Sea-level rise impacts on seawater intrusion in coastal aquifers: Review and integration
Graphical abstract
Introduction
Groundwater is generally the most important freshwater resource in many coastal regions which are threatened by seawater intrusion (SWI) (Ataie-Ashtiani and Ketabchi, 2011, Ketabchi and Ataie-Ashtiani, 2015b). Climate change impacts such as sea-level rise (SLR) and precipitation variations that change recharge rates are the influential climatic factors that affect SWI (Werner et al., 2013, Ataie-Ashtiani et al., 2013a). The Intergovernmental Panel on Climate Change (IPCC, 2013) predicts that the global mean SLR may rise between 0.26 m and 0.82 m by the year 2100. A SLR in the range of 0.18–0.59 m was predicted by IPCC (2007) for a similar period. This shows a significant upward revision for SLR prediction between IPCC (2007) and IPCC (2013) and highlights the potential importance of SLR impacts on SWI.
Based on the assessments of IPCC (2013), annual mean precipitation can vary up to ±50% in the world. This range includes the estimate of projected uncertainties. The high latitudes and the equatorial Pacific Ocean are likely to experience an increase in annual mean precipitation by the end of this century. In many mid-latitude and subtropical arid regions, mean precipitation will likely decrease, while in many mid-latitude wet regions, mean precipitation will likely increase by the year 2100 (IPCC, 2013, Horton et al., 2014, Bring et al., 2015). Larger uncertainties surround the projections of surface runoff and recharge rate to groundwater resources, which are affected by many climatic factors, include changes in mean precipitation and temperature regimes. Further assessments have been provided by e.g. Holman, 2006, IPCC, 2013, and Bring et al. (2015).
Ketabchi and Ataie-Ashtiani, 2015b, Ketabchi and Ataie-Ashtiani, 2015c developed the efficient and robust decision models which have the superior abilities in terms of both solution quality and computational time criteria. Using such decision models, they highlighted a need for an integrated study to address how the conceptualization of climate change impacts e.g. SLR, land-surface inundation (LSI), and recharge rate variations can be handled on prospective coastal groundwater management strategies. Gorelick and Zheng (2015) emphasized that global changes such as climatic effects led to multiple stresses that should be considered in groundwater management plans. Ojha et al. (2015) assessed the long-term potential influences of climate change, e.g. SLR impacts in aquifers and efficient management of these resources in many regions of the world. They concluded that such studies are yet open challenges concerning uncertainties in modeling and in defining climate change scenarios, heterogeneities, estimation of recharge rate to groundwater systems, data challenges, and addressing the increasing threats from competing demands and mounting hydrologic stresses on groundwater systems, which all indicated a pressing need to develop effective management strategies.
The main objective of this study is to provide a systematic review of numerous previous studies and to then undertake an analysis of the relative importance of the purported influential factors controlling SWI. We present the literature review in tabulated and diagrammatic formats so as to be easily comprehensible and to easily identify what factors previous studies have and have not included. This is the first study that highlights the impacts of all of known SLR-induced influential factors and thus directs us to evaluate the relative importance of these impacts on SWI using both analytical and numerical methods. The SLR impacts on the SWI interface and in particular seawater toe location are the focus of this study. Such an integrated assessment does not exist because each previous study has only assessed a (different) subset of the purported controlling factors.
Section snippets
A review of previous studies
In recent years, there has been a growing body of research relating to climatic and hydrogeologic controls on SWI. It is not easy to rapidly discern the similarities and differences in these studies. Furthermore, it is also not immediately clear where current knowledge gaps might exist. Even more importantly, it is not indeed evident that any previous studies have conducted an integrated assessment to analyze the relative importance of the purported range of influential factors.
Recently,
Future research challenges
Based on the review, a list of future challenges is identified in this section. All previous studies have investigated a subset of controlling processes without considering many other ones that may be expected to exert a significant control on SWI processes. Integration of all the purported controlling factors studies in previous literature into a single, unifying, framework is required to conduct the fully-integrated analyses. Simultaneously examining many of the controlling factors, within
Integrated assessments using analytical modeling
The main objective in this section is to propose a simple tool for seawater toe location sensitivity assessments that are based on fundamental SWI mathematics as applied to a variety of idealized settings. Here, sensitivity refers to the relative propensity for SWI to occur. The basis of the methodology is that the rates of change in seawater toe location assessments in response to stress changes, described as partial derivatives of equations, offer insight into the propensity for SWI (Werner
Integrated assessments using numerical modeling
In this section, the saturated-unsaturated density-dependent flow and transport USGS code SUTRA (Voss and Provost, 2010) is employed. SUTRA (Voss and Provost, 2010) has been previously used by e.g. Michael et al., 2013, Ketabchi et al., 2014, Mahmoodzadeh et al., 2014, and Ketabchi and Ataie-Ashtiani (2015c) for investigating the SLR-induced SWI in coastal aquifers.
Schematized unconfined aquifer cross-sections (based on the presented data in Table 2) used for modeling with finite-element mesh
Conclusions
In order to better understand the previous research, current knowledge gaps, and opportunities for advancing the understanding of SLR impacts on SWI in more general terms, we first conducted a literature review with a greater focus on SLR-induced phenomena and the associated impacts. Therefore, the first review of previous literature is provided in a diagrammatic and tabular form on SWI in coastal aquifers. This review demonstrates that all studies to date have only investigated a subset of
Acknowledgments
The author Craig T. Simmons acknowledges funding support of the National Centre for Groundwater Research and Training, a collaborative initiative of the Australian Research Council and the National Water Commission, Australia. The authors appreciate the constructive comments of the reviewer, Dr. Antonis D. Koussis, three anonymous reviewers, and Editor-in-Chief Dr. Geoff Syme, which helped to improve the final version of this paper.
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