1 © IWA Publishing 2016 Hydrology Research | in press | 2016 Uncorrected Proof Coastal aquifer hydrodynamics and salinity in response to the tide: case study in Lisbon, Portugal M. Mancuso, E. Carol, E. Kruse and F. Mendes Rodrigues ABSTRACT The variability in dynamics and salinity of the coastal alluvial aquifer on which the city of Lisbon is located was evaluated. Such an evaluation was based on the analysis of level and groundwater electrical conductivity fluctuations depending on the tide in the Tagus River. The results obtained made it possible to recognize three sectors. First, a littoral sector where the variations in level and salinity are larger on the coast and decrease towards the innermost sections of the alluvial fan. Second, a sector close to the docks where there is greater dynamic and salinity variability than in the coastal sectors, as the excavations of the docks favour the tidal propagation towards the aquifer. And third, a sector located towards the apex of the alluvial fan associated with the dynamics of the stormwater channel. In this sector, the largest periodical water table fluctuations in the aquifer occur, since the freshwater that cannot drain towards the river enters the aquifer at high tide, causing a slight decrease in salinity content. On the basis of these results, conceptual models of hydrogeological behaviour were used to describe the spatial and temporal variations in the hydrodynamic and salinity characteristics of groundwater. doi: 10.2166/nh.2016.203 M. Mancuso (corresponding author) Department of Agricultural and Environmental Sciences, Federal University of Santa Maria (UFSM), Linha Sete de Setembro s/n – BR386 KM40, Frederico Westphalen, Rio Grande do Sul, Brazil E-mail: malvamancuso@ufsm.br E. Carol Centro de Investigaciones Geológicas, Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata (UNLP), La Plata, Argentina E. Kruse Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Facultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata (UNLP), La Plata, Argentina F. Mendes Rodrigues Rede Ferroviária Nacional – REFER, EPE, Direção de Estratégia e Desenvolvimento da Rede, Gestão de Investimentos, Rua de Sta. Apolónia, Lisbon, Portugal Key words | coastal aquifer, conceptual models, Lisbon, salinity variability, tidal hydrodynamics INTRODUCTION The complexity of the problems related to coasts has increased in the last few decades as a result of the impact of different human activities and uses. It is estimated that 80% of the world population lives less than 100 km from the coast and this tendency continues to increase. This implies that there are millions of people who demand space for housing, work and recreation, construction materials, food, drinking water, and so on. Therefore, the major challenge today is undoubtedly to develop specific technological, scientific, economic and legal resources to successfully deal with the needs that such transformations create. Spatial and temporal variability in groundwater dynamics and salinity in coastal areas is a widely studied topic. The research carried out in different regions worldwide (e.g., Park et al. ; Ghabayen et al. ; Kim et al. ; Giambastiani et al. ; Rosenthal et al. ; Antonellini et al. ; Carol et al. , ; Silva- Filho et al. ; Currell et al. ; Jun et al. ; Shapouri et al. ; Surinaidu ) shows highly dissimilar beha- viours, which is evidence of the hydrological complexity of coastal aquifers. The city of Lisbon, situated at the mouth of the Tagus River, is the capital and largest city of Portugal, concentrat- ing 20% of the country’s population, approximately 600,000 inhabitants, in an 85 km² area (Alcoforado et al. ). The Lisbon Metropolitan Area is one of the fastest growing urban agglomerations in the European Union, and it is esti- mated that its population will grow to 4.5 million inhabitants by 2050. As in any coastal city, the underlying mailto:malvamancuso@ufsm.br Q1 2 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof aquifers show changes in dynamics and salinity, conditioned in this case by the tidal flow entering the Tagus River from the Atlantic Ocean. The objective of this work is to assess the variability in dynamics and salinity of the coastal alluvial aquifer in Lisbon on the basis of the analysis of the variations in groundwater electrical conductivity (EC) and level depend- ing on the tide in the Tagus River. Even though the water of the aquifer is not used for human consumption, the importance of recognizing such an influence mainly lies in that the dynamics and salinity of the groundwater con- dition the useful life of the structures and urbanizations that have been developed in the aquifer. In coastal aqui- fers, the influence of seawater causes premature deterioration of constructions (through alteration of the constitutive material of concrete, corrosion of metallic alloys, etc.), reducing their useful life (Gjørv & Vennesland ; Jinjie & Wei ). A detailed understanding of the tidal influence on the aquifer will make it possible to better define the environmental conditions that construc- tion materials are subjected to and to take precautions when developing construction projects. A knowledge of subsurface hydrodynamics may also be of help in urban planning, for instance allowing the demarcation of favour- able areas for rainwater infiltration or less favourable areas for the construction of underground structures which, due to the obstruction of the flow, may contribute to the flood- ing of lower-lying areas. Figure 1 | Location of the study area and sampling points. (Full colour version available online STUDY AREA The study area is in a coastal sector of the city of Lisbon, located in western Portugal. The climate is Mediterranean, influenced by the Gulf Stream, with an average temperature of 17 WC, maximum temperatures of over 27 WC in July and August, and a minimum temperature of 11.3 WC in January. Rainfall shows a mean annual value of 726 mm, with December being the wettest month (121.8 mm) and July the driest (6.1 mm). The estuary of the Tagus River is one of the largest on the European Atlantic coast with a length of 50 km and an area of 325 km2, about 40% of which is an intertidal area. The average depth is less than 10 m and the greatest depths (∼40 m) occur close to the mouth of the estuary. The mean river flow, which shows high seasonal and inter- annual variability, is 400 m3 s�1. Salinity ranges from 0‰ at 50 km upstream from the mouth to almost 37‰ at the mouth of the estuary (França et al. ). The city of Lisbon is built over alluvial sediments where an unconfined aquifer, whose saturated thickness is of up to 40 m, occurs. This alluvial aquifer is limited at its base by the Lisbon Volcanic Complex (basalts) and partially to the north by compact calcareous rocks (Almeida ) (Figure 1) (full colour versions of all figures are available in the online version of this paper, doi: xx.xxxx/xx.xxxx.xxx). It is mainly constituted by medium- to fine-grained sand with intercala- tions of silty-sandy levels. Groundwater in the coastal .) Roig Rectángulo 3 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof sector is related to the Tagus River which is under the tidal influence of the Atlantic Ocean and, in the central sector, to the Alcântara Channel, which is a channelized surface watercourse with a permeable base that flows across the sector from north to south and into the Tagus River. METHODOLOGY Study design The hydrogeological characteristics of the unconfined aqui- fer were studied on the basis of boreholes, taking samples and measuring the groundwater level and EC. The boreholes were drilled by percussion drilling until the base- ment was reached, so as to cover the total thickness of the aquifer. The wells are 4 inches in diameter and each of them has a well-selected siliceous gravel pre-filter and a continuous slot filter in PVC tubing. During the well dril- ling process, the lithological description of the sediments was carried out. In order to analyze the variations in groundwater level and salinity depending on the tide of the Tagus River, con- tinuous water level and EC measurement sensors were installed (records every 10 min) in the exploration wells, and level measurements were carried out in the river. In the wells, EC measurements were also undertaken in groundwater at different levels of the water column on the basis of continuous record sensors located in the basal, middle and superficial parts of the aquifer. Data analysis Between April and October 2009, continuous level, temp- erature and EC measurements were carried out by means of sensors that were installed in rotation in the different wells of the monitoring network. In order to analyze a simi- lar hydrological situation, the measurement period comprising April and May 2009 was chosen. The selection of this period was based on the fact that it concentrated most of the data from the sensors located at different depths, which made it possible to analyze the EC and temp- erature variations throughout the thickness of the aquifer. The groundwater level data, as well as those of the Tagus River, were set at a similar zero level for reference, expres- sing the values in metres below or above such level. The detailed study of the spatial variability in dynamics and salinity of the aquifer was carried out on the basis of measurements obtained from Wells 1, 2, 3, 4, 5, 6, 11 and S1 (Figure 1). The groundwater level, EC and temperature data were plotted in relation to time and compared with the level data obtained for the Tagus River in order to deter- mine the response of these parameters to the tidal oscillations of the river. So as to determine the location of the salt wedge and estimate quantitatively the area of tidal influence, the data obtained were incorporated into a geographic information system. Furthermore, the level and EC records of the wells at different levels were used to construct hydrogeological cross-sections in order to identify the dynamics of the groundwater flow in relation to the tide and the location of the salt wedge within the study area. RESULTS Tidal influence on groundwater level, EC and temperature In the period analyzed, the maximum tidal oscillations range between �1.74 and 1.61 m, showing ranges close to 1 m at quadrature and to 3 m at syzygy. At low tide, the groundwater levels in all of the studied wells provide evi- dence that the regional groundwater flow direction is from the apex of the alluvial cone towards the Tagus River. In most wells, groundwater level oscillations associated with the tidal flows were recorded, as well as the presence of sal- inity stratification, demarcating three sectors with different behaviours (Figure 1). Sector 1 Sector 1 (Figure 1) is located at the southwest end of the alluvial cone, in a zone where there are two large under- ground parking garages. The aquifer has a thickness of 13 m on the coast, becoming wedge-shaped towards the northwest until it disappears where the basaltic basement 4 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof crops out. The alluvial sediments are composed of sand with intercalations of silty to clayey lenses. The variations in level and EC of groundwater in a trans- ect perpendicular to the coastline, which includes Wells 1, 2 and 3 (Figure 1), make it possible to visualize the tidal influ- ence on the hydrodynamic behaviour and salinity of the aquifer. The groundwater level and EC records in the well farthest from the coastline (Well 1, installed 200 m from the Tagus River) show that there are no significant variations in such parameters depending on the tide (Figure 2). Through- out most of the period analyzed, the values of the water table remained close to 0.63 mand theECof the groundwater shows average values of approximately 1.50 mS/cm, with a slight increase in EC at some high spring tides. In Well 2, located 145 m from the river, slight oscil- lations in the water table and EC associated with the tidal flows in the river were recorded (Figure 2). At low tide, the water table has an average height of 0.57 m, with increases at the onset of the high tide. At high neap tides, the water level does not vary or shows variations below 0.05 m, whereas at high spring tides an average increase of 0.10 m with respect to its position at low tide was recorded. In turn, the average EC of the groundwater is 1.2 mS/cm, with maximum variations of 0.4 mS/cm between the low neap tide and the high spring tide. The groundwater EC in Well 2 recorded at different depths (Figure 3) shows evidence of salinity stratification, with average values of 11 mS/cm at the base and 1.5 mS/cm on the surface. At high tide, the EC of the groundwater tends to increase in the most superficial sections of the aqui- fer (records from depths of �0.5 and �3.7 m), whereas at depth it remains constant (records from a depth of �6.8 m). It should be noted that, while the maximum peaks of the water table occur before the high tide in the river, the highest values of EC in groundwater appear with a delay with respect to the high tide in the river. In the area closest to the coastline (Well 3, located 70 m from the river), a similar behaviour to that of Well 2 can be observed, although the former has larger amplitudes in the fluctuation of the water table and higher salinity content (Figure 2). At low tide, the average water table is 0.22 m and it increases at the onset of the high tide. At high neap tides, increases in the water table of 0.22 m with respect to its position at low tide are recorded, whereas at syzygy they reach 0.83 m. The average EC of groundwater is 9.30 mS/cm, with increases at high tide that are more noticeable at high spring tides. The EC of groundwater measured at different levels in the water column (Figure 3) shows salinity stratification with average values of 18.6 mS/cm at the bottom of the well and 9.2 mS/cm on the surface. At high tide, the EC of groundwater tends to increase in the shallower areas (records from depths of �1.2 and �4.4 m.a.s.l.), while at depth they remain constant (records from depths of �6.8 and �7.5 m.a.s.l.). Sector 2 Sector 2 (Figure 1) comprises a sector near the docks at the Santo Amaro port, where the surface water level fluctuates as the Tagus River fluctuates with the tide. The alluvial aqui- fer has a thickness of approximately 21 m, with sediments composed of sand, which become sandy-silty to clayey facies towards the base. Level and EC data of groundwater in Wells 5 and 6 (Figure 4) were analyzed. Well 5 is located 200 m from the docks and 370 m from the riverbank, and Well 6 is located 70 m from the docks and 240 m from the riverbank. The water table levels in Well 5 show that at low tide the average level is 0.22 m and that it rises periodically at high tide (Figure 4). During high spring tides, the increases reach 0.36 m with respect to their low-tide position, whereas during neap tides the increases are 0.10 m, with the highest groundwater level peaks occurring after the low tide peaks in the river. Groundwater EC also shows the tidal influence, with an average value at low tide of 2.60 mS/cm (at �13.4 m) and recorded increases of 0.1 and 0.2 mS/cm at high tide (Figure 4). The groundwater EC records at different levels (Figure 5) show salinity stratification, with average values of 5.5 mS/cm at �16 m, 2.9 mS/cm at �8 m and 2.5 mS/cm at �1.0 m. At the base of the aquifer, the EC of groundwater does not vary with the tide, whereas in the middle and shal- low sections of the water column, a slight increase is recorded at high tide (Figure 5). At Well 6, the average level at low tide is 0.17 m, which increases at the onset of the high tide. At high neap tides, the water table rises 0.18 m with respect to its position at low Figure 2 | Diagram of groundwater level and EC variations in Wells 1, 2 and 3, and of the tide in the Tagus River depending on the time. (Full colour version available online.) 5 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof Figure 3 | Diagram of groundwater level and EC variations at different depths in Wells 2 and 3, and of the tide in the Tagus River depending on the time. (Full colour version available online.) 6 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof tide, whereas at syzygy increases of up to 0.75 m are recorded (Figure 4). The highest groundwater level peaks occur before the high tide peaks in the river. The EC of groundwater at the base of the aquifer (�16.4 m) does not show large variations, with an average value of 42 mS/cm. This high value is only representative of the deepest sections of the aquifer, since the average EC records of groundwater are 4.2 mS/cm (at �8.2 m) and 3.0 mS/cm (at �2.5 m). At the base of the aquifer, no changes in the EC of groundwater depending on the tide are recorded, whereas at �8.2 m and at �2.5 m there is a tendency for EC to increase at low tide and when the high tide at the river is higher than the water table (Figure 5). The EC data at these depths show a similar behaviour, with larger variations in the shallower sections. Sector 3 This sector is located in the vicinity of the apex of the allu- vial cone, and comprises a section close to the Alcântara Channel, which is the main stormwater channel (Figure 1). The alluvial aquifer is composed of sand with limited clay intercalations that on the whole have a thickness of 7.6 m at Well 11 and 20 m at Well S1. The stormwater channel is permeable, so there is a direct connection between the aquifer and the water in the channel. In turn, the channel discharges into the river, which is why it constitutes a prefer- ential pathway for the tidal flow from the river at high tide. Well S1 is located 700 m from the riverbank and 2 m from the stormwater channel. The level records (Figure 6) show that at low tide the level remains at a stable value close to 0 m. At high tide, the water table begins to rise, until it reaches a position similar to the one of the river, with maximum water table peaks occurring before the high tide peaks of the river. Another characteristic that should be noted is the fact that at quadrature the maximum water table peaks tend to exceed the ones of the high tide at the river, whereas at syzygy the high tide peaks slightly exceed the ones of the water table (Figure 6). The EC values of groundwater obtained from three levels of the aquifer suggest the presence of brackish water with an average EC of 11 mS/cm at the base of the aquifer (�15.4 m), while in the middle and shallow sections (�8.1 and �0.9 m) the water has low salinity and an average EC close to 1.30 mS/cm (Figure 7). Even though there is salinity stratification, the EC of groundwater does not show signifi- cant variations depending on the tide. In the basal part of the aquifer, a decrease in salinity at high tide can be observed, with a maximum range of 0.5 mS/cm (Figure 7). Well 11 is located at the same distance from the coast- line as Well S1, but at approximately 50 m from the Figure 4 | Diagram of groundwater level and EC variations in Wells 5 and 6, and of the tide in the Tagus River depending on the time. (Full colour version available online.) 7 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof stormwater channel. At low tide, the water table is close to 0.50 m, and it increases as the level of the river increases at high tide. As in Well S1, at quadrature the maximum peaks of the water table tend to exceed those of the river at low tide, whereas, at syzygy, the peak levels in the river at high tide slightly exceed those in the water table (Figure 6). The groundwater EC has an average value of 1.20 mS/cm, with a slight decrease at high tide (Figure 7). Area of tidal influence and salt wedge The groundwater levels and EC depending on the tide in the river suggest spatial variations in the dynamics and salinity of the aquifer, with three sectors showing distinctive behaviours. In Sector 1, the propagation of the tidal wave in the aquifer reaches at least 150 m from the coastline, generating variations in the water table, whose amplitude decreases as the distance from the coastline increases (Figure 8). The sal- inity stratification and the increase in EC of groundwater at high tide, together with an increase in the salinity concen- trations towards the coast, are the characteristics that indicate the propagation of the tidal wave in the coastal aquifer. However, the increase in the water table as regards the groundwater–surface water interaction is associated with two processes. At the onset of the high tide, the level increase in the Tagus River prevents the groundwater Figure 5 | Diagram of groundwater level and EC variations at different depths in Wells 5 and 6, and of the tide in the Tagus River depending on the time. (Full colour version available online.) Figure 6 | Diagram of groundwater level and EC variations in Wells S1 and 11, and of the tide in the Tagus River depending on the time. (Full colour version available online.) 8 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof Figure 7 | Diagram of groundwater level and EC variations at different depths in Wells S1 and 11, and of the tide in the Tagus River depending on the time. (Full colour version available online.) 9 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof discharge towards the river, which causes groundwater to accumulate and the water table to rise, without increasing the salinity. Subsequently, when the high tide in the river rises over the level of the water table, the surface water flows from the river to the aquifer, causing the water table to continue rising and triggering an increase in the EC of groundwater. These two processes are evident in the relationship that exists between the high tide peaks in the river, and the level and EC peaks in the aquifer, with the water level peaks occurring before the tidal peak in the river and the EC peaks in groundwater being delayed with respect to the high tide in the river. In Sector 2, the tide has an influence over the innermost sections of the aquifer with respect to Sector 1. This is due to the fact that the docks of the Santo Amaro port is an area where the alluvial cone in which the aquifer occurs has been undermined and where the tide from the river may penetrate freely (Figure 8). Water table fluctuations are recorded up to 300 m from the riverbank, with the maxi- mum water table level occurring before the high tide in the river in the dock area, and after the high tide in the river in the more distant areas. In the vicinity of the docks, the aquifer shows a marked salinization at the base (over 40 mS/cm), but not on the surface and the more dis- tant sectors in which the EC is less than 5 mS/cm. In the zone near the docks, the EC of groundwater tends to decrease at high tide in the middle section of the aquifer, a tendency that is less noticeable on the surface. This charac- teristic, together with the occurrence of the maximum water table level before the high tide in the river, may be indicating that at the onset of the high tide there is an accumulation of freshwater in the aquifer, which cannot be discharged when the level in the river rises. In turn, the irregularity in the EC records in the superficial sections may be associated with the fact that the lateral cement walls of the docks cause an irregular propagation of the tidal wave towards the aquifer. Sector 3 is most distant from the coastline; however, the largest oscillations of the water table in response to the tide are recorded there. Groundwater salinity is low, and the EC of groundwater in the middle and shallow sections are below 1.3 mS/cm, with a slight decrease at high tide. This behaviour of the groundwater levels and EC is caused by the fact that the stormwater channel constitutes a preferen- tial pathway for the tidal flow towards the interior of the alluvial fan. Upon the entrance of tidal water at high tide, the rainwater discharge is blocked and it tends to accumu- late in the channel, causing a rise in the water level (Figure 8), which reaches a level similar to the one in the river during neap tides and exceeds the one in the river during spring tides. As the walls of the channel are per- meable, the freshwater that accumulates in the channel enters the aquifer, causing a decrease in EC and a rise in Figure 8 | Diagram indicating the relationship between the Tagus River water and the aquifer at low tide (LT) and high tide (HT) for the three sectors studied. (Full colour version available online.) 10 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof 11 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof the water table, which reaches values similar to the ones in the channel and the river. The distribution of the wells that show high EC values at depth and a significant level variation in relation to the tide allows the identification of the coastal area affected by saline intrusion, which reaches a surface of 0.38 km2 (Figure 9). The narrowest coastal strip affected by saline intrusion (150 m wide) is located to the west of the study area, in the sector with no docks. In the central and eastern sectors, the salinized strip includes the Santo Amaro and Alcântara docks, with the salinized area widening from the Tagus River bank up to a width of 350 m. In the image shown in Figure 9, it can be observed that the salinized sector com- prises urban areas that have foundations and underground constructions (parking garages) in contact with the salinized groundwater and in which significant tidal oscillations occur. To the north, and in the vicinity of the Alcântara Chan- nel, there is a non-salinized area but with strong tidal oscillations (Figure 9). This 0.06 km2 area also comprises an important number of constructions whose foundations are in direct contact with groundwater. Figure 9 | Location of area of tidal influence and of the salt wedge. The dotted lines bordering area with saltwater intrusion. The dotted line surrounding the Alcântara Channel in online.) DISCUSSION The study of groundwater behaviour in coastal areas requires undertaking fieldwork and detailed measurements in order to understand the spatial variations in hydrodyn- amics and salinity. The complexity that characterizes coastal aquifers, especially those associated with significant urban development, requires greater detail in the under- standing of such variables. This is an indispensable basis for a correct simulation that would enable, by means of a mathematical model, the planning and design of under- ground engineering works. In this regard, it should be taken into consideration that the urban development in large cities includes an increasing, intensive underground occupation due to the construction of different types of infrastructure that may be near or in contact with the water table. The stability of underground engineering works, such as parking garages, areas of com- mercial use, tunnels, urban buildings with deep foundations, underground transport constructions, etc., depends to a large extent on the characteristics of the sub- stratum on which they lie, with the thickness of the the Tagus River, Alcântara Dock and stretching a few hundred metres inland indicates the dicates the area of fresh water with strong tidal oscillation. (Full colour version available 12 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof non-saturated zone and groundwater salinity being par- ameters of importance to be considered in construction projects (Aitcin ). The presence of such underground structures interacts with groundwater (Vazquez-Suñe et al. ). This inter- action may occur in two ways. One is related to the effect that a structure with a different hydraulic conductivity causes when it modifies the natural groundwater flow. The other is associated with the effect that groundwater may have when it filters through such a structure (Carrera & Váz- quez Suñé ). In the latter case, the degree of influence depends on the variability in salinity and in the oscillations of the water levels, with a higher corrosion rate and chemi- cal attack of the structures built underground in the coastal areas with marine influence (Wegen et al. ; Costa & Appleton ; Aitcin ). The hydrodynamic and salinity variability as a conse- quence of the tidal influence shows different effects in the alluvial cone on which the city of Lisbon is located. The large underground parking garages related to Sector 1 con- stitute impermeable obstacles that locally modify the natural groundwater flow. The shallow depth of the water table causes such structures to maintain constant interaction with the groundwater. The stratification and salinity vari- ation of groundwater may affect the concrete structures, especially those of the parking garages located closer to the coastline. The construction of docks in Sector 2 altered the natural hydrological behaviour, favouring tidal propagation and, consequently, causing larger oscillations of the water table and the penetration of the salt wedge into the innermost sec- tions of the aquifer. These characteristics should be taken into consideration when construction works are undertaken in this sector, as well as in the construction projects for new docks, which may affect the existing constructions. The existence of a permeable drainage channel dischar- ging into the river constitutes a preferential pathway for the tide in sectors that are distant from the coastline (Sector 3). This modifies the natural water flow conditions, generating large variations in level in sections of the aquifer that are very distant from the coastline and where the water table oscillations as a consequence of the tide should be minimal. Even though in this sector the salinization of the ground- water is low, the periodical variation in the levels causes stress in constructions, with the possibility that the chemical attack of concrete by groundwater may be stronger than the one by seawater (Santhanam et al. ). An understanding of the hydrodynamic and salinity characteristics of the environment may contribute to the planning of underground space occupation, such as the scale and location of underground structures, the selection of alternatives and a project for hydrogeological impact miti- gation, as well as the selection of materials which would optimize the useful life of such structures in unsaturated/ saturated zone interface environments and in areas with sal- inity variability due to tidal influence. For instance, the data used in this work made it possible to generate a model of hydrological behaviour which was applied to the design of a future underground tunnel connecting two lines of subur- ban trains in Lisbon (Mancuso et al. , ; Mancuso & Pina ). Besides, both the data and the model are avail- able from the Lisbon City Council to be used by the engineers responsible for issuing building permits and pro- viding counsel for the construction of new buildings, underground parking garages, etc. CONCLUSIONS The groundwater level and EC data collected at different depths made it possible to observe the variability of such parameters within the alluvial aquifer concerning its three spatial components. The tidal fluctuation in the river and the anthropogenic alterations of the environment determine the existence of different behaviours in the dynamics of the aquifer. There are sectors in which the tidal wave typically propagates towards the aquifer (Sector 1), affecting to a larger extent the sectors closer to the coast. On the other hand, it can be observed that the inner sectors of the alluvial fan that are close to the docks (Sector 2) may have a larger dynamic and salinity variability than the coastal sectors, due to the fact that the excavations of the docks favour the tidal propagation towards the aquifer. In both cases (Sectors 1 and 2), the high tides are associated with rises in the water tables and salinity of groundwater. Similarly, in the sectors with a stronger tidal influence, the aquifer tends to have a marked salinity stratification, a characteristic which maxi- mizes the salinity variability of the aquifer. 13 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof Towards the apex of the alluvial fan, and associated with the dynamics of the storm water channel (Sector 3), the aquifer shows larger periodical oscillations in the water table as a result of the tides. In this sector, it is not the case that the saline tidal flow enters the aquifer, but rather that the freshwater that cannot drain towards the river enters the aquifer at high tide, causing a slight decrease in salinity content. Salinity stratification tends to decrease along the entire water column at high tide; the most basal sections of the aquifer, however, contain brackish water. Being aware of the dynamic and salinity variability of shallow aquifers in areas with significant urban develop- ment is essential to develop land use planning policies that would tend to minimize the impact on the water flows and maximize the durability of constructions in time. REFERENCES Aitcin, P. C.  The durability characteristics of high performance concrete: a review. Cement Concrete Comp. 25, 409–420. Alcoforado, M. J., Andrade, H., Lopes, A. & Vasconcelos, J.  Application of climatic guidelines to urban planning: the example of Lisbon (Portugal). Landsc. Urban Plan. 90, 56–65. Almeida, F. M.  Carta Geológica do Concelho de Lisboa. Direcção-geral de Geologia de Minas. Serviço Geológico de Portugal. Gravado e Composta em Geocart. Serviço Cartográfico do Exército. Escala 1, 10000. Antonellini, M., Mollema, P., Giambastiani, B., Bishop, K., Caruso, L., Minchio, A., Pellegrini, L., Sabia, M., Ulazzi, E. & Gabbianelli, G.  Salt water intrusion in the coastal aquifer of the southern Po Plain, Italy. Hydrogeol J. 16, 1541–1556. Carol, E., Kruse, E. & Mas Pla, J.  Hydrochemical and isotopical evidence of ground water salinization processes on the coastal plain of Samborombón Bay, Argentina. J. Hydrol. 365, 335–345. Carol, E., Dragani, W., Kruse, E. & Pousa, J.  Surface water and groundwater characteristics in the wetlands of the Ajó River (Argentina). Cont. Shelf Res. 49, 25–33, doi:10.1016/j. csr.2012.09.009. Carrera, J. & Vázquez Suñé, E.  Sobre la interacción entre acuíferos y obras subterráneas. El agua y las infraestructuras en el medio subterráneo. AIH-GE, Barcelona, 21–38. Costa, A. & Appleton, J.  Chloride penetration into concrete in marine environment – Part I: main parameters affecting chloride penetration. Mater. Struct. 32, 252–259. Currell,M., Cendón,D. I.&Cheng,X. Analysis of environmental isotopes in groundwater to understand the response of a vulnerable coastal aquifer to pumping: Western Port Basin, south-eastern Australia. Hydrogeol. J. 21, 1413–1427. França, S., Vinagre, C., Caçador, I. & Cabral, H. N.  Heavy metal concentrations in sediment, benthic invertebrates and fish in three salt marsh areas subjected to different pollution loads in the Tagus Estuary (Portugal). Mar. Pollut. Bull. 50, 998–1003. Ghabayen, S., McKee, M. & Kemblowski, M.  Ionic and isotopic ratios for identification of salinity sources and missing data in the Gaza aquifer. J. Hydrol. 318, 360–373. Giambastiani, B. M. S., Antonellini, M., Oude Essink, G. H. P. & Stuurman, R. J.  Saltwater intrusion and water management in the unconfined coastal aquifer of Ravenna (Italy): a numerical model. J. Hydrol. 340, 91–104. Gjørv, O. E. & Vennesland, Ø.  Diffusion of chloride ions from seawater into concrete. Cement Concrete Res. 9, 229–238. Jinjie, S. & Wei, S.  Effect of sulfate ions on the corrosion behavior of steel in concrete using electrochemical methods. Adv. Mat. Res. 163–167, 3049–3054. Jun, S., Bae, G. & Lee, K.  Factors causing dynamic variations in the saltwater–freshwater transition zone in a beach aquifer, Mangsang, South Korea. Hydrogeol. J. 21, 1355–1371. Kim, K. Y., Seong, H., Kim, T., Park, K. H., Woo, N. C., Park, Y. S., Koh, G. W. & Park, W. B.  Tidal effects on variations of fresh-saltwater interface and groundwater flow in a multilayered coastal aquifer on a volcanic island (Jeju Island, Korea). J. Hydrol. 330, 525–542. Mancuso, M. A. & Pina, B.  Modelo matemático do escoamento subterrâneo no vale de Alcântara. 1.W Relatório de Modelação da Fase 3: Actualização do modelo conceptual e Análise de sensibilidade. Proc. 0504/1/17219. Proc. Int. 0504/541/1293. Portugal. REFER. Relatório Técnico 407/ 2011 – NGEA. LNEC. p. 43. Mancuso, M. A., Lobo Ferreira, J. P., Oliveira, M. M., Novo, M. E. & Henriques, M. J.  Modelo Matemático do Escoamento Subterrâneo no Vale de Alcântara. Relatório Sectorial da Fase 1: Águas Subterrâneas. Relatório Técnico 378/2008 – NAS. LNEC. p. 59. Mancuso, M. A., Novo, M. E. & Lobo Ferreira, J. P. Modelo Matemático doEscoamento Subterrâneo noVale deAlcântara. 1 Relatório Sectorial da Fase 2: Águas Subterrâneas. Relatório Técnico 229/2009 – NAS. LNEC. p. 47. Park, S., Yun, S., Chae, G., Yoo, I., Shin, K., Heo, C. & Lee, S.  Regional hydrochemical study on salinization of coastal aquifers, western coastal area of South Korea. J. Hydrol. 313, 182–194. Rosenthal, E., Zilberbrand, M. & Livshitz, Y.  The hydrochemical evolution of brackish groundwater in central and northern Sinai (Egypt) and in the western Negev (Israel). J. Hydrol. 337, 294–314. Santhanam, M., Cohen, M. & Olek, J.  Differentiating seawater and groundwater sulfate attack in Portland cement mortars. Cement Concrete Res. 36, 2132–2137. http://dx.doi.org/10.1016/S0958-9465(02)00081-1 http://dx.doi.org/10.1016/S0958-9465(02)00081-1 http://dx.doi.org/10.1016/j.landurbplan.2008.10.006 http://dx.doi.org/10.1016/j.landurbplan.2008.10.006 http://dx.doi.org/10.1007/s10040-008-0319-9 http://dx.doi.org/10.1007/s10040-008-0319-9 http://dx.doi.org/10.1016/j.jhydrol.2008.11.041 http://dx.doi.org/10.1016/j.jhydrol.2008.11.041 http://dx.doi.org/10.1016/j.jhydrol.2008.11.041 http://dx.doi.org/10.1016/j.csr.2012.09.009 http://dx.doi.org/10.1016/j.csr.2012.09.009 http://dx.doi.org/10.1016/j.csr.2012.09.009 http://dx.doi.org/10.1007/BF02479594 http://dx.doi.org/10.1007/BF02479594 http://dx.doi.org/10.1007/BF02479594 http://dx.doi.org/10.1007/s10040-013-1017-9 http://dx.doi.org/10.1007/s10040-013-1017-9 http://dx.doi.org/10.1007/s10040-013-1017-9 http://dx.doi.org/10.1007/s10040-013-1017-9 http://dx.doi.org/10.1016/j.marpolbul.2005.06.040 http://dx.doi.org/10.1016/j.marpolbul.2005.06.040 http://dx.doi.org/10.1016/j.marpolbul.2005.06.040 http://dx.doi.org/10.1016/j.marpolbul.2005.06.040 http://dx.doi.org/10.1016/j.jhydrol.2005.06.041 http://dx.doi.org/10.1016/j.jhydrol.2005.06.041 http://dx.doi.org/10.1016/j.jhydrol.2005.06.041 http://dx.doi.org/10.1016/j.jhydrol.2007.04.001 http://dx.doi.org/10.1016/j.jhydrol.2007.04.001 http://dx.doi.org/10.1016/j.jhydrol.2007.04.001 http://dx.doi.org/10.1016/0008-8846(79)90029-2 http://dx.doi.org/10.1016/0008-8846(79)90029-2 http://dx.doi.org/10.1007/s10040-013-0995-y http://dx.doi.org/10.1007/s10040-013-0995-y http://dx.doi.org/10.1007/s10040-013-0995-y http://dx.doi.org/10.1016/j.jhydrol.2006.04.022 http://dx.doi.org/10.1016/j.jhydrol.2006.04.022 http://dx.doi.org/10.1016/j.jhydrol.2006.04.022 http://dx.doi.org/10.1016/j.jhydrol.2006.04.022 http://dx.doi.org/10.1016/j.jhydrol.2005.03.001 http://dx.doi.org/10.1016/j.jhydrol.2005.03.001 http://dx.doi.org/10.1016/j.jhydrol.2007.01.042 http://dx.doi.org/10.1016/j.jhydrol.2007.01.042 http://dx.doi.org/10.1016/j.jhydrol.2007.01.042 http://dx.doi.org/10.1016/j.cemconres.2006.09.011 http://dx.doi.org/10.1016/j.cemconres.2006.09.011 http://dx.doi.org/10.1016/j.cemconres.2006.09.011 Q2 Q3 14 M. Mancuso et al. | Coastal aquifer hydrodynamics and salinity in response to the tide Hydrology Research | in press | 2016 Uncorrected Proof Shapouri, M., Cancela da Fonseca, L., Iepure, S., Stigter, T., Ribeiro, L. & Silva, A.  The variation of stygofauna along a gradient of salinization in a coastal aquifer: the variation of stygofauna along a gradient of salinization in coastal aquifer. Hydrol. Res., doi:10.2166/nh.2015.153 (in press). Silva-Filho, E. V., Sobral Barcellos, R. G., Emblanch, C., Blavoux, B., Sella, S. M., Daniel, M., Simler, R. & Wasserman, J. C.  Groundwater chemical characterization of a Rio de Janeiro coastal aquifer, SE – Brazil. J. S. Am. Earth Sci. 27, 100–108. Surinaidu, L.  Role of hydrogeochemical process in increasing groundwater salinity in the central Godavari delta. Hydrol. Res. doi:10.2166/nh.2015.050 (in press). Vazquez-Suñe, S., Sanchez-Vila, X. & Carrera, J.  Introductory review of specific factors influencing urban groundwater, an emerging branch of hydrogeology, with reference to Barcelona, Spain. Hydrogeol. J. 13, 522–533. Wegen, G. V., Bijen, J. & Selst, R.  Behaviour of concrete affected by sea water under high pressure. Mater. Struct. 26, 549–556. First received 15 October 2015; accepted in revised form 12 January 2016. Available online 19 February 2016 http://dx.doi.org/10.2166/nh.2015.153 http://dx.doi.org/10.2166/nh.2015.153 http://dx.doi.org/10.2166/nh.2015.153 http://dx.doi.org/10.1016/j.jsames.2008.11.004 http://dx.doi.org/10.1016/j.jsames.2008.11.004 http://dx.doi.org/10.2166/nh.2015.050 http://dx.doi.org/10.1007/s10040-004-0360-2 http://dx.doi.org/10.1007/s10040-004-0360-2 http://dx.doi.org/10.1007/s10040-004-0360-2 http://dx.doi.org/10.1007/BF02472865 http://dx.doi.org/10.1007/BF02472865 Roig Resaltar Roig Rectángulo Roig Rectángulo Coastal aquifer hydrodynamics and salinity in response to the tide: case study in Lisbon, Portugal INTRODUCTION STUDY AREA METHODOLOGY Study design Data analysis RESULTS Tidal influence on groundwater level, EC and temperature Sector 1 Sector 2 Sector 3 Area of tidal influence and salt wedge DISCUSSION CONCLUSIONS REFERENCES