Groundwater residence time in Malta

The Maltese Islands are composed of Oligocene and Miocene coralline limestones which contain two aquifers, separated by the more poorly-permeable Globigerina Limestone. The lower ‘sea level’ aquifer exhibits a classic Ghyben-Herzberg lens which has been modified by heavy groundwater abstraction from sea-level pumping station galleries. Malta is dependent on groundwater for both public supply and irrigation as there is limited surface water. Water levels and spring flows in the upper ‘perched’ aquifer and in some areas of the lower aquifer respond quickly to rainfall and together with some visible karstic features has lead to the assumption that this is a predominantly rapid-flow system. However, tritium studies reported in 2003 by Bakalovich and Mangion and our recent residence time studies show that the bulk of water in the saturated zone moves slowly with a travel time of decades. This is in agreement with the estimated water balance which indicates that annual replenishment is low compared to the estimated storage. Microbiological work suggests that there are limited rapid pathways through the unsaturated zone and water movement must be primarily through the matrix. Where the unsaturated zone is thick the travel time may also be decades. This new model has profound implications for aquifer management, particularly with respect to the obligations of the Water Framework Directive.