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Coastal aquifer hydrodynamics and salinity in response to the tide: case study in Lisbon, Portugal

The variability in dynamics and salinity of the coastal alluvial aquifer on which the city of Lisbon is located was evaluated. The 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


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 live 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.

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 W C, maximum temperatures of over 27 W C in July and August, and a minimum temperature of 11.3 W C 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 km 2 , 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 interannual variability, is 400 m 3 s À1 . 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

METHODOLOGY Study design
The hydrogeological characteristics of the unconfined aquifer 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 basement 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 drilling 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, continuous 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, temperature 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 similar 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 temperature 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, expressing the values in metres below or above such a 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 determine 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 ranged between À1.74 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 provided evidence 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 salinity stratification, demarcating three sectors with different behaviours ( Figure 1). at À2.5 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 alluvial 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 preferential 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 Well 11 is located at the same distance from the coastline as Well S1, but at approximately 50 m from the 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   To the north, and in the vicinity of the Alcântara Channel, there is a non-salinized area, but with strong tidal oscillations ( Figure 9). This 0.06 km 2 area also comprises an important number of structures whose foundations are in direct contact with groundwater.

DISCUSSION
The study of groundwater behaviour in coastal areas requires and 2), the high tides are associated with rises in the water tables and the salinity of the groundwater. Similarly, in the sectors with a stronger tidal influence, the aquifer tends to have a marked salinity stratification, a characteristic which maximizes the variable salinity of the aquifer.
Towards the apex of the alluvial fan, and associated with the dynamics of the stormwater 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 development is essential to develop land use planning policies that would tend to minimize the impact on the water flows and maximize the durability of structures over time.