Groundwater residence time and movement in the Maltese islands – A geochemical approach

Abstract

The Maltese Islands are composed of two limestone aquifers, the Upper and Lower Coralline Limestone separated by an aquitard, the ‘Blue Clay’. The Lower Coralline Limestone is overlain in part by the poorly permeable Globigerina Limestone. The upper perched aquifers are discontinuous and have very limited saturated thickness and a short water level response time to rainfall. Frequent detections of coliforms suggest a rapid route to groundwater. However, the unsaturated zone has a considerable thickness in places and the primary porosity of the Upper Coralline Limestone is high, so there is likely to be older recharge by slow matrix flow as well as rapid recharge from fractures. Measurement of SF6 from a pumping station in a deep part of one of the perched aquifers indicated a mean saturated zone age of about 15 years. The Main Sea Level aquifers (MSL) on both Malta and Gozo have a large unsaturated thickness as water levels are close to sea level. On Malta, parts of the aquifer are capped by the perched aquifers and more extensively by the Globigerina Limestone. The limited detection of coliform bacteria suggests only some rapid recharge from the surface via fractures or karst features. Transmissivity is low and tritium and CFC/SF6 data indicate that saturated zone travel times are in the range 15-40 years. On Gozo the aquifer is similar but is more-extensively capped by impermeable Blue Clay. CFC data show the saturated zone travel time is from 25 years to possibly more than 60 years. Groundwater age is clearly related to the extent of low-permeability cover. The δ13C signature of groundwater is related to the geochemical processes which occur along the flowpath and is consistent with residence time ages in the sequence; perched aquifers<Malta MSL<Gozo MSL. The 18O and 2H enriched isotopic signature of post 1983 desalinated water can be seen in more-modern groundwater, particularly the urbanised areas of the perched and Malta MSL aquifers. In all aquifers, movement of solutes from the surface travelling slowly through the matrix provide a long term source of groundwater contaminants such as nitrate.