Groundwater discharge: A solution to Malta’s water problem?
The Malta Resources Authority refers to various articles on local media where Peter Gatt made a number of statements on coastal groundwater discharge.
The Authority considers such statements to merit clarification to the benefit of the general public, more so, when the tapping of groundwater discharge in Malta has been, in these contributions, described fleetingly as an easy solution to solving our water problems.
The occurrence of groundwater in Malta’s sea level aquifer systems is attributed to a small difference in density between fresh and saline (sea) water, which leads to freshwater percolating downwards from the surface to float on the denser sea water once it reaches the zone of saturation (commonly called aquifer). In small permeable islands, such as Malta, this freshwater body takes the shape of a lens floating on the underlying saltwater (See Fig. 1).
The shape of the freshwater lens is influenced by the size of the island, the recharge distribution and the permeability/porosity of the aquifer formation.
The relationship between freshwater and saltwater in coastal and island aquifers has been extensively studied since the first observations carried out by Badon Ghijben (1889) in Holland, and Herzberg (1901) in Northern Germany.
They developed a simple model based on a very basic principle: at any one point of a sharp interface between two immiscible fluids, the water pressure at both sides must be the same. This relationship led to the Ghijben-Herzberg equation describing the structure of a floating/coastal aquifer system, which indicates that as a first approximation, the depth of the interface below mean sea level is about 36 times the freshwater head (water table elevation) above local mean sea level. Further modifications to this equation, to take into account the fact that in reality freshwater and saltwater are not separted by a sharp interface but by a transition (mixing) zone, were carried out by Hubbert (1940).
Field measurements carried out in gauging wells in the sea level aquifer in Malta show that this aquifer follows closely the models described above. In fact conductivity logs carried out in deep gauging wells, show the thickness of the aquifer to closely follow the Ghijben-Herzberg model but with a relatively thick transition zone separating the freshwater from the sea-water.
Groundwater flows from the recharge areas of an island towards the coast, driven by the head potential (pressure) created by inland recharge. General flow patterns are directed downwards in the recharge areas and upwards near the discharge areas. Retention times in the saturated zone are also generally quite long, due to the slow movement of groundwater in the aquifer pore matrix. In the Malta sea level aquifer system, studies by the International Atomic Energy Agency (2002) and British Geological Survey (2009) indicate that the average age of groundwater in the saturated zone is of the order of 40 years.
Assuming a homogeneous carbonate rock-matrix, groundwater is discharged at the coast in diffuse form through the pores of the rock formation. However, in reality, groundwater flow is enhanced along open fissures and fractures in the rock matrix and thus discharge to the sea is expected, in some way or another, to follow these preferential routes. Therefore, discharge of freshwater to the sea is expected to follow two main routes: slow discharge through the pore matrix along all the coastal margin and fast discharge through open fractures.
However, one should consider that along the coastal area, mixing of discharging freshwater with saline water does occur within the transition zone and thus the freshwater discharged is in reality expected to be a mix of freshwater and saline water. The direct concentrated discharge of freshwater under pressure (subsurface springs) is expected only to occur under conditions of high piezometric heads (required to develop the necessary pressure to overcome the overlying seawater) and in developed karst systems (required to provide the necessary conduits in which the freshwater flow can concentrate). It seems pertinent to outline that current knowledge indicates that both conditions are absent in Malta.
Numerical models of Malta’s sea level aquifer system developed by the ATIGA Consortium (1969) and the French Bureau de Recherches Geologiques Et Minieres (1990) indicate the natural discharge of freshwater at the coast to reach levels as high as 50-60% of the mean annual recharge. Invariably this natural phenomenon limits the amount of groundwater available for abstraction to around half of what is potentially available.
Investigations aimed at the identification of points of concentrated coastal groundwater discharge are mainly based on thermographic surveys. Groundwater in Malta has a stable temperature in the range of 19-20°C whilst the temperature of seawater reaches annual minima and maxima of around 15°C and 26°C respectively. It is expected that at concentrated discharge points mixing between freshwater and seawater results in modifying the local seawater temperature, which modifications can be detected by the thermographic sensors. Such thermographic investigations in Malta were carried out by the ATIGA Consortium (1969) and later by the French Institut Geographique National (1990).
The IGN study identified around 190 thermal anomalies along the coast of the Maltese islands; with most of the stronger anomalies in temperature, as expected, being located near the major fault lines such as the Victoria fault and the Magħlaq fault. The said study also entailed the actual verification of these potential discharge points, on the ground, through direct measurements of the electrical conductivity of the out flowing water at the discharge point. The conductivity readings ranged between 31,000μS/cm and 49,200μS/cm. When compared to the average conductivity of sea-water of 54,000μS/cm; these results tend to imply that the freshwater outflow is not of the significance required to substantially lower the sea-water salinity content.
These low discharge volumes was also one of the main constraints identified in the 1969 report by the ATIGA consortium which notes that “The infrared survey was initiated to obtain a complete inventory of the submarine freshwater springs around Malta and Gozo. The survey was handicapped by the small volume of water involved in each flow.”
Available knowledge indicates that the direct abstraction of out flowing groundwater at the coast is at best problematic. Results indicate that, in the Maltese islands, this discharge is diffused along numerous points/areas at the coastal region. Moreover, such coastal abstraction points would be located at the edge of the freshwater lens, where the freshwater body thins out. Any direct abstraction of large volumes in this area would inevitably lead to the immediate upconing of the underlying sea-water inducing further salinisation of the already brackish water. Experience in other coastal regions has shown that such abstractions may be feasible where karstic features are highly developed and groundwater discharge occurs through natural conduits of sizeable dimensions..
The Authority notes that other possibilities for managing this natural outflow do however exist. The starting point for the formulation of such management actions is however the acceptance of the principle that coastal discharge being directly related to the piezometric head (height of the water table above mean sea level) cannot be completely stopped, since it is a natural consequence of the existence of a ‘floating’ freshwater body with no fixed boundary.
Experience in other coastal aquifers shows that possible measures to reduce the outflow of groundwater include:
(i) artificial recharge of the fringe region of the sea-level aquifers with treated wastewater and/or locally collected runoff for the creation of a freshwater barrier that would physically obstruct the discharge of groundwater from the central regions of the island. In such case, however, treated water would be lost in lieu of fresh groundwater.
(ii) allowing the regulated operation of shallow low-yield wells in the near coastal regions of the aquifer in order to manage the piezometric gradient of freshwater in this region and thereby reduce the pressure (push) on the outward moving water.
The implementation of such measures ensures an increase in the safe-yield of the aquifer systems; the increase in abstraction capacity being sourced from groundwater which otherwise would have been naturally lost to the sea.
This is the way-forward which is currently being followed by the Malta Resources Authority which in the Water Catchment Management Plan for Malta as required by the Water Framework Directive is amongst others advocating:
(i) the enactment of new legislation introducing licensing and best-practice in groundwater development with particular reference to the metering of abstraction from groundwater sources; and
(ii) an increased and more effective enforcement of existing water protection legislation.
This in order to create the necessary regulatory framework within which a more holistic approach at fully harnessing the islands’ groundwater resources can be undertaken; which framework would also include the necessary provisions to ensure that the long-term sustainability of these resources is not impaired.
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