MARSOL Demonstrating Managed Aquifer Recharge As a Solution to Water Scarcity and Drought

MARSOL Demonstrating Managed Aquifer Recharge as a Solution to Water Scarcity and Drought

Characterisation of the sea‐level aquifer system in the Malta South Region

Deliverable No. D10.1 Version 3.2.4 Version Date 11.06.2015 Author(s) Manuel Sapiano Dissemination Level PU Status Final

The MARSOL project has received funding from the European Union's Seventh Framework Programme for Research, Technological Development and Demonstration under grant agree‐ ment no 619120. MARSOL Deliverable D10.1

CONTENTS

1. Introduction

2. Characterisation of the Malta Mean Sea‐Level Aquifer System 2.1 Geological Formations of the Malta Mean Sea‐Level Aquifer 2.1.1 Lower Coralline Limestone 2.1.2 Globigerina Limestone Formation 2.2 Hydrochemical Characteristics

3. Regional Properties of the Mean Sea‐Level Aquifer in the South‐Eastern Region of Malta 3.1 Definition of aquifer boundaries 3.2 Structural Geology 3.3 Aquifer Characterisation

4. Qualitative Characterisation of the Southern Region of the Mean Sea‐Level Aquifer 4.1 Historical Chemical Data 4.2 Water Framework Directive Monitoring Network

5. Conclusions

References

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1. INTRODUCTION

The Maltese islands consist of three inhabited islands: Malta, Gozo and Comino, and a number of uninhabited islets scattered around the shoreline of the major islands. Their location is approxima‐ tely 96 km south of Sicily and 290 km north of Tunisia. They are located at latitudes 35°48’ and 36°05’ north and longitudes 14°11’ and 14°35’ east. The total surface area of the islands is approximately 316 km2; with Malta and Gozo, the two largest islands, occupying 246 and 67 km2, respectively.

The islands lie on the eastern edge of the North African continental shelf, geologically known as the Pelagian Block. This corresponds to an oceanic area in the central Mediterranean spanning from the shores of Tunisia in the southwest, to the shores of Sicily in the north and ending in an abrupt escarpment at the edge of the Ionian Sea. Late Cretacious and Tertiary movements gave rise to a series of horst and graben structures running in a NE‐SW direction and having a predominant regional dip to the northeast.

The geology of the Maltese islands comprises a succession of Tertiary limestones and marls with scarce Quaternary deposits. Essentially, the islands are geologically made up of a core of clays and marls, the Blue Clay and Globigerina Limestone formations stacked between two permeable lime‐ stone formations known as the Upper and the Lower Coralline Limestones. The oldest formation, the Lower Coralline Limestone is of Oligocene Age whilst the Maltese succession ends in the Miocene, with the top of the Upper Coralline Limestone being chronologically dated to the Upper Messinian age possibly extending into the early Pliocene.

Figure 1: Lithologic column of the Maltese rock formations.

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From a structural point of view, the Maltese islands can be subdivided into three regions, primarily consisting of two elevated blocks separated by the two major NE‐SW fault lines present in the islands, namely the Ghajnsielem‐Qala fault in the north and the Victoria fault in the south. Between these two faults a structural graben stretching between southern Gozo, Comino and northern Malta separates the two upthrown blocks.

In a significant part of the island of Malta, south of the Victoria fault line, the Upper Coralline Lime‐ stone and the Globigerina/Lower Coralline Limestone are stacked vertically. The Lower Coralline Limestone in this region occurs mainly at sea level and is thus in lateral and vertical contact with sea‐ water. The Upper Coralline Limestone formation outcrops mainly on the western site of the island, perched over the Blue Clay formation.

Figure 2: The main Groundwater Bodies in the Maltese Islands together with the two major NE‐SW fault lines.

The downthrown region of the islands, north of the Victoria fault, is divided by a NE‐SW fault system into a succession of horst‐ and graben‐like structures. This structure with parallel compartments separated by faults leads to the formation of relatively small aquifer blocks, which are independent from one another from a hydrogeological point of view.

The lithological different natures of the two main geological formations present in the islands together with their geological position gives rise to two broad aquifer types: the upper (perched) aquifers in the Upper Coralline Limestone and the lower (mean sea level) aquifers in the lower limestone units (the Lower Coralline Limestone, and where highly fractured the Globigerina Limestone). Due to the depressed structure of the central region of the islands, the Upper Coralline Limestone also hosts small sea level aquifers in the Northern region of Malta.

The Upper and Lower Coralline Limestones are thus considered to function as the main aquifer formations in the islands. The Globigerina Limestone functions only locally as an aquifer formation, only where it is fractured and/or is located at sea level, and is commonly expected to allow ground‐ water flow through fractures and fissures. The Blue Clay formation is normally impermeable and underlies the perched aquifer.

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On the basis of historical data, it can be noted that the quality of groundwater in the Maltese islands is highly variable, with the main influences arising from sea‐water salinity and from nitrate, mainly of agricultural origin. Groundwater sustained in the sea level aquifer formations in Malta has generally high levels of chloride and other seawater related parameters, typical of a groundwater body which occurs in direct lateral and vertical contact with seawater. In such a scenario any alteration to the flow of groundwater caused by both natural and anthropogenic factors will result in the intrusion of saline waters.

Furthermore, in an island with a high population density and a rapidly developing economy, the aquifers are subject to intense pressures and impacts that have led to a gradual depletion in the qualitative status over the years. Nitrate contamination is a source of particular concern since con‐ centrations in the aquifers currently exceed accepted limiting values.

As is typical for carbonate aquifers, groundwater in Malta has a relatively high degree of hardness whereas fluoride levels are close to limiting values for drinking water in places where groundwater flow occurs through the phosphorite conglomerate beds which define the transition between the various sub‐formations of Globigerina Limestone formation.

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2. CHARACTERISATION OF THE MALTA MEAN SEA‐LEVEL AQUIFER SYSTEM

The mean sea‐level aquifers are coastal aquifers mostly occurring in the Lower Coralline Limestone (Oligocene) and in few cases in the Upper Coralline Limestone when the latter is depressed to sea‐ level.

The Lower Coralline Limestone formation represents the most important aquifer formation of the Maltese islands, sustaining the major sea‐level groundwater bodies which by far are the primary sources of freshwater for the islands. As the formation is predominantly composed of an algal‐ fossiliferous limestone with sparse corals, it has a moderate, irregular and frequently layered or channel‐like permeability. In fact, the high permeabilities of coral reefs are absent and are replaced instead by an irregular permeability more characteristic of algal reefs. This heterogeneity is further accentuated by the presence of scattered patch‐reefs in lateral contact with lagoonal and fore reef facies.

The primary porosity of the formation is highly variable and varies from 7% to 20%. The different density indicates that a large part of the primary pore‐space is not interconnected, a fact which is also stressed by the fact that the primary permeability is rather low. The effective porosity of the formation is mainly connected with fracture permeability, since otherwise the pores are very poorly interconnected. Flow and dewatering of pore‐spaces rely on secondary permeability by tectonical fracturing and solution enlargement. The fractures range from microfissures to Karst solution cavities, frequently aligned in one direction. The secondary permeability is thus mainly fissure dependent and is estimated to range from 10% to 15%, whilst the average hydraulic conductivity as measured from pumping tests is 400 x 10‐6 m/s. The transmissivity of the formation is estimated to vary between 10‐4 and 10‐3 m2/sec.

Infiltration Upper 100–200 mm/yr Coralline Limestone Rapid infiltration via karst features & Poorly permeable, fractures? Impermeable Blue Borehole fractured Globigerina Enhanced Clay Limestone recharge at clay margin? Spring Rate of downwards Lower Pumping movement in matrix Coralline station 0.5–2.8 m/yr Limestone

F Porosity = 7–20 % Natural a F u a Borehole groundwater l u t flow lt Gallery

Saturated travel time 15-40 years

Groundwater drawn under perched SW Saline upconing NE Natural direction of aquifers by abstraction groundwater flow Saline

Figure 3: Cross Section of the Malta Mean Sea‐Level Aquifer.

The largest and by far the most important of these sea‐level bodies of groundwater is the mean sea‐ level aquifer system occurring in the island of Malta. The amount of water stored in the Maltese mean sea‐level aquifers was estimated by BRGM (1991) to be of the order of 1.5 x 109 m3. This aquifer stretches across an area of 216 km2, primarily south of the Victoria fault. However, intense

MARSOL Deliverable D10.1 fracturing along the fault plane allows horizontal communication of groundwater; and thus the effective boundary of this aquifer system is considered as the ‘sealing’ Pwales fault which is located further to the north. The body of groundwater sustained by the Malta mean sea level aquifer yields an estimated 66% of the total groundwater abstracted in the country.

The second largest mean sea‐level aquifer system is found in the island of Gozo, north of the Ghajnsielem‐Qala fault, stretching over an area of 50 km2. The groundwater body sustained in this aquifer system is the major source of groundwater on the island of Gozo and an exclusive source of drinking water. Other sea‐level aquifer systems occur in the northern region of Malta, north of the Pwales fault, but being very limited in extent these aquifers are considered as having only a relatively ‘local’ importance.

The groundwater bodies occurring in these mean sea‐level aquifer systems are in lateral and vertical contact with sea‐water. Due to the density contrast of fresh‐water and salt‐water a Ghyben‐Herzberg system is developed. The outcome is a lens shaped body of freshwater that is dynamically floating on more saline water, having a convex piezometric surface and conversely a concave interface, both tapering towards the coast where there is virtually no distinct definition between the two surfaces. Maximum hydraulic heads of 4‐5 m amsl were measured for the groundwater body sustained in the Malta mean sea level aquifer in the 1940’s when the system was still largely unexploited. The lens sinks to a depth below sea‐level roughly 40 times its piezometric head at any point, fading into more saline water across a transition zone, the thickness of which depends on the hydrodynamic charac‐ teristics of the aquifer formation. The limits of this transition zone are commonly defined by the surface of the 1% and 95% seawater content, based on the total dissolved solids or chloride content.

The groundwater in the mean sea‐level aquifers is not at rest but flows away more or less horizontally. Part of this lateral flow is recovered by public and private abstractions using galleries and boreholes, while the remaining part continues its outward journey towards the coast to be discharged into the surrounding sea. On a long term‐basis, the total recharged water that is not abstracted is flowing out to the sea.

This outflow has been estimated by ATIGA (1972) to account for about 50 percent of the recharge of the sea‐level aquifers. Aquifer modelling of the main mean sea‐level aquifer by the BRGM has quanti‐ fied outflow from this aquifer at 30 million m3/year, or about 60% of the recharge.

In practice, these mean sea‐level aquifer systems are very sensitive to point‐form saline upconing due to their hydrogeological characteristics and the relatively small piezometric head. Wells drilled to some depth below sea‐level are prone to localized upconing of saline water in response to the draw‐ down in the piezometric head caused by abstraction; with the direct result being an increase in the salinity of the abstracted water.

The groundwater in the mean sea‐level aquifer is characterized by the mixing of waters with two different infiltration processes: (1) a very slow infiltration, through the matrix porosity, which is the dominant recharge process of the aquifers and (2) a fast infiltration, through cracks and fractures, which is local and discrete flow occurring probably only during the main events and which most probably is responsible for the direct leaching of pollutants from the surface into the saturated zone.

According to Stuart et al. (2010) rapid infiltration of groundwater may be one of the reasons behind rapid changes in the level of the mean sea‐level aquifers. Some boreholes can also act as conduits for

MARSOL Deliverable D10.1 the rapid infiltration of water by connecting fractures. Surface waters disappear quickly after intense rainfall on outcrops of Lower Coralline Limestone and some rapid infiltration routes such as sinkholes are visible from the surface. Nonetheless the geological complexity of Malta with its vertically divided sequence of aquifers may limit the amount of recharge to parts of the mean sea‐level aquifers, thus increasing residence times (Stuart et al. 2010).

2.1 Geological Formations of the Malta Mean Sea Level Aquifer

2.1.1 Lower Coralline Limestone Outcrops of the Lower Coralline Limestone are limited to deep valley incisions and cliff faces where 140m of formation is exposed in southern Malta and Gozo. The formation consists of thickly bedded detrital limestone that can be distinguished into four members (Pedley 1978) namely the Maghlaq member (lowest), the Attard member, the Xlendi member and the Il‐Mara member.

Maghlaq Member (Chattian) The lowest member of the Lower Coralline Limestone consists of pale‐yellow beds of foraminiferal biomicrites composed of tests of benthonic forams that constitute 40% of the rock. Typical micro‐ fossils recorded by Felix (1973) are the Miliolid foraminifera, Austrotrillima paucialveolata and Praerhapydionina delicata, Borelis haueri, and Peneroplis Thomasi. The Maghlaq member gradually changes into the Attard member and extends uniformly beneath the whole territory.

Attard Member (Chattian) The Attard member consists of pale‐grey biosparites (wackestone and packstone) associated with large Archaeolithothamnium algal rhodolits. Rhodolite colonies consisting of Archeolitothamnium intermedium Ranier and Lithotamnium make up between 15% and 80% of the rock, while bethonic foraminifera may form up to 11% of the rock. These assemblages are locally associated with bryozoa and strombid gastropods, indicating a depositional environment typical of open shoal‐conditions with depths less than 25 m.

Xlendi Member (Chattian) The Xlendi member consists of cross‐stratified coarse‐grained limestone (packstone) with abundant foraminiferal fragments, and giant foraminifera including Amphistegina, Spiroclypeus and Hetero‐ stegina. At Ghar il‐Qamh in Gozo it reaches a maximum thickness of 22 m exposing low‐dipping forsets and predominantly west‐facing cross stratification.

Mara Member (Chattian) This is the highest member of the Lower Coralline Limestone and consists of pale‐yellow massive bedded biosparites and biomicrites. The basal contact is transitional and overlies the Xlendi member. Towards the upper levels the member becomes richer in Lepidocyclina, bryozoan fragments and the echinoids Echinolampas.

2.1.2 Globigerina Limestone Formation The Globigerina Limestone is the formation that covers the largest surface in Malta and Gozo. It consists of yellow to pale grey, fine‐grained limestones composed entirely of rests of globigerinid

MARSOL Deliverable D10.1 foraminifera. There are several phosphorite horizons and hard grounds, two of which are ubiquitous and can be traced throughout the islands. These horizons, often less than 0.5m thick and consisting of dark brown to black collophanite pebbles, have enabled the subdivision of this formation into three members (House et al. 1961); the Lower, Middle and an Upper Globigerina member. These are described below. In the course of this project a survey has been undertaken to determine the spatial variations of the phosphorite conglomerate beds.

Figure 4: Geological Map of the Maltese Islands.

Lower Globigerina Limestone (Aquitanian) This member consists of yellow to pale cream indurated, globigerinid packstones becoming wacke‐ stone one metre away from the basal contact with the Blue Clay. At outcrop it exposes a charac‐ teristic honeycomb weathering and a reddish‐yellow colour. Fossils, where present, include the molluscs Chlamys and Flabellipecten, the echinoids Schizaster and Eupatagus, pteropods such as Cavolina, and thallasinoidean burrows.

At the base, the member is transitional with the Lower Coralline Limestone whilst it ends at the top in a phosphorite conglomerate layer less than 1m thick. This phosphorite bed referred to by Pedley as the lower phosphorite horizon Mc1, is planar at the top and irregular at the base. It consists of rounded phosphatic pebbles that have been cemented by chemical and concretionary action of phosphates. Phosphatised clasts of molluscs and corals are locally common together with the shark teeth of Carcharodon megalodon, Odontaspis, Isurus and Hemeprestes, and tasks of marine mam‐ mals.

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The basal boundary of the Globigerina Limestone formation is taken to be the first bed above the Scutella bed that does not contain Scutella echinoids. The Lower Globigerina is thus considered to be transitional with the Mara member.

Middle Globigerina Limestone (Aquitanian ‐ Burdigalian) The Middle Globigerina Limestone member lies unconformably over the first phosphorite horizon in distinct beds of white to pale grey finely laminated marly limestones. At the base it is more whitish in colour and finely laminated with occasional seams of brown chert nodules. Towards the top, it be‐ comes grey in colour and more marly with sparse clay lenses. Fossils commonly found in this unit are the echinoids Brissopsis and Schizaster, the bivalves Chlamys and Flabellipecten, thallassinoidean burrows, and the remains of the turtle Tryonyx and the crocodile Tomistoma.

The Middle Globigerina Limestone ends with a phosphorite horizon referred to as Mc2 consisting of 0.5 m of reworked phosphorite pebbles, molluscs casts, corals, echinoids, shark teeth and casts of the nautiloid Aturia aturi. Below this bed, thallassinoidean burrows extend up to 0.75 m and are commonly filled with reworked phosphorite pebbles.

Upper Globigerina Limestone Member (Langhian) The Upper Globigerina limestone consists of a tripartite sequence comprising a lower division of pale yellow biomicrites, a middle pale‐grey marl and an upper cream coloured limestone. It lies conformable over the Upper Phosphorite Conglomerate bed Mc2 in eastern Gozo while it lies locally over an eroded surface west of Gharb.

Schizaster euryonotus, the gastropod Epitonium, the Pteropod Maginella and thallasinoidean bur‐ rows are frequently found in the upper division.

2.2 Hydrochemical characteristics

As water flows through an aquifer it interacts with the chemistry of the geological strata and builds up a typical chemical composition. In coastal and island aquifer systems, which are in direct contact with seawater, variations in the chemical composition of the groundwater due to seawater intrusion should also be taken into consideration. In fact, in this project, in order to fully evaluate the chemical composition of the groundwater and attribute the variations of certain parameters to the sea‐water intrusion and possibly also to other forms of anthropogenic pollution, it has proved necessary to investigate the interdependence between the various chemical species, particularly those charac‐ teristic of saline conditions.

The Na and Cl content of groundwater showed an excellent correlation and their ratio can be used as an indicator of direct or indirect seawater entry. The same conclusions could also be reached with regards to Mg and Cl. Moreover, plotting the Electrical Conductivity versus the chloride content, results in the expected direct correlation between there parameters, effectively demonstrating that the intruding saline water has be far the major responsibility for the high conductivity observed in groundwater.

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Figure 5: Chloride and Sodium content in groundwater.

The correlation between SO4 and Cl is also good. However, when analysing this correlation in more detail, a number of deviations have been identified with groundwater from certain sources exhibiting an anomalously high SO4/Cl ratio. These results suggest that the sulphate contained in groundwater is not derived exclusively from the intruding sea‐water, but in certain places (particularly regions of the aquifer with a relatively low depth to groundwater) the influence exerted by other anthropogenic sources such as fertilisers used in agricultural activities could be significant. Moreover, most of the monitoring stations in which a high SO4/Cl ratio was identified, also registered high values for other chemical elements or parameters.

Figure 6: Chloride and Sulphate content in groundwater

The typology of the groundwater in the mean sea level aquifer systems was further analysed through ternary (Piper) plots where, as expected, samples from these aquifer systems exhibited a general shift towards the apex characteristic of NaCl waters. Thus the Piper plot indicates that as a general rule, groundwater in the mean sea level aquifer system is shifting from the fresh recharging carbo‐ nate waters position towards the saline water position primarily as a result of the mixing process with intruding seawater in the aquifer system.

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Figure 7: Piper Plot for the quality parameters of groundwater samples.

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3. REGIONAL PROPOSERTIES OF THE MEAN SEA LEVEL AQUFIER IN THE SOUTH‐EASTERN REGION OF MALTA

3.1 Definition of regional aquifer boundaries

The Lower Coralline Limestone is the most extensive and the most important aquifer formation of the Maltese islands and hosts the mean sea level water table. In the island of Malta its maximum thickness above sea level is about 120m at Ghar Bittija, Dingli. It only dips below sea‐level in for limited extent in the Valletta basin where it is replaced by the Lower Globigerina Limestone and in the Marsasxlokk basin where at sea level one finds the Lower and Middle Globigerina Limestone.

Figure 8: Structural Map of the top of the Lower Coralline Limestone formation.

The Malta south region of the sea‐level aquifer system therefore lies between these two depressions in the Lower Coralline Limestone and can thus be considered as a central SW to NE oriented ‘corridor’ of Lower Coralline Limestone bounded by the less permeable Globigerina Limestone For‐ mation to the north and to the south, and the coastal region to the east. The Globigerina Limestone, where present at sea‐level therefore, acts as a protective barrier to the more permeable Lower Coralline Limestone aquifer formation, dampening the potential impact of saline intrusion from a northwards and southwards direction.

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Figure 9: Boundaries of the Malta south mean sea‐level aquifer system.

3.2 Structural Geology

The Malta south region is also dissected by a number of NE‐SW striking normal faults mostly evident along the coastline. Inland these faults are mostly difficult to trace. Minor NW‐SE tending faults be‐ longing to the second class of faults present on the Maltese islands lie along the northeast coastline of Eastern Malta from Zonqor point to il‐Kalaka tal‐Partrijiet.

Although most of the faults transect Eastern Malta from SW to NE the throw is minor, only a few meters and the faults are relatively of a meso‐scale nature. The most prominent fault in the area is the Marsascala Fault which has a throw of the order of 15 m.

Reverse faults are not known within the Maltese islands while wrench faults have been noted, within Eastern Malta these are only of minor displacement.

The structural geology predominantly comprises normal faults and regional dip, both of which have a strong bearing on the geomorphology, and drainage patterns of the valley systems that dissect Eastern Malta. Two generation of faults are recognised: a Mio‐Pliocene fault system oriented NE‐SW and a younger fault generation of a Plio‐Quaternary age oriented NW‐SE. Within Eastern Malta the latter fault system is not well represented and the few faults noted are of a very local nature. The second structural element is the regional dip of the Malta Horst. Within Eastern Malta this is relatively low, and generally does not exceed 6°. The two fault systems are described in the below paragraphs.

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Figure 10: Fault Map of the Maltese islands.

Northeast‐Southwest trending normal faults:

Although this group of faults is well represented, the throw of these faults is generally only of the order of a few meters. These faults are old; some bear evidence of synsedimentary activity but presently are no longer active. As a result the original fault scraps have receded and the only topo‐ graphic expression is usually a slight change in the slope of the terrain. For this reason evidence of these faults comes mainly from the cliff sections along the coastline and deep gorges cut in the Lower Coralline Limestone. In addition the depth to formation tops in water boreholes has been used to extrapolate the faults inland.

Probably one of the most notable faults is the il‐Maghluq fault that extends to the southwest beyond Zejtun and has a throw of about 7 m. Il‐Maghluq is a buried river channel now filled with Quaternary deposits and terra rossa. Its present watershed is too small to account for the size and depth of the channel.

The Migra Ilma fault is notable for its particular throw (over 30 m) which can be traced far inland as far as Tal‐Handaq and most probably extends to the Harbour Area.

Northwest‐Southeast trending faults:

This fault system is recent and fault scarps are reserved. The few faults that belong to this family are mostly seen close to the coastline and do not extend far inland.

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3.3 Aquifer Characterisation

An initial characterisation of the Malta south regional aquifer system can be undertaken through the use of geological data from the public groundwater abstraction wells driven in this aquifer system during the 1970s with the aim of augmenting the public water supply. A review of drilling records at the Water Services Corporation (Malta’s public water utility and also a partner within the MARSOL project) revealed the presence of 18 public groundwater sources in this regional aquifer. These groundwater abstraction sources are currently not in use, due to the deterioration in the quality of groundwater in the region. The list of identified groundwater abstraction sources is presented in Table 1 below.

Table 1: Public groundwater abstraction stations in the Malta South Mean Sea‐Level regional aquifer. Borehole Name ID No. x‐cordinate y‐coordinate San Anard 10296 460087 3970589 San Nikola 10298 460509 3970230 Zonqor 10293 460081 3969953 Salib (Sajjied) 10417 459405 3970169 Burzett 10412 459071 3970661 Soru 10415 459211 3971026 Hofra 1 10413 458985 3970877 Hofra 2 10414 458969 3970187 San Pietro 10299 458452 3971740 Zabbar 4 10040 458092 3971305 Bieb is‐Sultan 10372 457580 3970935 San Klement 10370 457471 3970712 Madrin 10368 457800 3970500 Hompesh Gate 10390 457379 3970091 Zabbar 8 10045 457840 3970019 Sant’Antnin 10279 459469 3696863 Hal Tmiem 10353 458998 3968313 Ramla Road 10301 460570 3968615

The drilling logs of these groundwater abstraction stations indicate the presence of the Lower Coral‐ line Limestone at mean sea level, all being drilled in the central region of the aquifer system.

This geological information therefore confirms the earlier hypothesis considering the Malta south regional aquifer as a central section of the Lower Coralline Limestone formation flanked to the north and south by the less permeable Globigerina Limestone formation.

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Table 2: Geological Characteristics.

Borehole Name Outcropping formation Depth to top of the Lower Coralline Limestone (m)

San Anard Upper Corraline Limestone Formation 45 meters (Gebel Imbark Member) San Nikola Middle Globigerina Limestone Formation 40 meters Zonqor Lowerl Coralline Limestone Formation At surface

(Mara Member) Salib (Sajjied) Lower Globigerina Limestone Formation 8 meters Burzett Lower Globigerina Limestone Formation 30 meters Soru Upper Corraline Limestone Formation 47.5 meters

(Gebel Imbark Member) Hofra 1 Middle Globigerina Limestone Formation 35 meters Hofra 2 Middle Globigerina Limestone Formation 35 meters San Pietro Lower Globigerina Limestone Formation 27.5 meters

Zabbar 4 Lowerl Coralline Limestone Formation At surface (Mara Member) Bieb is‐Sultan Lower Globigerina Limestone Formation 28 meters

San Klement Lower Globigerina Limestone Formation 19 meters

Madrin Lower Globigerina Limestone Formation 15 meters Hompesh Gate Lowerl Coralline Limestone Formation At surface (Mara Member)

Zabbar 8 Lower Globigerina Limestone Formation 32.5 meters

Sant’Antnin Lowerl Coralline Limestone Formation At surface (Mara Member) Hal Tmiem Lower Globigerina Limestone Formation 15 meters

Ramla Road Lowerl Coralline Limestone Formation At surface

(Mara Member)

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4. QUALITATIVE CHARACTERISATION OF THE SOUTHERN REGION OF THE MEAN SEA LEVEL AQUIFER

4.1 Historical Chemical Data

The Malta South region of the Mean Sea Level aquifer system includes 18 groundwater abstraction stations which have to various degrees been used for the abstraction of groundwater intended for public consumption and/or irrigation water. The first qualitative information from these boreholes date back to 1976, since most of these stations have been drilled in the 1970s with the aim of increa‐ sing the national water supply. Due to a general deterioration in quality (mainly salinity) abstraction from these groundwater abstraction stations for public purposes was practically discontinued by the mid‐1980s. The groundwater abstraction stations nearest to the MARSOL Managed Aquifer Recharge (MAR) demonstration site are: Zonqor BH, San Leonardo BH, San Nikola BH, Salib (Sajjied) BH, and Bursett BH.

The historic qualitative results from these abstraction stations have been collated and are represen‐ ted in Table 3 below. These results indicate the high saline content of this regional aquifer system even at the initial borehole operational stage – and therefore indicative of the high (background) saline content in this regional aquifer system. This could possibly be a result of the high prevalence of secondary permeability (fracture flow) in this aquifer system, compounded with the highly distribu‐ ted, but intense abstraction of groundwater for secondary purposes such as agricultural irrigation.

This indication is further outlined by the direct response in salinity in these public groundwater abstraction stations in response to groundwater abstraction when the boreholes were in operation.

Figure 12: Distribution of private groundwater abstraction stations in the Malta South Regional Aquifer system.

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Table 3: Qualitative Characteristics – Historical Data.

DATE__ SITE NAME E_Cond T_HARD Ca Mg Na K Cl NO3 SO4

uS/cm mg/l mg/l mg/l mg/l mg/l mg/l mg/l 09/02/1981 Bieb is-Sultan 1800 460 221.1 08/10/1981 Bieb is-Sultan 1600 500 160 24.0 275 9.0 400 223.7 65.8 14/05/1984 Bieb is-Sultan 2300 540 148 41.3 420 218.3 12/07/1984 Bieb is-Sultan 2300 470 144 27.0 410 225.5 29/07/1985 Burset 3400 600 148 55.9 880 145.7 05/08/1985 Burset 3100 710 143.9 06/08/1985 Burset 3200 680 141.3 07/08/1985 Burset 3000 720 137.8 08/08/1985 Burset 3000 720 140.4 09/08/1985 Burset 3000 700 138.7 11/08/1985 Burset 3000 700 167.0 19/08/1985 Burset 3100 720 142.2 28/08/1985 Burset 3100 760 135.1 03/09/1985 Burset 3100 800 165.2 04/09/1985 Burset 3100 780 11/09/1985 Burset 3100 700 135.1 21/11/1985 Burset 5800 820 168 97.2 1620 126.3 280.0 01/10/1986 Burset 4200 620 144 63.2 1080 123.6 139.6 12/06/1987 Burset 5200 1280 124.9 11/02/1988 Burset 5300 700 144 82.6 1340 126.3 185.0 07/04/1988 Burset 5700 26/03/1990 Burset 9200 1040 184 141.0 2580 116.5 12/06/1985 Hofra I 69.6 08/07/1985 Hofra I 1800 250 68 19.4 330 74.9 04/09/1985 Hofra I 1750 300 76 26.7 360 73.1 04/09/1986 Hofra I 30000 2950 660 316.0 8750 49.2 1120.0 12/06/1987 Hofra I 26400 8200 43.0 05/09/1985 Hofra II 860 260 130 86.4 20/01/1987 Hofra II 7100 980 224 102.0 2060 82.8 224.0 17/02/1988 Hofra II 8000 1050 200 133.7 2550 04/04/1988 Hofra II 6700 680 152 72.9 1840 62.5 237.0 07/04/1988 Hofra II 8100 26/03/1990 Hofra II 11500 1380 200 214.0 3300 81.1 06/10/1981 Hompesch Gate 1420 540 184 19.4 230 2.0 340 200.6 07/10/1981 Hompesch Gate 1500 510 184 12.2 370 190.0 56.8 08/10/1981 Hompesch Gate 1400 480 176 9.7 195 1.4 330 186.5 59.3 09/10/1981 Hompesch Gate 1300 520 184 14.6 185 1.4 320 186.5 65.8 05/02/1980 Madrin 2200 530 140 43.7 438 24.0 680 194.0 107.8 15/02/1980 Madrin 1420 370 196.7 08/10/1981 Madrin 1600 480 136 34.0 320 20.0 410 191.9 65.8 29/06/1982 Madrin 1600 460 410 213.9 14/05/1984 Madrin 2400 480 128 38.9 480 203.3 12/06/1985 Salib (Sajjied) 5500 1360 85.8 18/10/1985 Salib (Sajjied) 1270 370 128 12.2 190 80.2 09/02/1988 Salib (Sajjied) 28/03/1988 Salib (Sajjied) 5700 820 184 87.5 1520 71.3 220.5 07/04/1988 Salib (Sajjied) 4900

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11/04/1988 Salib (Sajjied) 21/11/1988 Salib (Sajjied) 6000 860 208 82.6 1660 90.8 227.0 09/01/1989 Salib (Sajjied) 5600 800 208 68.0 1520 91.7 227.0 22/02/1980 San Klement 3900 720 128 97.0 1360 145.0 03/03/1980 San Klement 2400 780 140.0 30/07/1980 San Klement 2250 480 116 46.0 720 150.6 11/08/1980 San Klement 142.6 12/08/1980 San Klement 3600 1360 137.3 13/08/1980 San Klement 3400 1220 133.8 22/08/1980 San Klement 3600 1280 128.4 09/09/1980 San Klement 3900 1340 130.2 18/12/1980 San Klement 3900 1340 145.7 11/02/1981 San Klement 5200 1900 112.9 17/02/1982 San Klement 5500 2050 115.6 07/04/1982 San Klement 5500 1750 14/05/1982 San Klement 5000 2000 28/06/1982 San Klement 5200 2200 21/07/1982 San Klement 4600 2100 13/08/1982 San Klement 2300 27/10/1982 San Klement 2260 18/11/1982 San Klement 2220 20/01/1983 San Klement 5800 2280 17/02/1984 San Klement 8200 1040 2120 115.6 07/11/1984 San Klement 8050 1000 2260 113.8 12/06/1985 San Klement 112.1 22/05/1986 San Klement 8200 1050 220 121.5 2450 113.0 335.0 15/02/1988 San Klement 7700 1020 160 150.0 2080 120.1 300.0 21/11/1988 San Klement 8700 1060 176 150.0 2460 113.9 560.0 26/03/1990 San Klement 8300 800 144 107.0 2440 106.8 07/10/1985 Soru 1200 340 104 19.4 220 71.3 50.0 04/09/1986 Soru 16000 740 184 68.0 5050 67.8 700.0 10/10/1977 Srina 2400 620 64.7 03/02/1976 Zabbar Road 2100 430 136 21.9 820 67.0 580 144.4 101.2 03/02/1976 Zabbar Road 1650 410 179.9 02/10/1979 Zabbar Road 1850 540 177.0 04/10/1979 Zabbar Road 2100 620 154.2 05/10/1979 Zabbar Road 2100 540 700 138.2 08/10/1979 Zabbar Road 2250 680 153.2 11/10/1979 Zabbar Road 2600 810 114.3 12/10/1979 Zabbar Road 2100 630 154.2 15/10/1979 Zabbar Road 2000 610 153.3 17/10/1979 Zabbar Road 2100 610 155.9 23/10/1979 Zabbar Road 2000 590 160.4 27/11/1979 Zabbar Road 2000 620 149.7 19/10/1981 Zabbar Road 2800 600 168 43.7 630 20.0 1020 138.7 128.4 17/02/1982 Zabbar Road 3300 1070 136.9 14/05/1982 Zabbar Road 3500 1280 28/06/1982 Zabbar Road 3500 1280 21/07/1982 Zabbar Road 3500 1360 13/08/1982 Zabbar Road 1600 27/10/1982 Zabbar Road 2080 18/11/1982 Zabbar Road 1800 17/02/1984 Zabbar Road 3200 600 800 142.2

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18/05/1984 Zabbar Road 7600 850 136 123.0 2250 50.1 12/06/1985 Zabbar Road 28.8 22/05/1986 Zabbar Road 11200 1400 280 170.0 3500 118.3 400.0 12/06/1987 Zabbar Road 7500 1980 122.7 09/02/1988 Zabbar Road 11/02/1988 Zabbar Road 5000 760 184 72.9 1340 137.8 160.0 07/04/1988 Zabbar Road 5000 18/10/1988 Zabbar Road 18/10/1988 Zabbar Road 02/03/1989 Zabbar Road 4000 680 180 55.9 1120 118.3 160.0 07/03/1989 Zabbar Road 26/03/1990 Zabbar Road 4800 760 184 73.0 1300 131.6 23/02/1978 Zejtun 780 350 116 14.6 80 2.3 140 55.8 26.3 31/01/1984 Zonqor 2700 500 180 12.2 540 59.8 03/02/1984 Zonqor 3700 580 224 4.9 780 57.1 270.0

Based on these historical results a chemical correlation was undertaken to assess the characteristics of groundwater in the region. The plot of electrical conductivity vs. chloride shows an excellent correlation between these two parameters, showing the important influence of sea‐water intrusion on the total salinity of the aquifer system. Furthermore, the plot of chloride vs. sulphate shows an excellent correlation, sulphate being the third major constituent of sea‐water.

Figure 13: Correlation plot for electrical conductivity and chloride content in monitoring stations located in the southern region of the Mean Sea‐Level Aquifer Region.

Figure 14: Correlation plot for chloride and sulphate content in monitoring stations located in the southern region of the Mean Sea‐Level Aquifer Region.

MARSOL Deliverable D10.1

A further correlation analysis was undertaken to assess the relationship between the chloride (sea‐ water intrusion derived) and nitrate (anthropogenically derived) content in groundwater from this regional aquifer system. The analysis shows that nitrate decreases with increasing saline content in the groundwater body. This relationship is indicative of mixing of groundwater with non‐nitrate containing intruding sea‐water.

Figure 15: Correlation plot for chloride and nitrate content in monitoring stations located in the southern region of the Mean Sea‐Level Aquifer Region

Finally, a temporal assessment of salinity content in response to groundwater abstraction in the pub‐ lic groundwater abstraction wells was undertaken. Under pumping conditions, significant increases in salinity were registered in these stations. In fact, abstraction for municipal purposes from all public groundwater stations in the regional aquifer was discontinued in the mid‐1990s due to high salinity levels.

Figure 16: Temporal assessment of salinity content in San Klement Borehole under pumping conditions.

Figure 17: Temporal assessment of salinity in Zabbar Road Borehole under pumping conditions.

MARSOL Deliverable D10.1

4.2 Water Framework Directive Monitoring Network As part of the implementation process of the Water Framework Directive an independent monitoring network has been established in all the groundwater bodies in the Maltese islands. The groundwater monitoring station in the Malta mean sea level aquifer system most representative of the southern regional aquifer system is the Zejtun Station.

Figure 18: WFD Monitoring Network in the Malta Mean Sea Level Aquifer System.

Groundwater qualitative results from this groundwater monitoring station were analysed in order to assess the relative quality levels and identify the occurrence of any trend in these qualitative results. The results show a significant variation in the range of results obtained, possibly indicative of regional groundwater abstraction activities undertaken in the region.

Table 4: Water Quality results for Zejtun Station from the WFD Monitoring Network (2009‐2014).

Parameter Unit Lowest Highest

Electrical Conductivity uS/cm 6200 7800

Nitrate mg/l 113 152

Chloride mg/l 1635 2433

Sulphate mg/l 200 338

Sodium mg/l 890 1050

Boron mg/l 0.45 0.575

Fluoride mg/l 0.25 0.71

Arsenic ug/l ND ND

Total Pesticides ug/l ND ND

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It is noted that although the indications are that the quality of the groundwater is deteriorating due the impact of human impacts such as water use (groundwater abstraction) and urban/agricultural activities (nitrates), a statistically significant trend at the 95% level of confidence could not be estab‐ lished. This situation calls for further long‐term studies to enable a comprehensive trend and status assessment, which will be undertaken during the WFDs .

Figure 19: Salinity (Electrical Conductivity) content in Zejtun Monitorng Station

Figure 21: Nitrate content in Zejtun Monitoring Station

Figure 15: Sulphate content in Zejtun Monitoring Station

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5. CONCLUSION

The overall objective of the Managed Aquifer Recharge (MAR) activities at the Malta South demon‐ stration site is the development of a sea‐water intrusion barrier within the Malta South regional sea‐ level aquifer system. Groundwater abstracted from the southern region of the Malta Mean Sea Level aquifer system exhibits characteristically high chloride contents. This deterioration in quality has resulted from the intrusion of saline waters in response to the historically high groundwater abstract‐ tion rates registered in the area, particularly from the dense and widely distributed private abstraction for agricultural purposes. This situation has resulted in groundwater abstraction for public purposes form this region being discontinued since the early 1990s.

Deliverable D10.1 under the MARSOL project has presented a characterisation of the regional sea‐ level aquifer system, undertaken through the development of a regional hydro‐geological assess‐ ment. Through this assessment the poor qualitative conditions of the aquifer system were confir‐ med, and baseline groundwater qualitative characteristics were identified prior to the establishment of the sea‐water intrusion barrier, thus permitting the identification of changes in groundwater status as a result of the MARSOL pilot project.

The regional hydro‐geological assessment also identified the aquifer system under consideration as being essentially composed of a central corridor of permeable Lower Coralline Limestone, flanked on the Northern and Southern sides by the less permeable Globigerina Limestone formation – which can thus be considered as acting as a barrier to sea‐water intrusion from these directions. This assessment thus indicates the MARSOL pilot MAR site to be ideally placed, on the eastern (coastal) region of the aquifer system for the development of a further hydraulic barrier aimed at offering increased protection to this aquifer system.

Through this pilot initiative, therefore an attempt will be made to raise the regional piezometric levels on the eastern coastal boundary of the aquifer system, to develop a sea‐water intrusion barrier with the aim of limiting the incidence of both lateral and vertical sea‐water intrusion, thereby resulting in an improvement in groundwater quality in the regional aquifer system.

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FAO (2006), Malta Water Resources Review. Rome.

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Pedley, H.M., House, M.R. and Waugh, B. (1976), The Geology of Malta and Gozo. Proceedings of the Geological Association 87, 325‐41.

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Stuart, M.E., Maurice, L., Heaton, T.H.E., Sapiano, M., Micallef Sultana, M., Gooddy, D.C. & Chilton, P.J. 2010. Groundwater residence time and movement in the Maltese islands – A geochemical approach. ‐ Applied Geochemistry, 25, 609‐620.