Hydrogeological changes in coastal aquifers due to sea level rise

Abstract

Global warming and climatic changes can lead to sea level rise (SLR) of dozens of cms over up-coming decades, along with groundwater permanent reserve losses (PRL). This study focuses on understanding the processes and estimating groundwater losses. A case study for such phenomena is Israel's Coastal aquifer. PRL estimation methodology is based upon a simple hydrogeological conceptual model. The results lead to estimation of two main components of an aquifer's PRL, and to key factors that can enhance or mitigate these losses. Such recommended measures as high-resolution topographic mapping and improved monitoring of sea level have been noted.

Introduction

Natural factors influence sea level rise (SLR) in a non-uniform way. Such geological processes as tectonic activities, including earthquakes, can supply material from below the oceans by means of eruption or intrusion of material from the earth's mantle, leading to the formation of new islands, contributing to a rise in sea level. On other hand, formation of tension faults can open canyons, rift valleys and augment deep relief, expanding the volume as well as the geometry of ocean and sea areas, resulting in a lowering of sea level. Natural processes can simultaneously supply material and reduce the water reservoir—thus raising seawater level, or enlarge the oceanic and sea volumes—lowering sea level [1].

Another natural cause for change of sea level is glacial isostatic adjustment (GIA). This factor can cause continental sinking in one area but a non-uniform rise in other areas [2].

An additional natural factor that can influence rise in sea level can result from coastal erosion. The alteration of lithological material at the stratigraphic front of a coastline transfers masses of eroded material from the shoreline front downwards to the sea bottom as well as upwards to form new inland ridging along the resultant coast [3], [4]. Sedimentilogical changes can damage natural environments along the shoreline [5], and may alter boundary conditions of hydrogeological systems connected to the sea.

It is therefore critical to determine what natural effects SLR might have, when they might occur, how significant they might be, and what their impacts might be upon groundwater and other natural resources, and the ambient environment of a shoreline.

Beside natural processes, adverse anthropogenic effects can change the SLR. Over-use of fossil fuels produces immense amounts of gases which build in the atmosphere to produce a “greenhouse effect”, contributing to atmospheric warming [6]. The latest reports of the World Meteorological Organization (WMO) and the United Nations Programme for the Environment (UNPE) indicate a sharp increase in world ambient temperature of between 1.4 and 5.8 °C by 2100 [7], [8]. Ocean and seawater temperatures may also increase significantly by 2100. This global warming could be responsible for climatic change that may lead to atmospheric instability by increasing the number and intensity of storms and hurricanes, amplifying inland intrusion of larger and higher seawater waves.

Another anthropogenic factor that can change the environment, enhancing or mitigating SLR is the over-exploitation of confined aquifers, which can cause long-term sinking of continental masses in some areas but rises in others. Another factor is the construction of dams. This reduces quantities of water and sediment flowing from the continent into the sea. On the one hand, this can diminish SLR by reducing inflowing volume of water into the sea. In USA, 11–13% of the total annual river runoff is presently sequestered behind large dams. Along the Mediterranean Sea's catchment area, such construction chiefly affects locally important alluvial discharge of such streams as the Rhone, Elbre and Nile. Discharge from the Nile has been reduced by more than 80% since the recent construction of a major dam. Reduction of sedimentation in deltaic regions can lead to severe erosion of coastal areas. Construction of dams can reduce sedimentation which could also lead to increased erosion, causing further regression of the seashore and a rise in SLR. The results of erosion could lead to further seawater intrusion that can damage coastal water resources and precious ecological and archeological sites, with adverse ancillary consequences on tourism and other economic concerns. The concern is thus the ultimate impact as regards SLR and intrusion of seawater into the continent [1], [9], [10], [11], [12].

The Inter-governmental Panel on Climatic Changes (IPCC) attributes the majority of recent SLR to thermal expansion of ocean water (steric effect), resulting from global warming and subsequent melting of glaciers ([1], [13]; Table 1).

The multitude of these natural and anthropogenic factors, along with the complexity of SLR measurement leads to uncertainty in accurately forecasting future SLR. Reports based on pre-1990 data indicate SLR of around 1–1.5 mm/yr. Later reports have been based on data between 1992 and 1998. Latest reports of the IPCC, based on data from the Topex—Poseidon satellite measurements and other facilities indicate that seawater warming is causing a rise in SLR of more than twice that noted in previous measurements (Table 1). Other recent reports in various areas in the world indicate SLR of between 5 and 20 mm/yr. This has been especially noticeable in the coastal areas of the Mediterranean Sea, measured over recent years (1992–2002). The focus has been upon the center and eastern portion of the sea, including the Israeli coasts of Tel Aviv, Ashdod, and Hadera. This trend has also been noted in other areas in the world. It is felt that SLR of between 50 and 100 cm could be expected by the end of the 21th century ([1], [5], [10], [13], [14], Table 1). Owing to non-uniformity, measurements in other areas of the world could also indicate trends in which no change at all are observed, or indeed, where sea levels might be dropping.

Rising ocean levels will enhance processes already on-going throughout the world, which include submergence of large portions of shallow relief areas in estuarine and deltaic areas of Pacific, Indian, and other ocean coastal islands. In Europe, this could include the swamping of shallow areas of the Netherlands, and some estuarine regions of the British Isles, which could include such large cities as London, as well as the Camargue area in France. In the US, coastal cities such as New York, Boston, Miami, and New Orleans, etc. could be threatened [1], [9]. A further question is not whether sea levels will rise, but rather how a change in sea level might alter the profile of coasts as regards coastal lithology, the integrity of coastal groundwater resources, and the ecology of the coastal environment [15]. Particular attention should be paid to coastal areas with low relief, where high waves could lead to significant inland damage. An extreme case, which occurred at the end of 2004, was the tsunami that struck coastal areas of the Indian ocean, leaving an apocalyptic disaster of more than a quarter of millions people dead and more than 1 million homeless in many countries [12], [16].

SLR could enhance seawater intrusion and inundation along the Mediterranean Sea's constricted basin. This basin is home to a population of around 500 millions habitants, having 45,000 km of coastline with numerous deltaic and estuarine areas in which a multitude of natural resources, such as groundwater and historic archeological sites, could be threatened [17], [18].

One of the attempts to develop a conceptual model to assess permanent losses that might result from SLR has been the DELFT3D three-dimensional model, which is used to estimate changes to be expected in the extent of seashore regression when considering lithology, stratigraphic parameters, and changes in tidal levels. The DELFT3D model can yield an approximate assessment of future coastline changes. The model indicates that even shorelines having a significant slope will suffer regression; a rise in sea level of 1 m could bring about an average regression of 100 m for a coast composed of sand, but only about 60 m where the coast is composed of harder material. The model further predicts a worldwide coastal regression of 10–100 m by the end of the 21st century. However, there is presently no model which can forecast SLR with an acceptable degree of accuracy, and which highlights the adverse effects SLR could have upon coastal groundwater reserves, lithology, stratigraphy, archeological remains of early coastal civilizations, ecological biota, and other environmental aspects [5], [11], [19].

The purpose of this study is to present the basis for a simple hydrogeological conceptual model relating seawater intrusion to SLR. Such a model ought to estimate groundwater losses that can occur owing to the SLR, which may take place over coming years. The study also focuses upon potential environmental changes and adverse effects which may occur along the sea shore in general, in the eastern portion of the Mediterranean Sea, and in particular, along Israel's coastal plain, which includes the Coastal aquifer, considered here as the case study. Recommendations are also made towards preventing, mitigating, or overcoming possible adverse effects.