Groundwater chemistry patterns in the phreatic aquifer of the central Belgian coastal plain

The Holocene geological evolution of the Belgian coastal plain is dominated by a transgression of the North Sea, silting up of the coastal plain and human intervention (impoldering). This has led to a typical pattern in groundwater quality which is discussed here for the central part of the coastal plain. Therefore, a database with available groundwater samples is composed. Water type according to the Stuyfzand classification is determined and different hydrosomes and their hydrochemical facies are identified. Based on this, the origin and evolution of the water types is explained using Piper plots and geochemical calculations with PHREEQC. Before the impoldering, salinising and freshening conditions alternated with a general salinisation of the aquifer after about 3400 BP. This results in a dominance of brackish and salt NaCl subtypes which are still found in the deeper part of the aquifer. The subsequent impoldering resulted in an major freshening of the aquifer leading to NaHCO₃, MgHCO₃ and CaHCO₃ subtypes. Overall, mixing, cation exchange, carbonate mineral dissolution and oxidation of organic matter are identified as the major processes determining the general water quality. The close link between geological evolution, water quality and what is still observable today is illustrated with this example of the Belgian coastal plain.     

试论深层地下水人工补给--SPD人工补给系统

本文提出了一种新的深层地下水回灌方式——SPD人工补给系统，并通过室内沙槽浑水模拟试验，对该系统和常规的直接井回灌法作了比较。研究结果表明，利用该系统补给深层地下水，具有回灌水质要求低、补给速度快、补给量多、不易淤堵等特点，是深层地下水人工补给的一条有效的新途径。     

The effectiveness of artificial recharge in combating seawater intrusion in Salalah coastal aquifer, Oman

The Salalah central sewage treatment plant has been designed to treat 20,000 m³/day at the first stage and two further stages to double the initial capacity. The plant currently (2005) treats more than 15,000 m³/day effluents to a tertiary level, and after chlorination phase, the effluents are recharged into tube wells in a line parallel to the coast. The process aims to help stabilize the seawater interface and a part to be recovered from hand-dug wells/boreholes further inland and downstream. A three-dimensional flow and solute advection transport model was developed to assess the effectiveness of the proposed recharge scheme and to track the solute transport with respect to the design system. The advection transport model predicted that in 2020 the maximum pathlines of the injection fluids would reach the abstraction wells that are located 600 m, southward of the injection bores in about 1-year travel time in the case of the no-management interference and more than that southward under management interference. The developed flow predicted the wedge of the saline intrusion in 2019 is tracked up to 2.7 and 3.4 km from the shoreline with the injection and without the injection, respectively under constant underflow. The injection scheme is effective in pushing back the saline zone front by 700 m. This study argues that the treated wastewater would help to increase the water levels at the vicinity of the injection line and to reduce the influence of saline inflows from the coast. The reclaimed sewage recharge scheme is examined in the case of the Salalah coastal aquifer using groundwater simulation, which can also be applied to other regions with similar conditions.     

Groundwater recharge and salinization in the arid coastal plain aquifer of the Wadi Watir delta,Sinai, Egypt

The Quaternary coastal plain aquifer down gradient of the Wadi Watir catchment is the main source of potable groundwater in the arid region of south Sinai, Egypt. The scarcity of rainfall over the last decade, combined with high groundwater pumping rates, have resulted in water-quality degradation in the main well field and in wells along the coast. Understanding the sources of groundwater salinization and amount of average annual recharge is critical for developing sustainable groundwater management strategies for the long-term prevention of groundwater quality deterioration. A combination of geochemistry, conservative ions (Cl and Br), and isotopic tracers (^87/86Sr, δ⁸¹Br, δ³⁷Cl), in conjunction with groundwater modeling, is an effective method to assess and manage groundwater resources in the Wadi Watir delta aquifers. High groundwater salinity, including high Cl and Br concentrations, is recorded inland in the deep drilled wells located in the main well field and in wells along the coast. The range of Cl/Br ratios for shallow and deep groundwaters in the delta (∼50–97) fall between the end member values of the recharge water that comes from the up gradient watershed, and evaporated seawater of marine origin, which is significantly different than the ratio in modern seawater (228). The ^87/86Sr and δ⁸¹Br isotopic values were higher in the recharge water (0.70,723 < ^87/86Sr < 0.70,894, +0.94 < δ⁸¹Br < +1.28‰), and lower in the deep groundwater (0.70,698 < ^87/86Sr < 0.70,705, +0.22‰ < δ⁸¹Br < +0.41‰). The δ³⁷Cl isotopic values were lower in the recharge water (−0.48 < δ³⁷Cl < −0.06‰) and higher in the deep groundwater (−0.01 < δ³⁷Cl < +0.22‰). The isotopic values of strontium, chloride, and bromide in groundwater from the Wadi Watir delta aquifers indicate that the main groundwater recharge source comes from the up gradient catchment along the main stream channel entering the delta. The solute-weighted mass balance mixing models show that groundwater in the main well field contains 4–10% deep saline groundwater, and groundwater in some wells along the coast contain 2–6% seawater and 18–29% deep saline groundwater.A three-dimensional, variable-density, flow-and-transport SEAWAT model was developed using groundwater isotopes (⁸⁷Sr/⁸⁶Sr, δ³⁷Cl and δ⁸¹Br) and calibrated using historical records of groundwater level and salinity. δ¹⁸O was used to normalize the evaporative effect on shallow groundwater salinity for model calibration. The model shows how groundwater salinity and hydrologic data can be used in SEAWAT to understand recharge mechanisms, estimate groundwater recharge rates, and simulate the upwelling of deep saline groundwater and seawater intrusion. The model indicates that most of the groundwater recharge occurs near the outlet of the main channel. Average annual recharge to delta alluvial aquifers for 1982 to 2009 is estimated to be 2.16 × 10⁶ m³/yr. The main factors that control groundwater salinity are overpumping and recharge availability.     

人工回灌条件下滨海砂质含水层渗透性的时空演变特征

人工回灌补给滨海含水层是海水入侵修复经常采用的工程措施,然而人工回灌条件下滨海砂质含水层经常出现渗透性显著降低的现象,其含水介质渗透性的时空演变规律仍不清楚。本文以青岛市大沽河下游咸水入侵区含水层砂样为研究对象,通过利用不同尺度的室内砂柱淡咸水驱替试验,对定水头和定流速条件下人工回灌咸淡水驱替过程中砂质含水层渗透性的时空演变特征进行了研究。结果表明：当淡水驱替咸水时含水介质的渗透性会发生显著变化,整个砂柱的渗透性先降低后回升,原因是回灌过程中砂柱中的黏土矿物发生释放、迁移、沉积;定水头条件下,在砂柱的前半段存在一个淘空区,渗透系数是初始渗透系数的1.6~2.0倍,砂柱其它部分渗透性是先降低再轻微回弹;定流速条件下,砂柱的渗透系数随时间的变化均呈现先降低后升高趋势,且砂柱各段离入水口的距离越远其渗透系数的值越小;砂柱黏粒含量的变化规律与渗透性的演化特征相吻合。研究成果可为人工回灌治理滨海含水层海水入侵提供一定的科学依据。     

Groundwater recharge formation in the Heihe intermountain artesian basin

The Heihe intermountain artesian basin is situated in northwestern China. The basic types of formation of groundwater recharge have been identified, viz., the percolation of river waters in the mountain regions and infiltration from irrigation canals and fields in the plain areas with a well-developed irrigation system. Such sources account for about 90% of the overall groundwater recharge.     

Groundwater flow model, recharge estimation and sustainability in an arid region of Patagonia, Argentina

The Península Valdés, in northeastern Patagonia, Argentina, is characterised by its arid climate and the lack of perennial watercourses; thus, all economic activities depend on the groundwater resources. Water demand is mainly associated with tourism, which is centralised in Puerto Pirámides and supplied by a water desalination plant, and to sheep farming, supplied by the local aquifer. Due to the exponential growth of tourism, the government is planning to exploit groundwater and convey it by aqueduct to the abovementioned locality. The objectives of this study were to corroborate the conceptual geohydrological model, to develop a mathematical model to simulate the response of the aquifer to different scenarios, and to assess the incidence of water input into the system as a variable—a function that poses difficulties in the models for arid regions. The Visual Modflow 4.1 code was used, calibrating it in trial-and-error mode, changing the recharge and hydraulic conductivity parameters with different variants in the recharge zone and in the inclusion or exclusion of the evapotranspiration module. Results indicate the importance of the recharge analysis by treating rainfall at daily time steps. The adjusted model was exposed to four scenarios with variations in water input and in output by pumping. It can be concluded that under different input conditions, but with a controlled extraction, the system responds in a sustainable manner.     

Seasonal variation in natural recharge of coastal aquifers

Many coastal zones around the world have irregular precipitation throughout the year. This results in discontinuous natural recharge of coastal aquifers, which affects the size of freshwater lenses present in sandy deposits. Temperature data for the period 1960–1990 from LocClim (local climate estimator) and those obtained from the Intergovernmental Panel on Climate Change (IPCC) SRES A1b scenario for 2070–2100, have been used to calculate the potential evapotranspiration with the Thornthwaite method. Potential recharge (difference between precipitation and potential evapotranspiration) was defined at 12 locations: Ameland (The Netherlands), Auckland and Wellington (New Zealand); Hong Kong (China); Ravenna (Italy), Mekong (Vietnam), Mumbai (India), New Jersey (USA), Nile Delta (Egypt), Kobe and Tokyo (Japan), and Singapore. The influence of variable/discontinuous recharge on the size of freshwater lenses was simulated with the SEAWAT model. The discrepancy between models with continuous and with discontinuous recharge is relatively small in areas where the total annual recharge is low (258–616 mm/year); but in places with Monsoon-dominated climate (e.g. Mumbai, with recharge up to 1,686 mm/year), the difference in freshwater-lens thickness between the discontinuous and the continuous model is larger (up to 5 m) and thus important to consider in numerical models that estimate freshwater availability.     

Groundwater recharge,flow and hydrogeochemical evolution in a complex volcanic aquifer system,central Ethiopia

Hydrochemical, multivariate statistical and inverse hydrogeochemical modeling techniques were used to investigate groundwater recharge, flow and the hydrochemical evolution within the Akaki volcanic aquifer system, central Ethiopia. The hydrochemical and multivariate statistical techniques are mutually supportive and the extracted information was analyzed together with environmental isotope data. Results reveal five spatial groundwater zones with defined hydrochemical facies, residence times, stable isotopic signals and hydrochemical evolution. These zones are designated as the (1) Intoto, (2) central, (3) Filwuha fault, (4) south zones and (5) a highly polluted sub-sector identified within the central zone. Both the hydrochemical and multivariate statistical analyses have shown the central sub-sector as being spite of differentially polluted by , Cl⁻ and and its tritium content shows recent recharge. Due to the fact that the main recharge source is precipitation, the hydrochemical and environmental isotope data clearly indicated that the central and southern sectors are also recharged from domestic waste water and leakage from water mains and reservoirs. Inverse hydrogeochemical modeling demonstrated reactions of silicate minerals in a CO₂ open system and precipitation of kaolinite, chalcedony, and rare calcite satisfy the observed change in water chemistry from north to south following the regional flow direction.

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Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.

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Résumé Des techniques relevant de l’hydrochimie, des statistiques multivariées et de la modélisation inverse hydrogéochimique, ont été utilisées dans le cadre de l’étude de la recharge des eaux souterraines, de l’écoulement et de l’évolution hydrochimique dans le système volcanique aquifère d’Akaki au centre de l’Ethiopie. Les techniques hydrochimiques et multivariées se supportent mutuellement et l’information extraite a été analysée avec les données isotopiques environnementales, des temps de résidence, des signaux isotopiques stables et une évolution hydrochimique. Ces zones ont été désignées comme le (1) Intoto, (2) le centre, (3) la faille de Filwuha, (4) les zones sud et (5) un sous-secteur fortement pollué identifié dans la zone centrale. Les analyses statistiques hydrochimiques et multivariées ont montré que le sous-secteur central a été différemment pollué par , Cl⁻ et , tandis que la teneur en tritium montre une recharge récente. Malgré le fait que la principale source de recharge soit les précipitations, les données hydrochimiques et isotopiques indiquent clairement que les secteurs centres et sud sont également rechargés par les eaux usées domestiques et les fuites de réservoirs et canalisations d’eau. La modélisation hydrogéochimique inverse a démontré les réactions des minéraux silicatés dans un système ouvert au CO₂, et la précipitation de kaolinite, de calcédoine, et la rareté de la calcite satisfont les changements observés dans la chimie de l’eau du nord vers le sud en suivant la direction régionale de l’écoulement.

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Resumen Se utilizaron la hidroquímica y técnicas de modelación hidrogeoquímica inversa y estadística multivariada, para investigar la recarga del agua subterránea, el flujo y la evolución hidroquímica, dentro del sistema acuífero volcánico Akaki, Etiopía Central. La hidroquímica y las técnicas estadísticas multivariadas se complementan entre si y la información así extraída se analizó junto con los datos de isótopos ambientales. Los resultados revelan cinco zonas diferentes de agua subterránea, con facies hidroquímicas, tiempos de residencia, improntas isotópicas estables y una evolución hidroquímica definidas. Estas zonas se designan como (1) Intoto, (2) Central, (3) Falla de Filwuha, (4) las Zonas del sur y (5) un sub-sector altamente contaminado identificado dentro de la zona central. Tanto los análisis estadísticos multivariados como la hidroquímica, han mostrado al sub-sector central como contaminado diferencialmente por , Cl⁻ y y su contenido de tritio muestra una recarga reciente. A pesar del hecho que la fuente principal de recarga es la precipitación, los datos de hidroquímica y de isótopos ambientales indican que los sectores central y del sur, también se recargan a partir de agua doméstica usada y del goteo de las conducciones del acueducto y de sus reservorios. El modelamiento hidrogeoquímico inverso demostró reacciones de minerales silicatados en un sistema de CO₂ abierto, y la precipitación de caolinita, calcedonia, y rara vez de calcita, satisfacen el cambio observado en la química de agua del norte a sur, siguiendo la dirección del flujo regional.

     

Groundwater nitrate pollution in the recharge zone of a regional Quaternary flow system (Wielkopolska region, Poland)

The present study examines the behaviour of nitrate in the recharge zone of a regional Quaternary flow system. The presence of contaminated groundwater with high nitrate content in the shallow part of the flow system was documented. Tritium analyses confirmed that these contaminants can migrate downward. A high downward gradient exists in the study area, extending into the regions of groundwater extraction. In the unconfined part of the flow system, which is the most vulnerable to pollution, a high concentration of nitrate was found to occur at great depth. However, denitrification processes limit nitrate migration. As a result, in the deeper parts of the flow system in regions under confined conditions, an absence of nitrate was observed, and a higher sulphate concentration and total hardness were evident. The denitrification was also confirmed by the existence of a high gaseous N₂ concentration. It was documented that denitrification occurs in both the confined and unconfined parts of the flow system, but the potential for denitrification is higher in the confined parts (leading to the disappearance of nitrate in deep aquifers). Autotrophic denitrification supported by sulphide compounds was indicated as the dominant denitrification process.     

沿岸地下水排放通量

海岸带地区,陆源物质由近岸向沿岸海洋输送的途径有很多,而在一些海岸或海湾,其中较普遍的一个过程就是海底地下水排放。它是发生在沿岸地区与毗邻海洋之间的又一个重要的陆海相互作用过程。目前,通过水文、直接测量和地球化学示踪法研究地下水排放的例子有很多,但对其进行定量估算仍面临着许多困难。本文主要介绍镭同位素地球化学示踪法的基本原理及应用,以海南东北部沿岸水域中镭同位素数据为例,计算得到相应的地下水排放通量为5.4×10-4dm3/(d.cm2)。     

Groundwater recharge in a sedimentary basin in semi-arid Mexico

Recharge mechanisms and the hydrochemical evolution of groundwater in a semi-arid, 6,840-km², intermountain basin in central Mexico were investigated using stable isotopes and major chemical constituents. Ionic ratio analysis helped to conceptualize and quantify in part the subsequent geochemical evolution in the aquifer system. Mass balance models (PHREEQC) were used to interpret and rectify the geochemical properties of the aquifer. The recharge conditions have not changed noticeably during the last several thousands of years. The recharge mechanisms are accompanied by leaching of meteoric salts on and near the ground surface during major rain events, which previously accumulated after minor rain events. Rapid and diffuse infiltration can be excluded. Indirect infiltration from wadis (arroyos) and depressions (playas) with little mixing in shallow groundwater contrasts with a high degree of mixing for water with deep circulation. The prevailing source of major cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) is weathering of carbonates and albite, followed by exchange reactions on clays and hydroxides. Ca²⁺/Na⁺ exchange may interchange along the flow path with reverse (Na⁺/Ca²⁺) exchange, although the Ca²⁺/Na⁺ option is prevalent. Meteoric Ca and Mg inputs are relatively small; however, meteoric Na is insignificant. Irrigation return flow plays an important role in the western part of the study area, giving rise to elevated sulfate and chloride concentrations.

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Resumen Se han investigado los mecanismos de recarga y la evolución hidroquímica de las aguas subterráneas en una cuenca semiárida e intermontanosa de México central, de 6.840 km². Para ello, se han utilizado isótopos estables y los constituyentes químicos mayores. El análisis de las relaciones iónicas ha servido para conceptuar y cuantificar en parte su evolución geoquímica posterior dentro del sistema acuífero. Se ha recurrido a modelos de balance de masas (PHREEQC) para interpretar y rectificar las propiedades geoquímicas del acuífero. Las condiciones de recarga no han cambiado de forma apreciable durante los últimos miles de años. Los mecanismos de recarga se ven acompañados por el lixiviado de las sales meteóricas sobre y cerca de la superficie del terreno durante los episodios principales de lluvia, las cuales son acumuladas en episodios menores de lluvia. Se puede excluir la infiltración rápida y difusa. La infiltración indirecta desde arroyos ( wadis) y depresiones ( playas), que apenas se mezcla con las aguas subterráneas someras, contrasta con un elevado nivel de mezcla con el agua de circulación profunda. La fuente dominante de cationes mayores (calcio, magnesio, sodio, potasio) es la meteorización de los carbonatos y albita, mientras que las reacciones de intercambio en las arcillas e hidróxidos son menos importantes. El intercambio ión calcio-ión sodio puede ser reemplazado a lo largo de una línea de flujo por el intercambio opuesto (ión sodio-ión calcio), aunque la primera es prioritaria. Las aportaciones meteóricas de calcio y magnesio son relativamente pequeñas, mientras que la de sodio es insignificante. Los retornos de riego desempeñan un papel importante en la parte occidental del área de estudio, dando lugar a concentraciones elevadas de sulfato y cloruro.

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Résumé Les mécanismes de recharge et lévolution hydrochimique de leau souterraine dans un bassin de montagne de 6 840 km² en zone semi-aride, dans le centre du Mexique, ont été étudiés au moyen des isotopes stables et des composés chimiques majeurs. Lanalyse des rapports ioniques a aidé à conceptualiser et à quantifier en partie lévolution géochimique qui en résulte, dans le système aquifère. Des modèles de bilan de masse (PHREEQC) ont été utilisés pour interpréter et corriger les propriétés de laquifère. Les conditions de recharge nont pas changé notablement au cours des derniers millénaires. Les mécanismes de recharge sont accompagnés, durant les épisodes majeurs de précipitation, dun lessivage, à la surface du sol et à son voisinage, de sels météoriques accumulés auparavant pendant les petits épisodes de pluie. Linfiltration rapide et diffuse peut être exclue. Linfiltration indirecte à partir des oueds (arroyos) et des dépressions (playas) avec un faible mélange dans la nappe superficielle contraste avec le degré élevé de mélange de leau avec les circulations profondes. La source prépondérante des cations majeurs (Ca²⁺, Mg²⁺, Na⁺, K⁺) est laltération des carbonates et des feldspaths ; léchange de cations avec les argiles et les hydroxydes est moins important. Léchange de Ca²⁺ avec Na⁺ peut sinverser le long des axes découlements pour donner un échange de Na⁺ avec Ca²⁺, bien que le cas Ca²⁺/Na⁺ soit prépondérant. Les apports météoriques de Ca et de Mg sont relativement faibles, cependant que celui de Na météorique est insignifiant. Lécoulement par retour dirrigation joue un rôle important dans la partie occidentale de la région étudiée, produisant un accroissement des concentrations élevées en sulfate et en chlorure.

     

工程降水中人工回灌综合技术

建筑工程降水往往伴生水资源浪费和地面变形等环境问题,人工回灌是解决这些问题的有效手段之一。影响工程降水中人工回灌的条件有回灌场地水文地质条件、回灌水源水质和水量及回灌方案的经济可行性等因素。就工程降水中出现的问题,提出资源补充型回灌和应力稳定型回灌方法,并给出了适用条件; 根据工程降水水质特点及现有的地下水人工回灌相关水质标准,提出工程降水回灌水质的控制指标主要为悬浮物、浊度、一般污染性指标、微生物指标及重金属等。针对目前人工回灌在工程中存在的可回灌性低的问题,进行了影响入渗速率因素研究,为今后开展促渗关键技术提供理论支持。     

Groundwater recharge and evolution in the Dunhuang Basin,northwestern China

Groundwater recharge and evolution in the Quaternary aquifer beneath the Dunhuang Basin was investigated using chemical indicators, stable isotopes, and radiocarbon data to provide guidance for regional water management. The quality of groundwater and surface water is generally good with low salinity and it is unpolluted. The dissolution of halite and sylvite from fine-grained sediments controls concentrations of Na⁺ and K⁺ in the groundwater, but Na⁺/Cl⁻ molar ratios >1 in all samples are also indicative of weathering of feldspar contributing to excess Na⁺. The dissolution of carbonate minerals yields Ca²⁺ to the groundwater, thereby exerting a strong influence on groundwater salinity. The δ¹⁸O and δ²H values in unconfined groundwater are enriched along the groundwater flow path from SW to NE. In contrast, confined groundwater was depleted in heavy isotopes, with mean values of −10.4‰ δ¹⁸O and −74.4‰ δ²H. Compared with the precipitation values, all of the groundwater samples were strongly depleted in heavy isotopes, indicating that modern direct recharge to the groundwater aquifers in the plains area is quite limited. The unconfined water is generally young with radiocarbon values of 64.9–79.6 pmc. In the northern basin, radiocarbon content in the confined groundwater is less than 15 pmc and an uncorrected age of ∼15 ka, indicates that this groundwater was recharged during a humid climatic phases of the late Pleistocence or early Holocene. The results have important implications for inter-basin water allocation programmes and groundwater management in the Dunhuang Basin.     

Groundwater recharge and flow in the Lower Cretaceous Nubian Sandstone aquifer in the Sinai Peninsula, using isotopic techniques and hydrochemistry

The present work was conducted in the Sinai Peninsula (1) to identify the recharge and flow characteristics and to evaluate the continuity of the Lower Cretaceous Nubian Sandstone aquifer; and (2) to provide information for the aquifer's rational appraisal. Isotopic and hydrochemical compositions combined with the geological and hydrogeological settings were used for this purpose. A considerable depletion in isotopic content (oxygen-18 and deuterium) and low d-excess values exist in the studied groundwater, reflecting the contribution of old meteoric water that recharged the aquifer in pluvial times. Modern recharge also occurs from precipitation that falls on the aquifer outcrops. The wide scatter of the data points around the two meteoric lines, the global meteoric water line (GMWL) and Mediterranean meteoric water line (MMWL), in the δ¹⁸O–δD diagram indicates considerable variation in recharge conditions (amount, altitude, temperature, air masses, distances from catchment, overland flow, etc.). The isotopic composition in the El-Bruk area is minimum (¹⁸O=–9.53‰), very close to the average value of the Western Desert Nubian Sandstone (¹⁸O=–10‰), where the local structural and lithologic conditions retard groundwater flow and the main bulk of water becomes noncyclic. The continuity of the aquifer in northern and central Sinai is evidenced by the isotopic similarity between samples taken from above and below the central Sinai Ragabet El-Naam fault, the distribution of potentiometric head, and hydrogeological cross sections. The combination of isotopic composition in terms of ¹⁸O and chemical composition in terms of TDS and salt contents is the basis for separating the studied groundwater into groups that reflect the recharge sources and isotopic and chemical modifications during flow. Electronic Publication     

Groundwater sustainability assessment in coastal aquifers

The present work investigates the response of shallow, coastal unconfined aquifers to anticipated overdraft conditions and climate change effect using numerical simulation. The groundwater flow model MODFLOW and variable density groundwater model SEAWAT are used for this investigation. The transmissivity and specific yield estimated from the existing database range from 10 to 810 m ²/day and 0.08% to 10.92% respectively. After successful calibration with Nash–Sutcliffe efficiency greater than 0.80, the values of horizontal hydraulic conductivity and specific yield of the unconfined aquifer were set in the range 1.85–61.90 m/day and 0.006–0.24 respectively. After validating the model, it is applied for forecasting the aquifer’s response to anticipated future scenarios of groundwater draft, recharge rate and sea level rise. The findings of the study illustrate that saltwater intrusion is intensified in the area adjoining the tidal rivers, rather than that due to the sea alone. Of all the scenarios simulated, the immense negative impact on groundwater quality emerges due to overdraft conditions and reduced recharge with the areal extent of seawater intrusion exceeding about 67% (TDS >1 kg/m ³). The study also arrives at the conclusion that, regional sea level rise of 1 mm/year has no impact on the groundwater dynamics of the aquifer.     

The recharge process in alluvial strip aquifers in arid Namibia and implication for artificial recharge

Alluvial strip aquifers associated with ephemeral rivers are important groundwater supply sources that sustain numerous settlements and ecological systems in arid Namibia. More than 70 % of the population in the nation’s western and southern regions depend on alluvial aquifers associated with ephemeral rivers. Under natural conditions, recharge occurs through infiltration during flood events. Due to the characteristic spatial and temporal variability of rainfall in arid regions, recharge is irregular making the aquifers challenging to manage sustainably and they are often overexploited. This condition is likely to become more acute with increasing water demand and climate change, and artificial recharge has been projected as the apparent means of increasing reliability of supply. The article explores, through a case study and numerical simulation, the processes controlling infiltration, significance of surface water and groundwater losses, and possible artificial recharge options. It is concluded that recharge processes in arid alluvial aquifers differ significantly from those processes in subhumid systems and viability of artificial recharge requires assessment through an understanding of the natural recharge process and losses from the aquifer. It is also established that in arid-region catchments, infiltration through the streambed occurs at rates dependent on factors such as antecedent conditions, flow rate, flow duration, channel morphology, and sediment texture and composition. The study provides an important reference for sustainable management of alluvial aquifer systems in similar regions.