Katerina Mazi

Tipping points for seawater intrusion in coastal aquifers under rising sea level

This study considers different projections of climate-driven sea-level rise and uses a recently developed, generalized analytical model to investigate the responses of sea intrusion in unconfined sloping coastal aquifers to climate-driven sea-level rise. The results show high nonlinearity in these responses, implying important thresholds, or tipping points, beyond which the responses of seawater intrusion to sea-level rise shift abruptly from a stable state of mild change responses to a new stable state of large responses to small changes that can rapidly lead to full seawater intrusion into a coastal aquifer. The identified tipping points are of three types: (a) spatial, for the particular aquifers (sections) along a coastline with depths that imply critical risk of full sea intrusion in response to even small sea-level rise; (b) temporal, for the critical sea-level rise and its timing, beyond which the change responses and the risk of complete sea intrusion in an aquifer shift abruptly from low to very high; and (c) managerial, for the critical minimum values of groundwater discharge and hydraulic head that inland water management must maintain in an aquifer in order to avoid rapid loss of control and complete sea intrusion in response to even small sea-level rise. The existence of a tipping point depends on highly variable aquifer properties and groundwater conditions, in combination with more homogeneous sea conditions. The generalized analytical model used in this study facilitates parsimonious quantification and screening of sea-intrusion risks and tipping points under such spatio-temporally different condition combinations along extended coastlines.

Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: application to the Akrotiri basin and aquifer, Cyprus

Abstract We investigate the general methodology for an intensive development of coastal aquifers, described in a companion paper, through its application to the management of the Akrotiri aquifer, Cyprus. The Zakaki area of that aquifer, adjacent to Lemessos City, is managed such that it permits a fixed annual agricultural water demand to be met, as well as and a fraction of the water demand of Lemessos, which varies according to available surface water. Effluents of the Lemessos wastewater treatment plant are injected into the aquifer to counteract the seawater intrusion resulting from the increased pumping. The locations of pumping and injection wells are optimized based on least-cost, subject to meeting the demand. This strategy controls sea intrusion so effectively that desalting of only small volumes of slightly brackish groundwater is required over short times, while ∼2.3 m3 of groundwater is produced for each 1 m3 of injected treated wastewater. The cost over the 20-year period 2000–2020 of operation is ∼40 M€ and the unit production cost of potable water is under 0.2 €/m3. The comparison between the deterministic and stochastic analyses of the groundwater dynamics indicates the former as conservative, i.e. yielding higher groundwater salinity at the well. The Akrotiri case study shows that the proposed aquifer management scheme yields solutions that are preferable to the widely promoted seawater desalination, also considering the revenues from using the treated wastewater for irrigation. Citation Koussis, A. D., Georgopoulou, E., Kotronarou, A., Mazi, K., Restrepo, P., Destouni, G., Prieto, C., Rodriguez, J. J., Rodriguez-Mirasol, J., Cordero, T., Ioannou, C., Georgiou, A., Schwartz, J. & Zacharias, I. (2010) Cost-efficient management of coastal aquifers via recharge with treated wastewater and desalination of brackish groundwater: application to the Akrotiri basin and aquifer, Cyprus. Hydrol. Sci. J. 55(7), 1234–1245.

Quantifying a Sustainable Management Space for Human Use of Coastal Groundwater under Multiple Change Pressures

In the densely populated coastal regions of the world, loss of groundwater due to seawater intrusion, driven by changes of climate, sea level, land use and water use, may critically impact many people. We analytically investigate and quantify the limits constraining a coastal aquifer’s sustainable management space, in order to avoid critical loss of the coastal groundwater resource by seawater intrusion. Limiting conditions occur when the intrusion toe reaches the pumping wells, well intrusion, or the marine-side groundwater divide, complete intrusion; in both cases the limits are functions of the seaward groundwater flow remaining after the human groundwater extractions. The study presents a screening-level approach to the quantification of the key natural and human-determined controls and sustainability limits for the human use of coastal groundwater. The physical and geometrical characteristics of the coastal aquifer along with the natural conditions for recharge and replenishment of the coastal groundwater are the key natural controls of the sustainable management space for the latter. The groundwater pumping rates and locations are the key human-determined controls of this space. The present approach to combining and accounting for both of these types of controls is simple, yet general. The approach is applicable across different scales and regions, and for historic, current and projected future conditions of changing hydro-climate, sea level, and human freshwater use. The use of this approach is also concretely demonstrated for the natural and human-determined controls and limits of the sustainable management space for two specific Mediterranean aquifers.

Corrected interface flow model for seawater intrusion in confined aquifers: relations to the dimensionless parameters of variable-density flow

Sharp-interface (or interface) flow models with Dupuit-Forchheimer approximation are widely used to assess, to first order, an aquifer’s vulnerability to seawater intrusion (SWI) and to evaluate sustainable management options for coastal groundwater resources at the screening level. Recognising that interface flow models overestimate SWI, corrections have been proposed to account for the neglected mixing and also for the outflow through a finite gap. These corrections, however, were introduced in the context of specific studies and may not be generally applicable as proposed. The interface model is revisited, placing its corrections in the context of variable-density flow (VDF) theory, by expressing them in terms of the dimensionless parameters governing VDF in schematised (aspect ratio = thickness/length) homogeneous confined coastal aquifers: the coupling parameter (α), a Péclet number (Pe), and the dispersivities ratio (rα). Interfaces are compared to the 50%-salinity lines of VDF numerical solutions and regression equations are developed for estimating the outflow gap and for correcting the length of the interface (terminating with a blunted edge); the dispersion correction, which modifies the interface curvature, is restated with a variable exponent. The corrections for dispersion and for the interface length appear to be the most effective; an outflow gap is important only at small α values (strong advection relative to vertical flow due to density differences). These concepts are applied successfully to calculate the interface position in the lowermost confined sub-unit of the Coastal Plain aquifer of Israel, as an estimate of SWI.RésuméLes modèles d’écoulement à interface abrupt (ou à interface) basés sur l’approximation de Dupuit-Forchheimer sont utilisés largement pour estimer, dans un premier temps, la vulnérabilité des aquifères à l’intrusion d’eau marine (IEM) et évaluer les options de gestion durable pour les ressources en eaux souterraines côtières au niveau des crépines. Reconnaissant que les modèles d’écoulement à interface surestiment l’IEM, des corrections ont été proposées pour prendre en considération le mélange négligé et aussi les sorties d’eau dans un espace fini. Ces corrections, cependant, ont été introduites dans un contexte d’études spécifiques et ne peuvent être applicables généralement comme proposé. Le modèle à interface est. revisité, introduisant les corrections pour un contexte de la théorie de l’écoulement à densité variable (EDF), en les exprimant avec des paramètres sans dimension gouvernant l’EDF pour des aquifères côtiers schématisés (rapport d’aspect = épaisseur/longueur) homogènes captifs: le paramètre de couplage (α), le nombre de Péclet (Pe), et le rapport de dispersivité (rα). Les interfaces sont comparées aux lignes de 50% de salinité des solutions numériques d’EDF et les équations de régression sont développées pour estimer l’espace de sorties d’eau et pour corriger la longueur de l’interface (terminant avec une arête émoussée); la correction de la dispersion, qui modifie la courbure de l’interface, est. reconsidéré avec un exposant variable. Les corrections pour la dispersion et pour la longueur de l’interface apparaissent comme étant les plus efficaces; un espace de sorties d’écoulement est. important seulement pour les petites valeurs α (forte advection relative à l’écoulement vertical due aux différences de densité). Ces concepts sont appliqués avec succès pour calculer la position de l’interface dans la sous-unité captive inférieure de l’aquifère de la plaine côtière d’Israël, une estimation de IEM.ResumenLos modelos de flujo de la interfaz con la aproximación Dupuit-Forchheimer se utilizan ampliamente para evaluar, en primer lugar, la vulnerabilidad del acuífero a la intrusión de agua de mar (SWI) y evaluar opciones de gestión sostenible para los recursos de aguas subterráneas costeras a nivel de prueba. Reconociendo que los modelos de flujo de interfaz sobreestiman SWI, se han propuesto correcciones para explicar la mezcla y también para el flujo de salida a través de un espacio finito. Sin embargo, estas correcciones se introdujeron en el contexto de estudios específicos y no pueden ser generalmente aplicables según lo propuesto. El modelo de interfaz se revisa, colocando sus correcciones en el contexto de la teoría de flujo de densidad variable (VDF), expresándolas en términos de los parámetros adimensionales que gobiernan VDF en acuíferos costeros confinados homogéneos esquematizados (relación de aspecto = espesor/longitud): el parámetro de acoplamiento (α), un número de Péclet (Pe) y la relación de dispersividades (rα). Las interfaces se comparan con las líneas de salinidad al 50% de las soluciones numéricas VDF y las ecuaciones de regresión se desarrollan para estimar el espacio del flujo de salida y para corregir la longitud de la interfaz (que termina con un borde romo); la corrección de la dispersión, que modifica la curvatura de la interfaz, se replantea con un exponente variable. Las correcciones para la dispersión y para la longitud de la interfaz parecen ser las más efectivas; un espacio de salida es importante solo a valores pequeños (fuerte advección relativa al flujo vertical debido a las diferencias de densidad). Estos conceptos se aplican con éxito para calcular la posición de la interfaz en la subunidad confinada más baja del acuífero de llanura costera de Israel, como una estimación de SWI.摘要棉线的分界面(或者说界面)水流模型与Dupuit-Forchheimer近似法广泛用来评价一阶含水层遭受海水入侵的脆弱性以及筛选级别上评估沿海地下水资源可持续管理的选项。认识到界面水流模型会过高估算海水入侵,因此,提出了校正以解释忽略的混合,以及通过有限差距的出流量。然而,这些校正被引入到特定的研究环境中,可能会不象推荐的那样普遍适用。再访界面模型,把校正数据放入可变密度水流理论中,用控制扼要表示的(纵横比 = 厚度/长度)均质承压沿海含水层中可变密度水流的无量纲参数表示:耦合参数(α)、一个Péclet数(Pe)以及分散性比(rα)。界面与可变密度水流数值解的50%-盐度线进行了对比,建立了回归方程,以估算出流量差,并校正界面的长度(用圆滑边缘终结);采用可变的成分重新开始进行修饰界面曲率的分散校正。分散和界面长度的校正似乎最有效;出流量差只有在α值很小时才非常重要(由于密度差,强烈的对流和垂直水流有关)。这些概念成功应用于计算以色列沿海平原含水层最低处的承压亚单元的界面位置,作为海水入侵的估算值。ResumoOs modelos de interface delgada (ou interface), utilizando aproximação de Dupuit-Forchheimer, são amplamente utilizados para avaliar, em primeira ordem, a vulnerabilidade de um aquífero à intrusão de água marinha (IAM) e avaliar opções de manejo sustentável para recursos de águas subterrâneas costeiras no nível de triagem. Reconhecendo que os modelos de fluxo de interface superestimam a IAM, correções foram propostas para analisar a porção negligenciada da mistura e obter o fluxo de saída através de um intervalo finito. Estas correções, no entanto, foram introduzidas no contexto de um estudo de caso e podem não ser aplicados, como aqui proposto, de forma genérica. O modelo de interface é revisto, sendo adicionadas correções no contexto da teoria do fluxo de densidade variável (FDV), expressando-as em termos dos parâmetros adimensionais que governam o FDV em aquíferos costeiros confinados homogêneos (razão de aspecto = comprimento/espessura): o parâmetro acoplado (α), número de Péclet (Pe) e razão de dispersividades (rα). Interfaces são comparadas com as isolinhas de 50% de salinidade das soluções numéricas das equações de FDV e equações de regressão são desenvolvidas para estimar a lacuna de vazão e para corrigir o comprimento da interface (terminando com uma borda grosseira); a correção de dispersão, que modifica a curvatura da interface, é reapresentada com um expoente variável. As correções para dispersão e para comprimento da interface parecem ser as mais efetivas; um intervalo de vazão é importante apenas em valores pequenos de α (forte advecção em relação ao fluxo vertical devido a diferenças de densidade). Estes conceitos são aplicados com sucesso para calcular a posição da interface na subunidade confinada inferior do aquífero da planície costeira de Israel, como uma estimativa da IAM.

Reverse flood and pollution routing with the lag-and-route model

ABSTRACT Reverse routing can be used to transfer flood- or pollution-related information monitored at a downstream gauging station to an ungauged upstream cross-section. This signal identification problem is ill-posed and, as such, is sensitive to perturbations in the data to be inverted; therefore, the amplification of errors, e.g., those befalling measurements, must be controlled. Storage routing models are parsimonious diffusion wave substitutes and well suited for conversion to direct reverse routers. We present efficient inversion frameworks based on the lag-and-route (single reservoir plus exact reverse lag-step) and the reservoirs-in-series models. In both cases we invert a centred finite difference scheme of the reservoir storage balance equation that involves only one value of the unknown signal; signal values identified in previous reverse time steps, which would carry perturbations, are absent. This simple structure endows the reverse scheme with robustness. Procedures are verified with perfect and with error-seeded data; solution oscillations caused by the latter are damped by low-pass filtering. Both inverse routing models regain the upstream signals with high fidelity. Reverse storage routing is exemplified in a demonstration of reservoir control and in a field case of solute transport in a stream. Editor M.C. Acreman; Associate editor X. Chen