Monitoring the development of the freshwater-saltwater interface in the Malta Mean Sea Level Aquifer system

The Malta MSLA is the largest aquifer system on the island, sustained in the permeable Lower Coralline Limestone formation. Groundwater develops as a floating lens system with freshwater in direct contact with sea water laterally and vertically, leading to a high vulnerability to saline intrusion. Here, traditional quantitative monitoring is not fully representative of the groundwater body, since the water level is relatively stable due to the floating nature of the groundwater body. New monitoring techniques are required that focus on the status of the whole freshwater zone which can comprehensively define the freshwater lens. A network of 16 stations (1 station/14sq km) traversing the whole freshwater zone, interface and into the underlying saline water was therefore developed with the support of the British Geological Survey. This facilitated the EC profiling of the water column. Results indicate that the relationship between hydraulic head and lens thickness also depends on the thickness of the transition zone, which can reach tens of meters at times, hence, piezometric levels do not fully represent the freshwater thickness. The changes in the transition zone can limit the actual volume of available freshwater. To better assess its development, a second network to monitor this zone was developed, with the support of KIGAM. This network comprises 15 boreholes, in which two “interface probes” were set to float at 17 mS/cm and 35 mS/cm, enabling the monitoring of the development of the thickness of the transition zone with time. This network enables the analysis of the response of the transition zone to changes in groundwater levels resulting from seasonal variations in rainfall and nearby abstraction. These networks will enable the assessment of the development of the freshwater and transition zones enabling the formulation of a more comprehensive groundwater protection strategy in view of emerging challenges and increased pressures from climate change.