Leanne K. Morgan

How important is the impact of land-surface inundation on seawater intrusion caused by sea-level rise?

The influence of sea-level rise (SLR) on seawater intrusion (SWI) has been the subject of several publications, which consider collectively a range of functional relationships within various hydrogeological and SLR settings. Most of the recent generalized analyses of SWI under SLR neglect land-surface inundation (LSI) by seawater. A simple analytical method is applied to quantitatively assess the influence and importance of LSI on SLR–SWI problems under idealized conditions. The results demonstrate that LSI induces significantly more extensive SWI, with inland penetration up to an order of magnitude larger in the worst case, compared to the effects of pressure changes at the shoreline in unconfined coastal aquifers with realistic parameters. The study also outlines some of the remaining research challenges in related areas, concluding that LSI impacts are among other important research questions regarding the SLR–SWI problems that have not been addressed, including the effects of aquifer heterogeneities, real-world three dimensionality, and mitigation measures.RésuméL’influence de l’élévation du niveau marin sur l’intrusion marine a fait l’objet de plusieurs publications, qui considèrent ensemble une série de relations fonctionnelles entre diverses configurations hydrogéologiques et marines. La plupart des analyses générales récentes portant sur les intrusions marines liées à la fluctuation du niveau marin négligent l’inondation des terres par la mer. Une méthode analytique simple est appliquée pour évaluer quantitativement, dans des conditions théoriques, l’influence et l’importance de l’inondation des terres sur l’élévation du niveau de la mer et l’intrusion marine. Les résultats montrent que l’inondation des terres induit de façon significative une intrusion marine plus étendue et, dans le pire des cas, une pénétration dans l’arrière pays plus importante que celle due aux effets des variations de pression sur les aquifères côtiers libres le long la ligne de rivage, calculée avec des paramètres réalistes. L’article souligne également quelques uns des défis encore posés à la recherche dans les domaines de ce genre, concluant que les impacts de l’inondation des terres figurent parmi les autres questions importantes de la recherche concernant les problèmes d’élévation du niveau de la mer et d’intrusion de l’eau de mer à n’avoir pas été abordées et qui comprennent l’incidence des hétérogénéités de l’aquifère, le caractère tridimensionnel du monde réel et les approches simplificatrices.ResumenLa influencia del ascenso del nivel del mar (SLR) en la intrusión de agua marina (SWI) ha sido objeto de varias publicaciones, que consideran colectivamente un rango de relaciones funcionales dentro de varias configuraciones hidrogeológicas y de SLR. La mayor parte de los recientes análisis generalizados de SWI bajo SLR desprecian la inundación de la superficie del terreno (LSI) por el agua de mar. Se aplica un método analítico simple para evaluar cuantitativamente la influencia y la importancia de LSI sobre los problemas de SLR–SWI bajo condiciones idealizadas. Los resultados demuestran que la LSI induce significativamente en SWI más extensas, con penetración tierra adentro de hasta un orden de magnitud más grande que en el peor de los casos, comparado con los efectos de los cambios de presión en la línea de costa en acuíferos costeros no confinados con parámetros realistas. El trabajo también esboza algunos de los desafíos de investigación que restan en áreas relacionados, concluyendo que los impactos de LSI, entre otras cuestiones importantes en relación a los problemas SLR–SWI, no han sido evaluados, incluyendo los efectos de las heterogeneidades del acuífero, tridimensionalidad del mundo real y las medidas de mitigación.ResumoA influência da elevação do nível do mar (ENM) na intrusão salina (IS) tem sido objeto de várias publicações, as quais consideram coletivamente uma gama de relações funcionais dentro de vários cenários hidrogeológicos e de ENM. A maior parte das análises recentes mais generalizadas de IS sob ENM negligenciam a inundação da superfície do solo (ISS) pela água marinha. É aplicado um método analítico simples para calcular quantitativamente a influência e importância da ISS nos problemas de ENM-IS sob condições ideais. Os resultados demonstram que a ISS induz uma IS significativamente mais extensa, com uma penetração continental até uma ordem de magnitude maior no pior dos casos, quando comparada com os efeitos das alterações de pressão na linha de costa em aquíferos livres costeiros com parâmetros realistas. O documento também enfatiza alguns dos restantes desafios da investigação em áreas similares, concluindo que os impactes da ISS estão entre outras questões importantes para a investigação relacionada com os problemas de ENM-IS que não foram ainda abordados, incluindo os efeitos das heterogeneidades dos aquíferos, a tridimensionalidade do mundo real e as medidas de mitigação.

Occurrence of seawater intrusion overshoot

A number of numerical modeling studies of transient sea level rise (SLR) and seawater intrusion (SI) in flux-controlled aquifer systems have reported an overshoot phenomenon, whereby the freshwater-saltwater interface temporarily extends further inland than the eventual steady state position. Recently, physical sand-tank modeling has shown overshoot to be a physical process. In this paper, we have carried out numerical modeling of SLR-SI to demonstrate that overshoot can occur at the field scale within unconfined aquifers. This result is contrary to previous conclusions drawn from a restricted number of cases. In addition, we show that SI overshoot is plausible under scenarios of gradual sea level rise that are consistent with conditions predicted by the Intergovernmental Panel for Climate Change. Overshoot was found to be largest in flux-controlled unconfined aquifers characterized by low freshwater flux, high specific yield, and large inland extent. These conditions result in longer timeframes for the aquifer to reach new steady state conditions following SLR, and the extended period prior to reequilibration of the groundwater flow field produces more extensive overshoot.

Water table salinization due to seawater intrusion

Seawater intrusion (SWI) is a significant threat to freshwater resources in coastal aquifers around the world. Previous studies have focused on SWI impacts involving salinization of the lower domain of coastal aquifers. However, under certain conditions, SWI may cause salinization of the entire saturated zone of the aquifer, leading to watertable salinization (WTS) in unconfined aquifers by replacing freshwater within the upper region of the saturated zone with seawater, thereby posing a salinity threat to the overlying soil zone. There is presently limited guidance on the extent to which WTS may occur as a secondary impact of SWI. In this study, physical experiments and numerical modelling were used to explore WTS associated with SWI in various non-tidal, unconfined coastal aquifer settings. Laboratory experiments and corresponding numerical simulations show that significant WTS can occur under active SWI (i.e., the freshwater hydraulic gradient slopes towards the land) because the cessation of freshwater discharge to the sea and the subsequent landward flow across the entire sea boundary eventually lead to watertable salinities approaching seawater concentration. WTS during active SWI is larger under conditions of high hydraulic conductivity, rapid SWI, high dispersivity and for deeper aquifers. Numerical modelling of four published field cases demonstrates that rates of WTS of up to 60 m/y are plausible. Under passive SWI (i.e., the hydraulic gradient slopes towards the sea), minor WTS may arise as a result of dispersive processes under certain conditions (i.e., high dispersivity and hydraulic conductivity, and low freshwater discharge). Our results show that WTS is probably widespread in coastal aquifers experiencing considerable groundwater decline sustained over several years, although further evidence is needed to identify WTS under field settings. This article is protected by copyright. All rights reserved.

Application of a Rapid-Assessment Method for Seawater Intrusion Vulnerability: Willunga Basin, South Australia

Seawater intrusion (SWI) causes degradation of water quality and loss of water security in coastal aquifers. Although the threat of SWI has been reported in all of the Australian states and the Northern Territory, comprehensive investigations of SWI are relatively uncommon because SWI is a complex process that can be difficult and expensive to characterise. The current study involves the application of a first-order method developed recently by Werner et al. (Ground Water 50(1):48–58, 2012) for rapidly assessing SWI vulnerability. The method improves on previous approaches for the rapid assessment of large-scale SWI vulnerability, because it is theoretically based and requires limited data, although it has not been widely applied. In this study, the Werner et al. (Ground Water 50(1):48–58, 2012) method is applied to the Willunga Basin, South Australia to explore SWI vulnerability arising from extraction, recharge change and sea-level rise (SLR). The Willunga Basin is a multi-aquifer system comprising the unconfined Quaternary (Qa) aquifer, confined Port Willunga Formation (PWF) aquifer and confined Maslin Sands (MS) aquifer. Groundwater is extracted from the PWF and MS aquifers for irrigated agriculture. In the Qa aquifer, the extent of SWI under current conditions was found to be small and SWI vulnerability, in general, was relatively low. For the PWF, SWI extent was found to be large and SWI is likely to be active due to change in heads since pre-development. Anecdotal evidence from recent drilling in the PWF suggests a seawater wedge at least 2 km from the coast. A relatively high vulnerability to future stresses was determined for the PWF, with key SWI drivers being SLR (under head-controlled conditions, which occur when pumping controls aquifer heads) and changes in flows at the inland boundary (as might occur if extraction increases). The MS aquifer was found to be highly vulnerable because it has unstable interface conditions, with active SWI likely. Limitations of the vulnerability indicators method, associated with the sharp-interface and steady-state assumptions, are addressed using numerical modelling to explore transient, dispersive SWI caused by SLR of 0.88 m. Both instantaneous and gradual (linear rise over 90 years) SLR impacts are evaluated for the Qa and PWF aquifers. A maximum change in wedge toe of 50 m occurred within 40 years (for instantaneous SLR) and 100 years (for gradual SLR) in the Qa. In the PWF, change in wedge toe was about 410 and 230 m after 100 years, for instantaneous and gradual SLR, respectively. Steady state had not been reached after 450 years in the PWF. Analysis of SLR in the MS was not possible due to unstable interface conditions. In general, results of this study highlight the need for further detailed investigation of SWI in the PWF and MS aquifers. Establishing the extent of SWI under current conditions is the main priority for both the PWF and MS aquifers. An important element of this involves research into the offshore extent of these aquifers. Further, predictions of SWI in the PWF should consider future extraction and SLR scenarios in the first instance. A field-based investigation of the Willunga aquifer is ongoing, and the current study provides guidance for well installation and for future data collection.

Application of an Analytical Solution as a Screening Tool for Sea Water Intrusion

Sea water intrusion into aquifers is problematic in many coastal areas. The physics and chemistry of this issue are complex, and sea water intrusion remains challenging to quantify. Simple assessment tools like analytical models offer advantages of rapid application, but their applicability to field situations is unclear. This study examines the reliability of a popular sharp-interface analytical approach for estimating the extent of sea water in a homogeneous coastal aquifer subjected to pumping and regional flow effects and under steady-state conditions. The analytical model is tested against observations from Canada, the United States, and Australia to assess its utility as an initial approximation of sea water extent for the purposes of rapid groundwater management decision making. The occurrence of sea water intrusion resulting in increased salinity at pumping wells was correctly predicted in approximately 60% of cases. Application of a correction to account for dispersion did not markedly improve the results. Failure of the analytical model to provide correct predictions can be attributed to mismatches between its simplifying assumptions and more complex field settings. The best results occurred where the toe of the salt water wedge is expected to be the closest to the coast under predevelopment conditions. Predictions were the poorest for aquifers where the salt water wedge was expected to extend further inland under predevelopment conditions and was therefore more dispersive prior to pumping. Sharp-interface solutions remain useful tools to screen for the vulnerability of coastal aquifers to sea water intrusion, although the significant sources of uncertainty identified in this study require careful consideration to avoid misinterpreting sharp-interface results.

Comment on “Closed-form analytical solutions for assessing the consequences of sea-level rise on groundwater resources in sloping coastal aquifers”: paper published in Hydrogeology Journal (2015) 23:1399–1413, by R. Chesnaux

Recently, the impact of sea-level rise (SLR) on seawater intrusion (SWI) has received considerable attention. The subject article (Chesnaux 2015) used Fetter’s (1972) analytical solution for the steady-state location of seawater in unconfined aquifers beneath oceanic islands to explore changes in water-table height, seawater wedge toe position, groundwater travel time and freshwater volume in sloping-shore coastal unconfined aquifers. A number of limitations to this work associated with the choice of mathematical model and inland boundary condition should be highlighted to avoid misinterpretation of the Chesnaux (2015) results. For example, the case presented by Chesnaux (2015) is specific to the inland boundary conditions, whereas a more generalizable solution is possible by considering alternative inland boundary conceptualisations such as those used in previous studies (e.g. Werner et al. 2012; Ataie-Ashtiani et al. 2013). This Comment article describes these extensional concepts, and highlights conclusions regarding SWI in response to SLR that differ to those obtained by Chesnaux (2015), who considered only a subset of common aquifer conditions. The conceptual model used by Chesnaux (2015) involves a predefined no-flow boundary location, which is commonly used to represent the symmetry of an oceanic strip-island lens (e.g. Fetter 1972; Morgan and Werner 2014), where the noflow boundary is assumed to be at the centre of the lens. Also, a fixed-location, no-flow boundary in a continental unconfined aquifer setting applies to situations where the landward geological limit of the aquifer is used as the inland boundary condition. In other cases, for example where the inland water table is controlled by head-dependent pumping rates, land surface effects (e.g. evapotranspiration), or surface water features (e.g. wetlands, rivers, drains, etc.), a fixed-location, noflow boundary condition typically does not apply under conditions of SLR, as discussed by Werner and Simmons (2009) and Werner et al. (2012). Hence, the reference by Chesnaux (2015) to: BThe groundwater divide position (i.e., a no-flow condition)...estimated from data defining the position of the water-table mound^ describes a feature of coastal aquifers (i.e. the peak of a water-table mound) that is often not fixed in position or head level, particularly in aquifers subjected to SLR. Additionally, a means of obtaining the groundwater divide location using existing theory is not given. In the analysis that follows, an approach to defining the location of a groundwater divide based on a known head and other hydrogeological parameters (e.g. aquifer geometry, hydraulic conductivity, recharge, etc.) is demonstrated. The location of the divide is shown to move in response to stress changes, such as SLR, under typical situations. The movement of the divide has significant bearing on the behaviour of the aquifer under SLR. The approach adopted by Chesnaux (2015) can be extended by employing the widely applied Strack (1976) single potential approach, which has been used previously by Cheng and Ouazar (1999), Werner et al. (2012) and Ataie-Ashtiani et al. (2013). Using this approach avoids the need to predefine the inland location of the no-flow boundary, and allows consideration of Bfixed-head^ situations, such as those encountered by Werner and Simmons (2009) and Michael et al. (2013), amongst others. Unlike Fetter’s (1972) equation, as * Leanne K. Morgan leanne.morgan@flinders.edu.au