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 2 MARSOL Deliverable D10.1 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. 3 MARSOL Deliverable D10.1 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. 4 MARSOL Deliverable D10.1 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. 5 MARSOL Deliverable D10.1 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 6 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.

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