Environmental isotope investigation of submarine groundwater discharge in Sicily, Italy

An important part of the continental water balance involves submarine groundwater discharge (SGD). Current estimates range from less than 10% to nearly 50% of the total riverine discharge into the oceans. As a result, SGD is increasingly being recognized as an important factor in the understanding and sustainable management of coastal freshwater aquifers in many areas of the world. In addition, SGD is a significant pathway for contamination of the near-shore marine environment from land based activities. Estimation of groundwater fluxes into the marine environment is complicated due to the fact that direct measurement is not possible by conventional means. Measurement of a range of isotopic tracers at the aquifer-marine interface provides the possibility to produce integrated flux estimates of discharge not possible by other non-nuclear methods. Within the framework of a coordinated research project of the IAEA and in cooperation with UNESCO, research work was initiated to develop isotope techniques for characterization of SGD. Together with the University of Palermo, Italy, the Sicilian coast was chosen for a pilot study based on various factors like availability of background information on geological setting, clear manifestation of SGD, local logistical support, and the Mediterranean Sea, which is a particularly vulnerable marine environment to fluxes of fresh and/or contaminated groundwater. A reconnaissance sampling campaign was undertaken in 2001 and a few samples were collected and analysed for environmental isotopes. Twenty-three water samples from various sources like seawater, wells and springs were drawn in 2003 from different locations on the southwestern and southeastern parts of the Sicilian coast for isotope and chemical analyses. The samples were analyzed for 2H, 18O, 3H/3He, 87Sr, CFCs, etc. to better understand the submarine groundwater discharge process. The plot of δ18O versus δ2H of Sicilian samples is shown. Groundwater and some of the springs lie close to the Mediterranean meteoric water line with δ18O values in the range of -4.5 to -6 per mille (VSMOW). Coastal springs and wells, representing submarine groundwater discharge, have a value of about -2 to -3 per mille δ18O and fall on a mixing line between groundwater and seawater. These samples may consist of about 30 - 40% fresh groundwater, implying high submarine groundwater discharge in the coastal areas. Tritium content varies from 1.47 ± 0.24 to 4.08 ± 0.30. The residence time of groundwater in the limestone formations of Sicily, based on 3H/3He measurements and CFCs, is estimated to range from about 2 to 30 years