Seawater rise and its adverse impact on stressed coastal aquifers and their groundwater reserves

Transcription

1 Water Resources Management III 185 Seawater rise and its adverse impact on stressed coastal aquifers and their groundwater reserves A. Melloul & M. Collin Water Commission, Hydrological Service, Jerusalem, Israel Abstract When considering theories of climatic changes, some alarming scenarios forecast a sea level rise of dozens of cms over up-coming decades. Consequently, future hydrological and environmental planning must take these concerns into consideration and weigh their impact upon water resources and environment, especially in those areas where continental fresh water is in contact with sea water. This study focuses on Israel s Coastal aquifer, which is a significant component of Israel s national water system, located along the eastern Mediterranean coast, where seawater intrusion into the aquifer due to many years of over-pumpage has been noted. There are still pristine areas not far from the coast, where fresh water can be found. Considering the long shoreline of this aquifer and the presence of fresh water in pristine areas, seawater rise can have a potentially significant adverse effect upon this aquifer and its environment. Based upon a simple geometric and hydological model and the assumption that the coastal aquifer can be subject to a rise of sea level due to global warming of between 10 and 50 cm over the coming 50 years, results highlight key parameters that can contibute to significant adverse changes, and include estimation of values and rates of potential fresh water loss from the aquifer s permanent reserves for the two cases noted above. The recommendations include: high-resolution topographic mapping, improvement of monitoring of coastal aquifer wells to provide critical information for effective water resource management, and long-term land-use planning. This case study can be considered representative of many coastal aquifers throughout the world. Keywords: global warming, sea level rise, Coastal aquifer, seawater intrusion, over-pumpage, hydological model, Mediterranean coast, seawater monitoring.

2 186 Water Resources Management III 1 Introduction 1.1 Factors that can influence the rise of sea level The factors of rising sea level can be either natural or artificial. The natural factors can be connected with such tectonic activities as earthquakes and other geological processes, which can supply upwelling material to the ocean floor. Such features include eruptions of volcanic material, and/or intrusive material from the earth s mantle, leading to the formations of new islands and chains of islands. Another factor that can influence changes in seawater level is glacial isostatic adjustment, which causes continental sinking. Artificial causes that can change seawater level are mainly linked to anthropogenic sources, such as emission of fossil fuel gases, which can influence global warming due to the greenhouse effect. This can lead to such a rise in world temperatures as to cause the melting of glaciers, and the corresponding steric effect, by which water volume expands IPCC [1]. 1.2 Expectation of sea level rise The Inter-governmental Panel on Climatic Changes (IPCC) estimates the rate of sea water level rise expected for the 21 st century to be between 1 to 2 mm per year IPCC, [1]. From some measurements taken along the Mediterranean coast (Spain, Greece, Israel, etc.) during recent years, especially along Israel s coasts, a rate of rise of as much as 10 mm per year has been noted Emery and Aubrey, [2]. Measurements taken over recent years indicate an accelerating trend in sea level rise as compared to rises over earlier years. Owing to the varying factors which can influence sea level rise, agreement has not been reached as to specific forecasts of water level rise Coast of Israel, [3]. Thus, there is presently no model which can forecast sea level rise with a high degree of accuracy. However, field data and the world s present way of life point to a logical and sound basis for expectations of global warming, rather than global cooling National Geographic Magazine [4]. 1.3 Expected changes of coastline The question is therefore not expectation of sea level rise but rather the degree to which this rise will harm global ecology and the coastal environment. Without regard to specific causes, the fact remains that a general rise in sea water level is expected. The question is not the rise itself, but rather about the degree of this rise, as well as whether such a rise can alter the environment of a shoreline. The rise of sea level can cause flooding in low-lying areas along the coast, and the lower the degree of slope of these coastlines, the greater will be the degree of flooding. The rise in sea level also causes changes in the profile of the coast. In Israel, some authors foresee even shorelines having a significant slope suffering regression. A rise in sea level of one meter will bring about an average regression of 100 m when the coast is composed of sand, but only about 60 m when the coast is composed of more consolidated material Porat [5].

3 Water Resources Management III 187 An additional change which can occur with a rise in sea level is alteration of the lithological material and stratigraphy fronting the coastline (see fig. 1). A global rise in sea level can have a significant effect upon the regimes of all coastal aquifers around the world. Changes in erosion patterns can impact the shoreline borders of these aquifers and alter their freshwater storage capacity Rosen [6]. The impact of sea level rise is the more significant when taking into consideration that most of the mega-cities of the world are located close to the coast line. In Israel, this is precisely the concern, in light of the fact that the highly populated Tel Aviv Metropolitan region is located along the Mediterranean Sea coastline. Its demography and economy stand to be directly impacted. The effect upon Israel s shoreline will also significantly alter the state of the country s most important water resource the Coastal aquifer. Approximately 500 million cubic meters of water are extracted from this aquifer in the course of a single drought year, which represents about a quarter of all the water extraction of the country IHSR [7]. In light of the range of anticipated expected scenarios of sea level rise, the object of this study is to present the anticipated impact upon Israel s coastal groundwater resources, chosen as a case study. 2 Hydrogeological situation of Israel s Coastal aquifer Israel s Coastal aquifer extends from the Mt. Carmel horst/graben in the north to the Sinai in the south, and from the foothills of the central Mountain aquifer on the east to the Mediterranean Sea coast on the west. The aquifer is approximately 150 m thick along the seacoast and feathers out to a few meters along its eastern border. The aquifer is composed of numerous sandstone and calcareous sandstone layers, having high conductivity, and silt and hamra (sandy silt) having moderate permeability; and impermeable clay wedges which are often found within the aquifer s stratigraphy up to 5 km from the seashore. These impermeable clay layers differentiate the aquifer into subaquifers. Some of the deepest clay layers become thicker towards the sea, and thus are able to disconnect the subaquifer segments below them from influence of seawater Tolmach [8]. Owing to this lithological characteristic of the aquifer, rise in sea water level will have greatest impact upon the uppermost layers. The assessment of seawater intrusion is delineated by a network of observation wells located along the entire length of the Coastal aquifer. The seawater interface is a transition zone separating seawater from fresh water of the aquifer. The toe is the maximum distance from the seashore into which seawater intrudes the aquifer. In a number of locations along the length of the Coastal aquifer, this toe intrudes around 1000 m from the seashore (fig. 1). Where seawater has intruded deeper into the aquifer, a significant number of pumping wells has become salinated and therefore closed as sources of drinking and even of irrigation water IHSR [7]. Today, more than 50% of fresh water

4 188 Water Resources Management III pumped from the aquifer is taken from the western storage reservoir of the upper aquifer, between 1 5 km from the coast. North Range of sea water intrusion in 2002 in the upper layer subaquifer (B) of the Coastal aquifer Areas South hydrological strips Distance of Sea water intrusion from the seashore in meters Hadera Sharon north and south Tel Av iv Rishon Yavne Ashdod Ashquelon Figure 1: Seawater intrusion in 2002 in the Coastal aquifer. In high focal areas of pumpage, water levels have dropped, and formed cones of depression. Water levels in the central portion of these cones of depression have reached 1 to 3 m below sea level. These cones of depression can be found mostly 1 to 3 km from the coast. Thus, intrusion of seawater more than 1 km from the sea can produce significant damage to the Coastal aquifer s freshwater reservoir. The most critical of these cones of depression are in the areas of Hadera, Ramat Gan, Tel Aviv, Holon, between Ashdod to Erez Shiqma, and Nir Am. In these areas, where water level drawdown has accompanied early warning signs of seawater intrusion, hundreds of millions of cubic meters of drinking water are in danger as a result of further seawater intrusion Goldenberg and Melloul [9]. In order to mitigate the negative impact of seawater intrusion, pumpage levels have been reduced, and fresh water has been recharged into the aquifer from a variety of sources characterized by salinities greater than around 200 mg/l Cl. So that, while this recharge contributes to a rise in groundwater level, it also raises ambient salinity of Coastal aquifer water. However, the rate of rise in salinity from these recharge sources is small compared to the potential impact of salinity rise due to seawater intrusion. Therefore, because seawater intrusion is highly connected to a rise in seawater level, it is of critical importance to focus on it and on the variety of scenarios regarding potential sea level rise over the short- or long-term.

5 Water Resources Management III Components and estimation of the potential permanent storage loss of Israel s Coastal aquifer Figure 2 presents a visualization of various scenarios which could lead to permanent storage loss, and table 1 gives an estimation of the quantity of such losses under various scenarios. Each scenario will be determined by the degree of sea level rise. In table 1, scenario A involves a case where virtually no sea level rise occurs. Scenario B is a case where sea level rises approximately 10 cm over 50 years, which is not far from IPCC estimation. Scenario C is a case where sea level rises approximately 50 cm over 50 years, which is higher than the estimation given by the various researchers dealing with the Mediterranean Sea Coast of Israel [3]. Figure 2: Illustration of loss of permanent reserves due to seawater level. For the purpose of estimation it can be assumed that sea water intrusion in all such cases will take a linear form, and will intrude at the rate of approximately 10 m/yr., in aquifer media of approximately 25% porosity. In the case of scenario A, the shoreline is represented in Fig. 2 by position O. In this case, seawater intrusion is illustrated in figure 2 with a toe of intrusion of I 1 (note: I 0 stands for the initial point of interface toe of seawater and fresh water on the seashore), currently an average of approximately 1000 m from the seashore. Fifty years from now, if overpumpage continues at the same rate as today, the assumption is that the toe will have advanced 500 m, to position I 2, or an average of approximately 1,500 m from the sea shore. In this case, figure OI 0 I 2

6 190 Water Resources Management III encompasses a cross-sectional area, which represents the area that will be filled by seawater. Estimation of permanent loss is thus obtained by multiplying this area by the aquifer porosity for a lateral width of 1 km. Thus, the estimation comes to 18.7 million cubic meters per km of shore line (column 5, table 2). In the case of scenarios B and C, position of the shoreline moves from position O to position O. Thus, the line of seawater interface, in the absence of sea level rise (scenario A) is represented by line OI 2. Correspondingly, for both scenarios, the illustrated toe of additional intrusion into the aquifer with depth is represented by line O I 3. Some assumptions have been made in order to obtain an approximate calculation of total aquifer storage loss, given a rise of either 10 or 50 cm in sea level over the coming 50 years. These assumptions lead to considering the ultimate figure formed by OO I 3 I 0 for purposes of calculation as a trapezoid, composed of triangle OI 0 I 2 and parallelogram OO I 3 I 2. Table 1: Estimation of the loss of permanent reserve in the upper coastal aquifer for various scenarios. Additional loss of permanent reserve due only to seawater rise ( MCM)* Total loss of permanent reserve with porosity of 0.25 ( MCM)* Sea water intrusion (m) for the case of a topographic shoreline slope of / % 1 Thickness of the western border of the aquifer (m) Scenarios ( 6) ( 5) ( 4) ( 3) ( 2) ( 1) A Without seawater rise B With seawater rise of 10 cm (MCM) = Million Cubic Meters C With seawater rise of 50 cm The latter is, in fact, the area which will be filled by sea water due to a rise in sea level. Therefore, the area filled by sea water, if there will be no sea level rise (Scenario A), is seen as the area encompassed by the triangle OI 0 I 2, whilst the area filled by sea water with a maximum rise (Scenarios B and C) in sea level is represented by the trapezoid OO I 3 I 0. Subtracting the triangular area from that of the trapezoid enables one to estimate the area of the remainder, parallelogram OO I 3 I 2. It is important then to note that the ultimate size of parallelogram OO I 3 I 2 is a function of the distance inland which seawater will intrude on the coastal surface, which is, itself, a function of the slope of that coast. For the

7 Water Resources Management III 191 purposes of this calculated estimate, a range of slopes has been presumed of 0.1% and 1%. The calculation of permanent loss of storage capacity to the aquifer as a result of a rise in sea level has been accomplished by multiplying the area of the parallelogram OO I 3 I 2 by the porosity of the aquifer. For a 50 cm sea level rise, presuming a topographic slope of the coastline of 0.001, OO = 500 m, O I 0 = m, and I 0 I 3 = 2000 m (table 1). In the case of a topographic slope of the coastline of 0.01, OO = 50 m, O I 0 =100.1 m, and I 0 I 3 = 1550 m (table 1). 4 Results and discussion Results given in column 6, table 1, indicate that for a shoreline having a topographic slope of 0.01 and a rise of sea level of 10 cm., the additional permanent loss of storage capacity of the aquifer would be about 2.5 MCM per km of shoreline. For a rise of sea level of 50 cm, the additional permanent loss of storage capacity of the aquifer would be about 12.5 MCM per km of shoreline. This additional loss of permanent reserve may prove much larger if these areas are located in the vicinity of hydrological cones of depression. Another scenario must be envisioned in the case where the coastline is bordered by steep topographic cliffs. In such situations, the degree of consolidation and erodibility of cliff lithology at the water line is critical. Where this lithology involves easily erodible rocks, seawater will slowly undermine the cliff, forcing it inexorably inland. Additionally, alteration of the shoreline can progressively interface with different lithological components of a coastal aquifer s stratigraphy. This can change the hydrogeological characteristics of the coastal aquifer. An additional consideration, presuming a rise in sea level, can result from a change in the basis of drainage. Especially in rapid rate, this may be involve a loss of inland water storage necessary for establishing new groundwater table head configuration and new hydrologic equilibrium state. However, beside these adverse effects, some mitigating factors can come into play in parallel to a global rise in sea level. Such mitigating factors can include: erosion of the shoreline, altering the lithostratigraphic border of the aquifer along the coast, in a way that decreases the rate of seawater intrusion into the aquifer; retarding shoreline erosion because of ridges of cemented sand agglomerate (kurkar) bordering the sea coast along portions of the length of the Coastal aquifer; presence of only a few shorelines along the coast having gentle slope lower than 0.1%; and in the case where global warming proves greater than predicted today, as affecting the area of the Mediterranean Sea, this warming could conceivably lead to increased evaporation, which could lead to a net lowering of the degree of sea level rise along the eastern coast of the Sea. 5 Conclusions and recommendations Global rise in sea level and warming of the oceanic water may exert potentially drastic effects upon the climate of various countries in the world, and especially

8 192 Water Resources Management III in regions such as Israel bordering the eastern shoreline of the Mediterranean. There thus remains a significant probability that within 50 years from today a sea level rise could impact the fresh water resources of the Coastal aquifer along the seashore. It can therefore be concluded that some form of danger exists, and future long-term water resources planning must, at the earliest stage possible, factor in this danger and its potential negative impact. For this purpose, it can be recommended that: - high-resolution, accurate mapping of the shoreline be carried out in order to delineate critical areas; - where reserves of fresh water stand to be depleted as a result of a rise in sea level, alternatives of fresh water recharge, seawater desalinization, etc. ought to be part of long-term water resources planning; - sea level rise must be more accurately measured. - finally, the work of various governmental and private offices ought to be merged into that of a single agency, which could include personnel from governmental ministries of Infrastructure, Health, Environmental Quality, Economics, as well as engineering and economic institutes. References [1] IPCC, Climate change the scientific basis Technical Summary, pp , [ 2] Emery, K.O. & Aubrey DG.,, Sea level, land levels and tides gauges, Springer Verlag, NY, 237p 1991 [3] Coasts of Israel, The sea level rise and the situation on the Mediterranean sea coasts. Report no. 5 of the Society of Nature Protection and the Coast. Edition Meir PaPay Organization Forum of under the support of "Brakha" 65p. 2004: (In Hebrew) [4] National Geographic Magazine; Global Warming. Vol. 206 (3), pp [5] Porat, Itamar, The influence of Sea Level rise on the strip and cliff the Israel Coastal aquifer. Report. No 35, 40p, 2004 (in Hebrew) [6] Rosen, D.S., Long term remedial measures of sedimentological impact due to Coastal Developments on the South-Eastern Mediterranean coast. Proc. Littoral 2002, The Changing Coast EUROCOAST/EUCC. Ed. EUROCOAST, paper 40, (2): pp, Porto, Portugal, Sept 2002 [7] IHSR, Development of groundwater resources in Israel up to Autumn Hydrological Service Jerusalem, Report 5/01, ISSN pp 2003, (in Hebrew) [8] Tolmach, Y., Hydrogeological atlas of Israel, Coastal aquifer, Areas of Tel Aviv through Hadera. Volumes 3, 4, and 5, Hydrological Service report, Jerusalem, 70 p (in Hebrew). [9] Goldenberg, L.C. and Melloul, A.J., Hydrological and chemical management in the rehabilitation of an aquifer. Journal of Environmental management, 42:pp , 1994.