大沽河下游咸水入侵区含水砂层水敏感性评价

通过室内砂柱的水平渗流试验,对大沽河下游成水入侵区含水砂层的水敏感性进行了评价。对海水入侵治理的注淡压成和抽成方案过程中含水层的渗透性变化进行了研究．提出了控制和减小方案实施过程中含水砂层渗透性降低的方法和措施。研究结果表明：该含水层具有显著的水敏感性特征,注淡压成过程导致其渗透性下降幅度超过了两个数量级;注淡和抽成的流速分别超过21ml／min和42ml／min时,亦能引起含水砂层水敏感性的发生,称为临界流速;淡水驱替咸水过程中存在一个临界离子强度-7638us／cm,驱替水的离子强度不宜小于该值以避免水敏性的发生。     

控制海水入侵的地下水多目标模拟优化管理模型

为实现滨海含水层地下水开采-回灌方案优化、控制海水入侵面积和降低海水入侵损失等多重管理目标,建立了海水入侵条件下地下水多目标模拟优化管理模型SWT-NPTSGA。模拟模型采用基于变密度流的数值模拟程序SEAWAT来模拟海水入侵过程。优化模型采用小生境Pareto禁忌遗传混合算法NPTSGA来求解,该算法在保证多目标权衡解的收敛性和计算效率的前提下,能维护整个进化群体的全局多样性。将SWT-NPTSGA程序应用于一个理想滨海含水层地下水开采方案和人工回灌控制海水入侵的优化设计中,结果表明该管理模型能够同时处理最大化总抽水流量、最小化人工回灌总量和最小化海水入侵范围等3个目标函数之间的权衡关系。通过采用人工回灌海水入侵区的减灾策略,既能增加滨海地区的供水量,又可减少海水入侵的范围,由此进一步验证了模型的有效性和可靠性。     

地下咸水储能回灌含水层胶体释放动力学与渗透性变异

为了揭示天津滨海地区咸水储能回灌过程中含水层渗透性变化的机制,采用室内土柱试验的方法研究不同温度下咸水储能回灌过程中含水层胶体的释放量、释放速率以及胶体释放对含水层渗透性的影响,同时探讨了胶体释放过程中Ca2+的变化特征。研究结果表明,咸水储能回灌过程中含水层胶体释放是脉冲式的,表现为突然增加,然后缓慢降低,大约20个孔隙体积为一个周期;胶体累积释放量随着孔隙体积数的增加而增加;胶体释放量与孔隙体积数为分段函数,不同阶段胶体累积释放量增长幅度和释放速率发生变化。胶体释放过程中含水层的渗透系数随着孔隙体积数的增加呈“S”型曲线;0℃、10℃和20℃情况下咸水含水层渗透系数分别增加了4.02倍、12.21倍和6.63倍;不同温度下虽然含水层渗透系数开始有差异,但60个孔隙体积数后渗透系数均接近15 cm/d左右。胶体释放过程中也存在着Ca2+-K+、Na+ 阳离子交换作用,不同的温度下这种交换作用动态特征不同。     

井周堵塞对承压含水层定流量回灌渗流场影响的半解析研究

为分析定流量条件下回灌堵塞对井周含水层渗流响应的影响,采用指数形式的渗透系数衰减方程反映堵塞作用下井周含水层渗透特性的时变效应,建立了考虑堵塞效应的定流量非完整回灌井流力学模型,采用变量代换、Laplace变换和有限余弦Fourier变换得到了井周含水层中水位抬升在Laplace空间的解,应用Stehfest数值逆变换方法获得了实时空间内的水头抬升和回灌压力。参数分析结果表明,较小的渐近渗透系数Kr,∞会增加回灌过程中水头抬升和回灌压力,并加剧含水层达到拟稳态时的渗流响应;较大的渗透系数衰减指数λ仅增加回灌过程中水头抬升和回灌压力,但不影响含水层达到拟稳态时的渗流响应;因Kr,∞和λ不同而引起的水头差在井筒处达到最大值并沿径向距离逐渐减小。研究结果可以为定流量回灌中井周含水层堵塞的识别与预测提供理论依据。     

咸淡水过渡带的多组分离子交换行为研究

以大沽河下游海水入侵砂质含水层为研究对象,采集地下淡水、海水和含水介质样品,并测定其组成和性质。通过渗流装置模拟咸淡水驱替过程,测定驱替过程中含水介质的渗透性参数和主要离子的变化规律,并尝试用数值模拟方法定量研究水敏感性影响下的多组分离子交换过程。研究结果表明,海水驱替淡水情况下,Na+能将砂土吸附的Ca2+、Mg2+、K+交换出来,Na+-Ca2+、Na+-K+交换速度很快,而Na+-Mg2+交换反应持续时间较长;在淡水驱替海水的过程中,溶液中的Ca2+、Mg2+将介质吸附的Na+、K+交换出来,但Ca2+-Na+交换起主导作用。含水介质的动力学参数和突破曲线的拐点变化均进一步验证了咸淡过渡带的水敏感性现象,多组分离子的交换行为导致了水敏感现象的发生。     

咸淡水驱替过程中含水介质渗透性变化的试验研究

在野外水文地质调查的基础上,采集青岛市大沽河下游咸水入侵区砂样,首先对含水介质的粒度和矿物组成进行了分析,然后通过室内砂槽模拟试验,对咸淡水驱替过程中含水介质的渗透性变化规律进行了研究。结果表明,当咸淡水相互驱替时,含水介质的渗透性会发生显著变化,这种变化主要是由于驱替液盐浓度的变化,使得伊利石、高岭石和绿泥石等非膨胀性粘土矿物经释放、迁移、絮凝和沉积,从而重新分布所引起的。含水介质渗透性的变化具有明显的非均质性,即位于距入水处不同水平距离以及不同高度处的含水介质,其渗透性变化规律有所不同。     

地下水人工回灌气相堵塞特征的试验研究

地下水人工回灌可以高效地利用雨季丰沛的水量来缓解地下水过量开采造成的海水入侵、泉水断流等环境水文地质问题,但回灌堵塞一直是制约人工回灌效率的关键问题。针对雨洪水回灌携带的大量气泡问题,设计室内砂柱试验模拟装置,利用曝气水进行人工回灌,定时记录试验过程中测压管读数及出流流量,利用达西定律计算各层渗透系数,研究气相堵塞的发展过程和规律。结果表明:回灌过程中由于气相堵塞导致含水层渗透系数随时间呈指数衰减,气相堵塞主要发生在介质浅表层（0~30 cm）,且随时间有向下发展的趋势,堵塞速率随深度的增加逐渐减小,在回灌过程中适时停灌进行排气有利于减小气相堵塞对回灌效率的影响。     

水力屏障和截渗墙在海水入侵防治中的数值模拟研究

基于SEAWAT-2000程序构建室内二维砂箱试验中咸水入侵数值模型,利用该模型分析了针对不同补给井井位、补给井流量、截渗墙位置及贯穿深度等多种情景下的咸淡水界面运移规律。二维砂箱实验模拟结果表明,当注水井位于盐水楔前锋附近,距咸水边界40 cm、砂箱顶部边界5 cm处时,注水井工程措施能达到最佳海水入侵驱退效果,回退系数达21.5%。当截渗墙布设于距咸水边界10 cm处,贯穿深度为35 cm时,截渗墙工程措施能达到最佳海水入侵驱退效果,回退系数达81.1%。在此基础上,结合实际场地条件,构建山东龙口地区滨海含水层中某典型二维剖面的海水入侵数值模型,探讨了不同截渗墙布设情景模式下海水入侵状况。模拟结果表明,当截渗墙布设于距海岸线600 m处,贯穿深度为18 m时,截渗墙工程措施达到最佳海水入侵驱退效果,回退系数达28.4%。研究结果揭示了补给井井位、补给井流量、截渗墙位置及贯穿深度等因素对咸淡水界面运移规律的影响,可为场地条件下滨海含水层海水入侵防治中的工程管理措施优化提供参考依据。     

运用三维变密度潮汐效应模型确定滨海含水系统的海底等效边界——山东烟台夹河中下游地区为例

确定咸淡水界面的位置是滨海地区海水入侵研究的主要任务之一。对于天然条件承压含水层而言，含水层顶板向海底延伸的距离直接影响了咸淡水界面的位置，它可以通过承压含水层中地下水的潮汐效应信息来确定。考虑到咸淡水之间密度的差异，建立了山东省夹河中下游地区滨海含水系统地下水三维变密度潮汐效应模型。通过反复对比潮汐效应观测中的地下水水头波动与模型计算出的水头波动，确定了滨海承压含水系统的海底边界。同时，也初步估计出海区与近海陆区含水层的水文地质参数。     

不同粘土矿物水敏性特征

通过改变砂样中的粘土矿物的种类和含量,结合咸淡水之间的单一和驱替实验,研究不同粘土矿物的水敏性特征。在单一的咸水和淡水情况下,当砂柱中的粘粒含量≤1.5%(重量比)时,其渗透系数基本不发生改变,即使粘粒含量从1.5%增加10%,其渗透系数也仅下降约一个数量级。各种粘土矿物中,蒙脱石导致含水介质渗透系数的下降最为明显,高岭土和伊利石约是其影响值的一半。咸淡水驱替实验中,加入蒙脱石颗粒的砂柱的渗透系数急剧下降,当蒙脱石含量达3~4%时,渗透系数已经下降的极低,可以看作不透水层,而加入高岭土和伊利石的砂柱的渗透系数没有急剧变化。对于这种现象的解释是:不同类型的粘土颗粒吸附不同数量的水分子,形成数量和大小不同的胶体团,使得介质渗透系数的变化量不同。     

A thirty-year artificial recharge experiment in a coastal aquifer in an arid zone: The Teboulba aquifer system (Tunisian Sahel)

With the increased demand for groundwater resulting from fast demographic growth, accelerated urbanization, economic and agricultural activity diversification, and the increase of per capita consumption, ground water resources, in particular in coastal regions, remain relatively low, compared to demand. The groundwater quality and piezometric variations result mainly from intensive exploitation, agricultural activities and the intrusion of seawater. This phenomenon is observed mostly in semi-arid areas, such as the oriental Sahel of Tunisia, where an apparent reduction in rainfall in recent years can be seen. Groundwater becomes overexploited especially as its natural recharge by rainwater does not succeed in maintaining the hydrologic balance. The imbalance between water demand and resources induces the degradation of the water quality. In such a case, the artificial recharge of water-table aquifers by water from dams is a credible alternative to improve the hydrodynamic and physicochemical conditions of the groundwater. Like most coastal aquifers, the Teboulba water-table aquifer is threatened by overexploitation for at least three decades. This threat appears by a considerable piezometric level drop and by water salinisation, due to seawater intrusion. Given this alarming situation, since 1971, artificial recharge through wells with surface water from a dam was tested in order to restore the water levels and to improve water quality. The piezometric and chemical surveys of the Teboulba aquifer permitted one to describe the temporal and spatial piezometric and geochemical conditions of the aquifer and to show the effect of the artificial recharge. Indeed, the artificial recharge undertaken since 1971 made the geochemical and piezometric conditions of the Teboulba aquifer improve. This example is a rare, well-documented case-study of the benefits of artificial recharge in a coastal aquifer, over the long term.     

Managed aquifer recharge (MAR) by the construction of subsurface dams in the semi-arid regions: A case study of the Kalangi river basin, Andhra Pradesh

Overuse of groundwater in coastal areas, due to high population and agricultural activity results in seawater intrusion into the coastal aquifer. This paper presents the control measures taken to manage aquifer recharge (MAR) and also to overcome the problem of seawater intrusion into the coastal aquifer along the Kalangi river, Nellore district of Andhra Pradesh, India having connectivity with Pulicat (saltwater) lake estuary. Due to overexploitation of groundwater and less rainfall in past years, adjacent seawater has started intruding in the Kalangi river sub-surface and deteriorating groundwater quality up to 11.6 km from the confluence of the river with Pulicat lake. To prevent this situtation, subsurface dams were constructed in traditional manner using local earth material in three different places across the Kalangi river near Sullurpet town. The water storage capacities calculated after the sub-surface dams’ construction are 1.28 mcft at GK Engineering College, 6.23 mcft at Challamagudi and 3.143 mcft at Holy Cross School sites. The Holy Cross School sub-surface dam is the first full scale dam-cum-check dam constructed to prevent salt water intrusion in the Kalangi river at Sullurpet, Nellore district, Andhra Pradesh. At the Kalangi river estuary portion (at the mouth of sea) a groyne was reconstructed over old groyne site with the introduction of clay bed and wooden sheet piles at down stream. Apart from prevention of sea water entry into Kalangi river sub-surface (during seasons) the groyne top level was raised to prevent mixing of high sea water tides with fresh water and ensuring additional storage of fresh water at upstream side. The reconstructed groyne was serving the purpose of obstructing the surface seawater entry in the Kalangi river and water quality has improved in the river as well as in the wells. After construction of sub-surface dam, as per the Simpson ratio classification, there is substantial improvement of water quality in the SHAR infiltration well situated near the Holy Cross School sub-surface dam.     

Hydrogeological investigation of groundwater artificial recharge by treated wastewater in semi-arid regions: Korba aquifer (Cap-Bon Tunisia)

Korba aquifer is one of the most typical examples of overexploited coastal aquifer in the Mediterranean countries. In fact, from 1985, a considerable piezometric level drop, water salinization, and seawater intrusion were registered in the aquifer. In December 2008, Tunisian authorities initiated a general plan to groundwater management in order to augment groundwater resources, restore the piezometric levels, and improve water quality. The plan consists of artificial recharge of groundwater used treated wastewater through three infiltration basins. During the first 4 years (from December 2008 to December 2012), 1.41 Mm³ of treated wastewater was injected to the Korba aquifer. This study presents a hydrogeological assessment of groundwater evolution during the recharge processes. In this study, 32 piezometric and chemical surveys of 70 piezometers and observed wells are used to present hydrogeological investigation and water quality evolution of wastewater reuse through artificial recharge in Korba coastal aquifer. The piezometric evolution maps are used to specify the positive effect in groundwater level that exceeding 1.5 m in some regions. The interpretation of salinity evolution maps are used to indicate the improving of groundwater quality.     

A simulation‐optimization model to control seawater intrusion in coastal aquifers using abstraction/recharge wells

Seawater intrusion is one of the most serious environmental problems in many coastal regions all over the world. Mixing a small quantity of seawater with groundwater makes it unsuitable for use and can result in abandonment of aquifers. Therefore, seawater intrusion should be prevented or at least controlled to protect groundwater resources. This paper presents development and application of a simulation‐optimization model to control seawater intrusion in coastal aquifers using different management scenarios; abstraction of brackish water, recharge of freshwater, and combination of abstraction and recharge. The model is based on the integration of a genetic algorithm optimisation technique and a coupled transient density‐dependent finite element model. The objectives of the management scenarios include determination of the optimal depth, location and abstraction/recharge rates for the wells to minimize the total costs for construction and operation as well as salt concentrations in the aquifer. The developed model is applied to analyze the control of seawater intrusion in a hypothetical confined coastal aquifer. The efficiencies of the three management scenarios are examined and compared. The results show that combination of abstraction and recharge wells is significantly better than using abstraction wells or recharge wells alone as it gives the least cost and least salt concentration in the aquifer. The results from this study would be useful in designing the system of abstraction/recharge wells to control seawater intrusion in coastal aquifers and can be applied in areas where there is a risk of seawater intrusion. Copyright © 2011 John Wiley & Sons, Ltd.     

A conceptual and numerical model for groundwater management: a case study on a coastal aquifer in southern Tuscany, Italy

Ongoing hydrogeological research aims to develop a correct management model for the Plio-Pleistocene multi-aquifer system of the Albegna River coastal plain (southern Tuscany, Italy); overexploitation of this aquifer for irrigation and tourism has caused seawater intrusion. The conceptual model is based on field and laboratory data collected during the 1995–2003 period. Meteoric infiltration and flows from the adjoining carbonate aquifer recharge the aquifer. Natural outflow occurs through a diffuse flow into the sea and river; artificial outflow occurs through intensive extraction of groundwater from wells. Water exchanges in the aquifer occur naturally (leakage, closing of aquitard) and artificially (multiscreened wells). The aquifer was represented by a three-dimensional finite element model using the FEFLOW numerical code. The model was calibrated for steady-state and transient conditions by matching computed and measured piezometric levels (February 1995–February 1996). The model helped establish that seawater intrusion is essentially due to withdrawals near the coast during the irrigation season and that it occurs above all in the Osa-Albegna sector, as well as along the river that at times feeds the aquifer. The effects of hypothetical aquifer exploitation were assessed in terms of water budget and hydraulic head evolution.     

Topical Collection: Coastal aquifers in the Middle East and North Africa region

The Middle East and North Africa (MENA) region suffers from low precipitation and high evaporation. Coastal areas of the MENA region are usually densely populated; hence, the coastal aquifers are easily over-exploited beyond their safe yield, and seawater intrusion and aquifer salinization have been caused by the groundwater-level declines. Four studies in MENA coastal aquifers, on seawater intrusion modeling and aquifer recharge and salinization using isotopes, have been brought together in a topical collection and are discussed in this essay. A brief overview is given of managed aquifer recharge as an effective method to combat groundwater-level decline, seawater intrusion and aquifer salinization in MENA counties.     

Groundwater recharge dams in arid areas as tools for aquifer replenishment and mitigating seawater intrusion: example of AlKhod, Oman

Groundwater depletion and seawater intrusion constitute major challenges along coastal aquifers in arid areas. This paper assesses the role of groundwater recharge dams constructed to replenish aquifers and fight seawater intrusion with reference to AlKhod dam, Oman, sited 7 km from the coast on a gravely unconfined aquifer. Water table rise in piezometers located downstream from the dam shows regular patterns correlating with magnitude of wadi flow, whereas upstream piezometers show irregular patterns. Controlled release of water captured by the dam optimizes water percolation and enhances artificial recharge which was estimated in the wet years 1997, 2003 and 2005 as 15, 22 and 27 Mm³, respectively, using water table fluctuation method. Recharge contributed 40–60 % of the total annual abstraction. Groundwater salinity increased in the 1980s and 1990s and the saline/freshwater interface advanced inland, but has receded partially after 1997 (highest rainfall) and completely after 2005 indicated by reduction in electrical conductivity and thickening of freshwater lens. The recession is attributed to the dam’s induced recharge and reduction of pumping in 2004 following the commissioning of Barka desalination plant. Integrating artificial recharge with groundwater resources management is therefore an effective measure to replenish aquifers in arid areas and mitigate seawater intrusion along the coasts.     

Hydrochemical and statistical studies of the groundwater salinization in Mediterranean arid zones: case of the Jerba coastal aquifer in southeast Tunisia

Coastal aquifers are considered as major sources for freshwater supply worldwide, especially in arid zones. The weak rainfall as well as the intensive extraction of groundwater from coastal aquifers reduce freshwater budget and create local water aquifer depression, causing both seawater intrusion and a threat to groundwater. This phenomenon was observed in the Jerba Island which is located in southeast Tunisia. Jerba??s unconfined aquifer shows high values of groundwater salinity reaching, locally, 17?g/l and a strong contrast between some zones of the aquifer. High pumping rates and weak recharge disturb the natural equilibrium between fresh and saline water causing water salinization in most areas of the island. This study aims at establishing the salinity map of the aquifer and identifying the origin of groundwater salinization. The salinity map shows that zones characterized by low groundwater salinity are located in the center of the study area. High groundwater salinities are observed near the coast and in some parts having low topographic and piezometric levels. Groundwater geochemical characterization, and Br/Cl and Na/Cl ratios suggest that the origin of abnormal salinity is seawater intrusion. Considering groundwater salinity values and Br concentrations, a seawater intrusion map is established. It shows that many areas of the unconfined aquifer are contaminated by mixed groundwater and seawater. The statistical analysis demonstrates that high mineralization of the groundwater is due to gypsum and carbonate dissolution coupled with the mixed groundwater and seawater in many areas.     

Seawater intrusion in the Salalah plain aquifer,Oman

Salalah is situated on a fresh water aquifer that is replenished during the annual monsoon season. The aquifer is the only source of water in Salalah city. The rainfall and mist precipitation in the Jabal AlQara recharges the plain with significant renewable fresh groundwater that has allowed agricultural and industrial development to occur. In Salalah city where groundwater has been used extensively since the early 1980s for agricultural, industrial and municipal purposes, the groundwater has been withdrawn from the aquifer more rapidly than it can be replenished by natural recharge. The heavy withdrawal of large quantities of the groundwater from the aquifer has led to the intrusion of seawater. Agricultural activities utilize over 70% of the groundwater. For the study of the saltwater intrusion, the area has been divided into four strips, A, B, C and D, on the basis of land-use in the area. Water samples were collected from 18 water wells. Chemical analysis of major ions and pollution parameters in the groundwater was carried out and compared to the previous observed values. The electrical conductivity and chloride concentrations were highest in the agricultural and residential strips and Garziz grass farm. Before 1992 the aquifer was in a steady state, but presently (2005) the groundwater quality in most of the agricultural and residential strips does not meet drinking water standards. In addition, model simulations were developed with the computer code MODFLOW and MT3DMS for solute transport to determine the movement of the freshwater/saltwater interface. The study proposes the protection of the groundwater in Salalah plain aquifer from further encroachment by artificial recharge with reclaimed water, preferably along the Salalah coastal agricultural strip. This scheme can also be applied to other regions with similar conditions.     

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.