The particulate 7Be/ 210Pb xs and 234Th/ 210Pb xs activity ratios as tracers for tidal-to-seasonal particle dynamics in the Gironde estuary (France): Implications for the budget of particle-associated contaminants

Abstract

The short-lived natural radionuclides 7Be ( T 1/2 = 53 days), 234Th xs ( T 1/2 = 24.1 days) and 210Pb xs ( T 1/2 = 22.3 years), i.e. 234Th and 210Pb in excesses of that supported within particles by the decay of their parent isotopes, were analysed in suspended particulate matter (SPM) to study the particle dynamics in the Gironde fluvial estuarine system (France), strongly impacted by heavy metal pollution. From surveys of this land–ocean interface in 2006 and 2007, we established a times series of these radioisotopes and of their activity ratios ( 7Be/ 210Pb xs and 234Th/ 210Pb xs ARs) in particles sampled under different hydrological conditions. The particulate 7Be/ 210Pb xs AR varies along the fluvial estuarine system mainly due to variations in 7Be activities, controlled by riverine, oceanic and atmospheric inputs and by resuspension of old 7Be-deficient sediments. These processes vary with river discharge, tidal cycle and season. Therefore, seasonal particle transport processes can be described using variations of the SPM 7Be/ 210Pb xs ARs. During high river discharge, the SPM 7Be/ 210Pb x ARs decrease from river to the ocean. The turbidity maximum zone (TMZ) is dispersed and the particles, and the associated contaminants, are rapidly transported from river to coastal waters, without significant retention within the TMZ. During low river discharge, the TMZ intrudes into the fluvial estuary, and the lowest 7Be/ 210Pb x ARs are observed there due to resuspension of 7Be-deficient sediments. Away from the TMZ, from the middle to lower estuary, SPM 7Be/ 210Pb x ARs increase, indicating that the particles have been recently tagged with 7Be. We explain this trend as being caused by marine input of dissolved radionuclides, as traced by SPM 234Th/ 210Pb xs ARs, followed by scavenging in the estuary. This result indicates that particle transport models based on 7Be and trace-metal budgets must consider oceanic dissolved inputs as an additional source of 7Be and, possibly, of contaminants to estuaries.