的主导因素。因而开源节流、人工回灌地下淡水、阻隔水流等措施在防治海水入侵方面是行之有效的。     

Research on Migration Features of Salt-Fresh Water Interface on the North China Plain

Underground water is the important water resources for the North China Plain, but due to long-time exhaustive exploitation, a series of problems about environmental geology including salt water migration appears. The salt groundwater that distributed widely moved downward vertically, triggering a strong impact on the water supply safety in this area; simultaneously, its large storage capacity brought great resources potential. Thus it had practical guiding significance to develop research on migration of salt-fresh water interface in the North China Plain and discuss the space distribution and migration features for improvement of groundwater environment and mitigation of water resource shortage status. This paper described migration features of salt-fresh water interface through changes in boundary line of salt water and fresh water and vertical interface of salt water and found out major causes for migration of salt-fresh water interface in the area were water level difference between salt water and fresh water area, saline concentration difference in salt water and fresh water, stratigraphic structure and artificial skylight based on analysis on the influence factors     

滨海盐碱地区包气带中淡水透镜体维持机理

大气降水是滨海盐碱地区浅层地下水的重要补给来源。滨海地区浅层地下水多为咸水且埋藏较浅,在不同包气带岩性渗透性差异下,在大气降水入渗补给过程中,一定时间内潜水面以上一定范围内存在淡水分布,即淡水透镜体,它能局部隔离地下咸水对上层土壤和植物的危害,并在一定程度上供给植物吸收利用。采用自制的室内物理模拟装置,通过控制土层结构,模拟了大气降水入渗补给条件下包气带中淡水透镜体的形成与消退过程,探讨了不同土壤类型中淡水透镜体的维持情况;并利用吸水管模拟客土上所种植物根系吸水,研究了不同吸水量条件下土壤中淡水透镜体的变化规律。结果表明:上层中砂、底层粉砂质黏土的双层土体结构中,淡水透镜体的维持性最好,在降水入渗补给条件下,透镜体形成时间在1500 min左右可达最大厚度(约15 cm),若补给源消失,透镜体完全消退需7500 min,能较长时间地阻隔地下咸水;双层土中模拟形成的稳定淡水透镜体在无补给条件下,能够隔离地下咸水的同时亦能为上层植物提供243.5 mL淡水资源。     

咸淡水混合灌溉技术应用与实践

咸水与淡水混合灌溉技术,将两种矿化度不同的灌溉水混合使用,目的是降低灌溉水的含盐量或改变其盐分组成.咸淡水混合灌溉在提高灌溉水水质的同时,也增加了可灌水的总量,使以前不能使用的高盐渍度的咸水得以利用.通过对邢台市平原区咸水灌溉条件的实验研究、咸淡水混合灌溉技术分析,邢台市咸淡水混合后矿化度应控制在3g/L以下.结合近年来开展咸淡水混合灌溉实践,在咸水区开展咸淡水混合灌溉,充分利用咸水资源,既减少了淡水的开采量,又对改善当地的生态环境发挥了重要作用.     

Brine transport in porous media: On the use of Von Mises and similarity transformations

In this paper we use a Von Mises transformation to study brine transport in porous media. The model involves mass balance equations for fluid and salt, Darcy's law and an equation of state, relating the salt mass fraction to the fluid density. Application of the Von Mises transformation recasts the model equations into a single nonlinear diffusion equation. A further reduction is possible if the problem admits similarity. This yields a formulation in terms of a boundary value problem for an ordinary differential equation which can be treated by semi‐analytical means. Three specific similarity problems are considered in detail: (i) one‐dimensional, stable displacement of fresh water and brine in a porous column, (ii) flow of fresh water along the surface of a salt rock, (iii) mixing of parallel layers of brine and fresh water. This revised version was published online in July 2006 with corrections to the Cover Date.     

环境同位素特征对滨海岩溶地区海水入侵过程的指示意义

大连大魏家水源地位于中国北方典型滨海岩溶地区。近30年来,地下淡水的不合理开采造成的地下水位降落漏斗引发了严重的海水入侵。以大魏家水源地为研究对象,通过大量的水文地质调查和水化学及同位素采样测试分析,探讨海水入侵形成的水动力条件,通过分析滨海岩溶含水层中地下水主要水化学和多种同位素(δ2H-δ18O,δ34S,δ13C)组成特征,识别了海水入侵过程中发生的主要水文地球化学作用,并对其进行了定量模拟,从而阐明了岩溶含水层中的海水入侵机理。研究结果表明:大连大魏家海水入侵主要通道为大魏家地区存在的导水断裂、岩溶裂隙以及第四系松散地层。对δ2H-δ18O同位素的组成分析表明,研究区地下水主要来自大气降水补给,结合Cl-浓度分布,认为除海水入侵淡水含水层后增加了地下水中的盐分外,浅层地下水的蒸发也对地下水中盐分的累积起到了重要作用。根据不同水体中δ34SSO4,δ13CHCO3等同位素特征,结合水化学成分(如SO2-4,Cl-)分析认为,研究区微咸水和咸水并不是地下水淡水和海水简单混合而成。利用反向水文地球化学模拟揭示了控制滨海岩溶含水层中水化学演化的主要水文地球化学反应有方解石、蒙脱石和石膏的溶解作用,伊利石的沉淀作用以及Ca-Na离子交换作用,伴随着CO2的释放。     

一种可增加海岛地下淡水资源储量的方法研究

淡水透镜体作为海岛上珍贵的地下淡水资源,对于满足居民生活用水与维护生态系统具有重要作用,增加海岛地下淡水资源储量可有效缓解海岛地区普遍存在的水资源短缺问题。本研究提出通过在海岛外部区域采用低渗透性介质材料增加海岛地下淡水资源储量的方法,采用室内物理模型实验与基于变密度地下水流溶质运移模型的数值模拟相结合的方法对提出方法的有效性进行了验证,并通过野外尺度的数学模型分析工程实施可能性。实验与数值模拟结果较吻合,表明该方法具有抵御海水入侵、增加海岛地下淡水资源储量的巨大潜力。通过基于野外尺度的数值模拟分析工程实施的可能性,发现对于降雨入渗补给强度为0.005 7 m/d、长度为200 m,孔隙度为0.3的狭长形均质海岛,地下淡水储量为66.5 m3/m,在海岛外部区域采用渗透系数为1 m/d、厚度为5 m的低渗透性介质材料后,淡水透镜体经过约8 a时间可再次达到稳态,地下淡水储量为343.8 m3/m,增加约4倍,工程实施具备一定可能性。本研究为缓解海岛地区缺水问题、实现淡水资源可持续利用提供了一种新方法。     

Analytical solution to one-dimensional consolidation in unsaturated soils under loading varying exponentially with time

This note presents an analytical solution to one-dimensional consolidation in unsaturated soils with a finite thickness under confinement in the lateral direction and vertical loading varying exponentially with time. The boundary conditions are that the top surface is permeable to water and air and the bottom is impermeable to water and air. The transfer relationship between the state vectors at the top surface and any depth is gained by applying the Laplace transform and Cayley-Hamilton mathematical methods to the governing equations of water and air, Darcy’s law and Fick’s law. The excess pore-air and pore-water pressures and settlement in the Laplace-transformed domain are obtained by using the Laplace transform with the initial and boundary conditions. By performing the inverse Laplace transforms, the analytical solutions of the excess pore-air and pore-water pressures at any depth and settlement are obtained in the time domain.