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。     

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。     

边坡防渗增加灰沙岛地下淡水的试验与数值模拟研究

地下淡水是支撑海岛居民生活用水保障和生态岛屿建设的重要战略资源。目前海岛淡水资源十分匮乏,鉴于我国灰沙岛的现实复杂性,亟需实施低成本、易操作且对岛礁稳定性及生态系统影响较小的增加地下淡水的措施。本研究提出了采用在海陆边坡处铺设防渗材料人为干预地下淡水体形成、增加淡水储量的措施,并通过砂箱试验和数值模拟相结合的方法在实验室尺度下分析了边坡防渗对灰沙岛淡水透镜体形成的影响,评估了不同因素在边坡防渗条件下增加地下淡水储量的效果,并以永兴岛为例定量评估了边坡防渗对地下淡水储量的影响。研究表明,边坡防渗通过改变地下水流场、增加淡水水头的方式增加了淡水储量。淡水体所需的稳定时间随之增加。在长、宽、高分别为50,5,35 cm的砂箱中,在35°的边坡处铺设14 cm长的隔水材料,淡水透镜体达到稳定后最大厚度由原有的13.7 cm增加至24.9 cm,淡水储量由561.8 cm3增加至1 592.3 cm3,所需稳定时间由120 min增加至150 min。增加的淡水储量随降雨强度增加、砂体渗透系数减小、边坡防渗深度增加、防渗材料渗透系数减小而增加。若在永兴岛海陆边坡铺设2 m深的隔水材料,在未来30...     

钙质砂水理性质及对岛礁淡水透镜体形成的影响

钙质砂的水理性质对岛礁地下淡水透镜体的形成至关重要。通过室内试验的方法从透水性、容水性和给水性3个方面对南海钙质砂的水理性质进行了研究,并在此基础上进一步结合数值模拟手段,探讨了上述相关性质对岛礁淡水透镜体形成的影响。结果表明：南海钙质砂多具有级配不良且不均匀的特征;其渗透系数通常在0.023~110 m/d之间,区域间差异较大,而孔隙度和给水度则主要集中在0.40~0.55和0.012~0.310之间,与同粒径范围的陆源砂相比,呈现出容水性好但给水性偏低的特征。钙质砂的水理性质对岛礁地下淡水透镜体形成的影响主要体现在流速、厚度、资源储量以及形成时间等方面,其中钙质砂的透水性越好,地下水中的流速越快,咸淡水间的混合作用越强,导致淡水透镜体的厚度越薄,储量越少;给水性则主要影响钙质砂中淡水透镜体的资源储量,对淡水透镜体的厚度及形状影响较小。     

中国东部沿海基岩海岛地下水资源分布特点——以庙岛群岛为例

本文依据对庙岛群岛基岩裂隙水进行的地质构造、水文地质、物探等方面的研究,提出了适用于我国东部沿海基岩海岛的地下淡水蘑菇体新模式。以新构造控水为指导思想,探讨了基岩海岛裂隙水的赋存条件及富集规律,指出了地下水资源开发方向,并且结合生产实际,比较有效地解决了研究区变质岩海岛找淡水难的问题。     

非均质包气带三氮累积转化模拟砂箱试验

为研究包气带非均质结构对氮素迁移转化的影响,采用室内砂箱实验模拟了1.72 mm/h降雨强度下NH₄-N污染非均质包气带的过程。通过观测出水及土壤溶液中NH₄-N、NO₂-N和NO₃-N质量浓度变化,分析探讨了NH₄-N在含有两个亚粘土透镜体的非均质包气带介质中的迁移转化和累积时空分布特征。结果表明：经过89 d物理吸附和微生物作用,出水NH₄-N首先迅速增加然后逐渐减小,而出水NO₃-N含量持续增加,其间NO₂-N出现累积峰,系统最终达到平衡稳定状态;土壤溶液中NH₄-N在两亚粘土透镜体上表面累积较多,NO₃-N和NO₂-N在上透镜体中的累积含量大于下透镜体;而砂槽上部土壤中出现明显的NO₃-N累积,NH₄-N和NO₂-N则在下部土壤和亚粘土透镜体中累积较多。包气带介质质地、粗 细交互结构及降雨淋溶、水分运移对“三氮”的迁移转化和富集有重要影响。     

下辽河平原南部明化镇组咸水体赋存条件及分布特征

下辽河平原南部自上世纪七十年代以来陆续开采了上第三系明化镇组深层地下水,开采至今暴露出一系列的问题,如区域地下水头下降较快形成大面积水头降落、局部出现咸水体扩张及淡水咸化问题,严重威胁该地区居民的淡水资源安全.通过对下辽河平原南部上第三系地下水明化镇组含水层咸水体赋存的地质地貌、补迳排条件、动态特征等进行研究,掌握其赋...     

太平洋典型海岛淡水透镜体特征与地下水开发利用关系分析——以Saipan海岛Ocean Vista Resort项目为例

淡水透镜体是位于岛屿的地下像凸透镜一样,中间厚四周薄的淡水层,是岛礁环境的重要组成部分。以位于西太平洋北马里亚纳群岛的Saipan岛为研究区域,基于为Ocean Vista Resort酒店项目寻找稳定水源中的相关地层与构造勘探成果,综合岛上现有水文地质资料,合理概化该岛屿的水文地质概念模型,对淡水透镜体的赋存状态、取水井中氯化物的浓度变化与井位选取、淡水透镜体厚度变化规律进行分析,在此基础上对岛屿商业开发供水水源类型及供水潜力进行评价,明确限制饮用水开发可利用性的主要因素,提出Ocean Vista Resort项目水源地的开发建议。结果认为：Saipan岛上地下淡水唯一补给为大气降水,降雨随高渗透性的礁灰岩渗透到地下,并在地下向海边运移,最终在海岸带排出,形成悬浮在咸水上的淡水透镜体,淡水透镜体的形状呈凸圆状,其厚度的变化受季节性交替,岛中心位置厚,朝向海岸线方向相当薄,研究区限制饮用水可利用性的因素主要是海水入侵、井群干扰导致的地下水位降低和向海洋排泄;在为Ocean Vista Resort项目进行地下水开发时应合理布置钻孔,找出饮用淡水透镜体最厚的井的位置,确认井深没有揭穿相...     

涠洲岛海水入侵模拟

通过岸边地下水动态分析，弥散度尺度效应的分形特征讨论及局部海水入侵模型校正，获取涠洲岛含水层的有关参数。结合全岛三维有限元数值模型，模拟了海水入侵的动态过程，对制订涠岛地下淡水透镜体的开采规划和控制海水入侵提供了依据．     

包气带弱渗透性黏土透镜体对降雨入渗补给影响的数值模拟

大气降雨是华北平原浅层地下水的主要补给来源。长期过量开采地下水造成地下水位持续下降,使原来处于饱水带的透镜体位移到包气带中,形成了厚度大、非均质性更为复杂的包气带。厚层包气带中弱渗透性黏土透镜体对于降雨入渗补给的影响是关系到降水入渗过程及补给量评价的基本问题。用HYDRUS软件建立数值模拟模型,模拟探讨单次降雨条件下,透镜体埋深、宽度比、厚度等要素对入渗路径、入渗补给时间和入渗补给量的影响。结果表明:入渗过程中弱渗透性黏土透镜体两侧会形成较快的绕流;透镜体会减小补给峰值并延长总体补给时间,但不改变补给起始时间;透镜体埋深与极限蒸发深度的相对关系决定了潜在补给量的大小,透镜体埋深或因透镜体形成的上层滞水处于极限蒸发深度以上会减少潜在补给量。     

Interpretation of pumping test data from large diameter wells on an oceanic island

 Estimation of aquifer parameters is vital for the assessment of groundwater potential and groundwater flow regime. On an oceanic island where fresh water lens is fragile and sensitive to various stresses, it is even more essential that in order to assess the potential of fresh water lens and the effect of various stresses on the fresh groundwater regime the parameters should be representative to the field hydrogeological set up. Pumping tests conducted on existing large diameter wells on an oceanic island have been analyzed. A finite difference method has been used to take into account the well storage, partial penetration and upconing effect into the aquifer. Forward modeling has been carried out to estimate aquifer parameters from the pumping test data. Field examples are described. Received: 15 June 1998 · Accepted: 25 August 1998     

Seasonal dynamic of a shallow freshwater lens due to irrigation in the coastal plain of Ravenna,Italy

Irrigation in low-lying coastal plains may enhance the formation of fresh groundwater lenses, which counteract salinization of groundwater and soil. This study presents seasonal dynamics of such a freshwater lens and discusses its influence on the salinity distribution of the unconfined aquifer in the coastal plain of Ravenna, Italy, combining field observations with numerical modeling (SEAWAT). The lens originates from an irrigation ditch used as a water reservoir for spray irrigation. The geometry of the freshwater lens shows seasonal differences because of freshwater infiltration during the irrigation season and upconing of deeper saltwater for the remainder of the year. The extent of the freshwater lens is controlled by the presence of nearby drainage ditches. Irrigation also results in a temperature anomaly in the aquifer because of the infiltration of warm water during the irrigation season. The surficial zone in the vicinity of the irrigation ditch is increased considerably in thickness. Finally, different irrigation alternatives and the influence of sea-level rise are simulated. This shows that it is necessary to integrate irrigation planning into the water management strategy of the coastal zone to have maximum benefits for freshening of the aquifer and to make optimal use of the existing infrastructure.     

Geophysical characterization of the complex dynamics of groundwater and seawater exchange in a highly stressed aquifer system linked to a coastal lagoon (SE Spain)

Anthropogenic pressure influences the two-way interactions between shallow aquifers and coastal lagoons. Aquifer overexploitation may lead to seawater intrusion, and aquifer recharge from rainfall plus irrigation may, in turn, increase the groundwater discharge into the lagoon. We analyse the evolution, since the 1950s up to the present, of the interactions between the Campo de Cartagena Quaternary aquifer and the Mar Menor coastal lagoon (SE Spain). This is a very heterogeneous and anisotropic detrital aquifer, where aquifer–lagoon interface has a very irregular geometry. Using electrical resistivity tomography, we clearly identified the freshwater–saltwater transition zone and detected areas affected by seawater intrusion. Severity of the intrusion was spatially variable and significantly related to the density of irrigation wells in 1950s–1960s, suggesting the role of groundwater overexploitation. We distinguish two different mechanisms by which water from the sea invades the land: (a) horizontal advance of the interface due to a wide exploitation area and (b) vertical rise (upconing) caused by local intensive pumping. In general, shallow parts of the geophysical profiles show higher electrical resistivity associated with freshwater mainly coming from irrigation return flows, with water resources mostly from deep confined aquifers and imported from Tagus river, 400 km north. This indicates a likely reversal of the former seawater intrusion process.     

Assessing the impacts of climate change on sustainable management of coastal aquifers

Water is a vital resource for the survival of not only human population, but also almost all ecosystems. Constituting 30 % of all freshwater, groundwater is the main source of available freshwater. Coastal aquifers, which serve as the major freshwater source for densely populated zones, are of vital importance and quite vulnerable to climate change. This paper examines the significant consequences of climate change, decreasing recharge rates, sea-level rise and increasing freshwater demand on the sustainable management of coastal aquifers, via a hypothetical case study. A 3-D numerical model is developed using SEAWAT, to simulate a circular island aquifer in the form of a freshwater lens surrounded by saltwater. Issues such as sloping land surface resulting in landward migration of the coastal boundary and transient response of the system due to pumping are considered through a set of predictive simulations. To assess the sensitivity of the model results to important parameters, a sensitivity analysis is performed. Results of this research, revealing the effects of mentioned pressures on the long-term sustainability of the freshwater resource, are evaluated on the basis of groundwater reserves and intrusion of the freshwater–saltwater interface in lateral and vertical directions. These outcomes are further used to determine the sustainable pumping rate of the system, considering both quantity and quality of the groundwater resources.     

Groundwater mixing in a sand-island freshwater lens: density-dependent flow and stratigraphic controls

This paper focuses on a small back-barrier sand-island on the southeast coast of Queensland. The freshwater lens in the study area exhibits anomalously high short-range salinity gradients at shallow depths, which cannot be explained using a standard seawater intrusion model. The island groundwater system consists of two aquifers: a semiconfined aquifer hosting saline to hypersaline groundwater and an overlying unconfined freshwater aquifer. The deeper aquifer is semiconfined within an incised paleovalley, and groundwater flow is restricted to an east – west direction. Tidal response observations show that the tidal signal propagates far more rapidly and is of much higher magnitude in the semiconfined aquifer than the unconfined aquifer. The tidal wave-pulse amplitude is also subject to greater attenuation in the unconfined aquifer. A conceptual hydrogeological model illustrates how upwelling of hypersaline groundwater, induced by density-dependent flow and tidal pumping, has contaminated the shallow groundwater resource. Salinisation at shallow depths is restricted to an area proximal to the paleovalley aquifer. The spatial distribution of lithological heterogeneity is an initial limiting control on the movement of the upwelling saline plume. The extent of shallow groundwater contamination is also limited by the presence of a baroclinic field, resulting from lateral variations in fluid density. Hydrochemical signatures have been used to support the model hypothesis and link the salinisation of fresh groundwater with the semiconfined aquifer as opposed to the surrounding estuarine surface water. The geometry and thickness of the freshwater lens are further controlled by the presence of the largely impermeable bedrock paleosurface between 9 and 12 m depth. The combination of hypersaline groundwater and hydraulically restrictive lithology at shallow depths has produced excessive thinning of the freshwater lens, demonstrating that the application of a model such as the Dupuit – Ghyben – Herzberg relationship would grossly overestimate the available groundwater resource.     

Optimal pumping from skimming wells from the Yamuna River flood plain in north India

This report examines the problem involving the pumping of groundwater from a group of 90 existing wells along the banks of the Yamuna River, northwest of Delhi (India), underlain with geologically occurring saline water. It is known that unregulated pumping will lead to upconing of saline water and therefore it is necessary to determine optimal rates and associated well locations (from an existing group of candidate wells that supply drinking water to the city of Delhi) that will minimize the total salinity. The nonlinear, non-convex problem is solved by embedding the calibrated groundwater model within a simulation-optimisation (S/O) framework. Optimisation is accomplished by using simulated annealing (SA), a search algorithm. The computational burden is primarily managed by replacing the numerical model with a surrogate simulator-artificial neural network (ANN). The model is applied to the real system to determine the optimal pumping schedule. The results of the operational model suggest that the skimming wells must be operated from optimal locations such that they are staggered in space and time to obtain the least saline water.     

海水入侵研究现状与展望

文中讨论了海水入侵的研究进展：海岸带海水入侵界面或过渡带的模拟，海岛地下淡水透镜体的动力学研究，井孔在界面或过渡带上部抽水所引起的升锥问题以及岸边地下水位动态研究及海平面变化对海水入侵的影响。海水入侵研究经历了从理想假定到合理概化，从室内实验模型、理想模型到数值模型这一过程。数值方法已成为模拟和求解海水入侵问题的最有力工具，海水入侵的过渡带模型成为主要的研究方向。分析了研究中存在的困难，展望了海水入侵研究的未来趋势：边界条件确定，水文地质参数选取，含水层结构估计，海水入侵输运与化学作用的耦合以及海水入侵预报。