Balance and residence times of 210Pb and 210Po in surface waters of the northwestern Mediterranean Sea

Concentrations of the naturally occurring radionuclides Pb-210 and Po-210 were determined in both the dissolved (<0.2 mum) and particulate (>0.2mum) fractions in surface waters of the northwestern Mediterranean Sea. About 4-8% of the Pb-210 activity was observed to be associated to particles, which were determined as being mostly biogenic. The Pb-210 content in the particulate fraction was generally greater than that of Pb-210, accounting for about 20% of the total activity. Total activities of Pb-210 and Po-210 were found to be in equilibrium. Ra-226 concentrations were also determined in an effort to investigate its occurrence in Mediterranean waters. Ra-226 was in excess to both Pb-210 and Po-210 with a mean concentration of 1.72+/-0.03 Bq m(-3). Steady-state balance equations of the three radionuclides permit calculation of residence times for Pb-210 and Po-210 in surface waters (0.8 and 3.0 yr, respectively). Po-210 residence times were almost one order of magnitude higher than mostly reported values. Polonium is known to manifest higher affinity and different binding mechanisms (by entering the organic cycle) to biogenic material than Pb-210. Several processes, such as Po-210 uptake by buoyant particles and recycling of organic matter in the surface layer, could lead to such long residence times of Po-210 and, therefore, of organic particles in surface waters. Larger Po-210 atmospheric fluxes than those here considered and inputs from rivers and/or fine-grained sediment resuspension could also help to explain the observation, although presumably to a lesser extent. (C) 2002 Elsevier Science Ltd. All rights reserved.