Radium isotopes as tracers of submarine groundwater discharge in Sicily

Abstract

The results derived from radium isotopes of a submarine groundwater discharge (SGD) intercomparison on the southern coast of Sicily are reported. Radium isotope samples were collected from seepage meters, active springs, shallow wells and a hole on the beach, and Mediterranean surface waters. Samples collected within a boat basin were highly enriched in all four radium isotopes with respect to offshore samples. This implies that there are strong nearshore sources of radium. Samples collected from seepage meters in the boat basin were about a factor of 2–3 higher in radium activity than overlying waters and had higher 224Ra/ 223Ra activity ratios (AR). A beach spring (presumed to originate in the artesian freshwater aquifer) was enriched in 226Ra relative to 228Ra. The “pier wells” and beach hole (presumed to represent the salty surficial aquifer) were enriched in 228Ra compared 226Ra. The different 228Ra/ 226Ra AR clearly defines these aquifers. Samples collected in the boat basin and offshore as well as samples collected in seepage bags within the boat basin fall between the isotopic composition of these sources. These observations imply that there are at least two sources of SGD supplying radium isotopes to the coastal waters. Seepage from the shallow aquifer contributes more SGD than the offshore springs. Overall, the surficial aquifer contributes 81% of SGD and the artesian aquifer contributes 19%. The estimated SGD flux for this coastline is 10 6 m 3 km −1 d −1. This is at least a factor of 5 greater than the flux based on a 222Rn model within the boat basin. Buoyancy associated with the artesian aquifer may facilitate exchange between the ocean and the surficial aquifer.