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

The short-lived natural radionuclides (7)Be (T(1/2)=53 days), (234)Th(xs) (T(1/2)=24.1 days) and (210)Pb(xs) (T(1/2)=22.3 years), i.e. (234)Th and (210)Pb 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 ((7)Be/(210)Pb(xs) and (234)Th/(210)Pb(xs) ARs) in particles sampled under different hydrological conditions. The particulate (7)Be/(210)Pb(xs) AR varies along the fluvial estuarine system mainly due to variations in (7)Be activities, controlled by riverine, oceanic and atmospheric inputs and by resuspension of old (7)Be-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 (7)Be/(210)Pb(xs) ARs. During high river discharge, the SPM (7)Be/(210)Pb(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 (7)Be/(210)Pb(x) ARs are observed there due to resuspension of (7)Be-deficient sediments. Away from the TMZ, from the middle to lower estuary, SPM (7)Be/(210)Pb(x) ARs increase, indicating that the particles have been recently tagged with (7)Be. We explain this trend as being caused by marine input of dissolved radionuclides, as traced by SPM (234)Th/(210)Pb(xs) ARs, followed by scavenging in the estuary. This result indicates that particle transport models based on (7)Be and trace-metal budgets must consider oceanic dissolved inputs as an additional source of (7)Be and, possibly, of contaminants to estuaries.

Spatiotemporal variation of dissolved (238)U in the Gironde fluvial-estuarine system (France).

Dissolved uranium ((238)U(D)) was measured within the Gironde fluvial-estuarine system (France), which is well known for its heavy metal pollution and its subsequent environmental effects. Dissolved (238)U activities exhibit the highest levels and variability in the small watersheds of the Isle and Riou-Mort rivers, ascribed to low discharges and the occurrence of peculiar features (local U ore deposits; Permian layers). The major tributaries of the Gironde Estuary: Garonne, Lot and Dordogne, show rather stable (238)U(D), higher than the global mean riverine uranium activity. In the Gironde Estuary (238)U(D) behaves conservatively along the river-estuary-ocean transect. Dissolved (238)U distribution in the Gironde fluvial-estuarine system can be explained by the characteristics of considered drainage basins. There is no evidence of anthropogenic pollution.

Potential impact of former Zn ore extraction activities on dissolved uranium distribution in the Riou-Mort watershed (France).

The industrial basin of Decazeville (Riou-Mort watershed, South-West France) is well known for its heavy metal pollution and its subsequent environmental effects on the Lot-Garonne River system. The source of this pollution is the Riou-Mort River, which drains smelting waste areas. A first survey after remediation works has revealed elevated dissolved uranium (U(D)) concentrations in the outlet of the Riou-Mort River. The objective of this research is to identify the origin of U(D) in the Riou-Mort watershed and to evaluate the impact of industrial activities on this element. Uranium was measured at 10 water sampling sites, located upstream and downstream the industrial basin, and in three smelting waste deposits. Uranium concentrations in the smelting waste deposits reach up to 14.4 mg kg(-1) and (234U/238U) activity ratios (AR) are near unity. Dissolved U concentrations in the Riou-Mort River and its main tributaries ranges over two orders of magnitude from 0.02 to 6.1 microg L(-1). The highest levels were measured in a site with no anthropogenic pollution, upstream from the industrial area. This observation suggests that U(D) is mainly linked to weathering; the elevated U concentrations originate from the naturally occurring radioactive materials (NORM) located in the Permian bedrock and no significant U pollution exists, at present, in the Riou-Mort watershed. This work demonstrates that spatial monitoring coupled with a long time-series are an essential prerequisite in assessment of spatiotemporal variations of U(D), prior to a diagnostic of pollution in a small watershed.