Spatial-temporal dynamics of salinity profiles measured in the freshwater lens system of the Malta Mean Sea Level Aquifer (MSLA)

The Mean Sea Level Aquifer (MSLA) of the island of Malta is a freshwater lens system sustained in a carbonate formation, floating on seawater in the bedrock. Given the specific hydrogeological and climatic conditions, the water table today reaches its maximum elevation at around 3 m amsl at the centre of the 316 Km² island, with a maximum thickness of freshwater lens being about 90 m. Seawater intrusion occurs as an unavoidable effect of groundwater abstraction and the situation is further exacerbated during the dry summer period when water demands are higher.

Groundwater plays a major role in meeting the water demand of the Maltese islands and in this regard, is a strategic resource which needs to be preserved in terms of quality and quantity. It is therefore critically important to have an accurate understanding of the volume of fresh groundwater stored in the aquifer and how it is changing in response to changes in recharge, withdrawal and climatic regimes, to support an effective management which ensures the sustainability of this resource.

The status of fresh groundwater in the MSLA is assessed through vertical profiles of salinity along the water column of Deep Monitor Boreholes (DMBs) which penetrate partly or entirely through the brackish-water transition zone that separates freshwater from the underlying higher density seawater in freshwater lens systems. Salinity profiles were measured using a multiparametric probe (SEBA HYDROMETRIE KLL-Q-2 with MPS-D8 probe) lowered from the water table till the bottom of the DMBs measuring electrical conductivity (as a proxy for salinity), temperature, pressure and pH in three DMBs on a weekly basis over one year during the wet seasons.

The monitoring of salinity profiles over time in these DMBs allowed the detection of typical patterns of fresh/sea-water interface fluctuations according to the occurrence of external driving forces like precipitation and/or local abstraction.  The profiles were correlated with aquifer characteristics such as, fractures and orientation of strata in the DMBs which were determined through high resolution images captured with an optical televiewer probe (MOUNT SOPRIS QL40-OBI-2G).

The results show that the thickness of the transition zone varies in the DMBs according to the succession of dry and wet periods with maximum fluctuations of about 8 m. Furthermore, the interface depth results about 32 times the freshwater head inferring a deviation from the standard Ghyben-Herzberg coefficient of 40 for sharp interfaces. By analysing local geological conditions and time-series of total rainfall, groundwater abstraction, piezometric levels and salinity profiles, we identified occurrence mechanisms of three typical transition zones: (i) sharp interface, (ii) diffused interface, and (iii) step-like changes of salinity with depth. These types of interfaces, which are rather recurrent in space and time, lead us to gain a clearer understanding of the seawater intrusion dynamics triggered by variable abstraction conditions and drought periods.

The outcomes of this study illustrate the value of DMBs in establishing an effective monitoring framework for island groundwater bodies status, since the development of the transition zone is an important factor for managing freshwater abstraction from near-coastal and island aquifer systems.