Spatio-temporal distribution and behaviour of antimony in the Gironde Estuary: a preliminary approach to regional nuclear accident scenarios

Supervisor: Jörg Schäfer (Univ. Bordeaux)
Antimony (Sb) is a highly toxic trace element for which environmental biogeochemical cycles are still relatively poorly known. In addition, Sb is a fission product in nuclear power plants (NPPs), with non-negligible radioactive emissions. The fate of radioactive elements can be extrapolated from their natural stable isotopes due to their analogous biogeochemical behaviour. In the present work, several approaches describe the dynamics of total Sb in aquatic and biological compartments of the Gironde Estuary and fluvial system. Specifically, this work addresses: (i) dissolved (Sbd) and particulate (Sbp) Sb concentrations and fluxes entering the Gironde Estuary via the Garonne River (from 2004 to 2014), (ii) Sbd and Sbp distribution along the salinity and turbidity gradients of the Gironde Estuary (during intermediate discharge in March 2014 and during high discharge in March 2015), (iii) Sb adsorption onto particulate phases experimentally assessed using stable isotope spikes, (iv) historic Sb concentrations in wild oysters Crassostrea gigas from the estuary mouth (from 1981 to 2010) and (v) recent bioaccumulation factors of Sb in wild oysters from both the Gironde Estuary (2014) and the Arcachon Bay (2015).

Results clearly suggest that Sbd and Sbp fluvial fluxes into the Gironde Estuary are dominated by seasonal discharge, whereas estuarine distribution and transformations depend on water and particle residence times, salinity and turbidity gradients. These factors explain the observed non-conservative, additive behaviour in the estuary, more evident in intermediate water discharges. The kinetic adsorption experiments support the field observations, suggesting a relatively slow exchange between dissolved and particulate forms. Over the past 34 years, Sb discharges into the Atlantic Ocean appear relatively stable, with punctual increases probably due to erosion events in the watershed or anthropogenic releases. These Sb inputs are registered in wild oyster tissues, especially in their gills, yet with relatively low bioaccumulation factors (BAF) compared to other metals. Oysters from the Gironde Estuary show lower condition indices (CI) than those from the Arcachon Bay, probably due to the influence of other metals in the estuary (e.g., Ag and Cd).

Based on these findings, we propose a first qualitative approximation (scenario) to a possible accidental release and dispersion of Sb radionuclides from the Blayais (in the estuary reaches) and Golfech (upstream the Garonne River) NPPs. Combining Sb partitioning and sorption behaviour together with known hydrodynamic and sedimentary features of the Gironde Estuary, we suggest that (i) high river discharge conditions favour Sb radiotoxicity on the coast or Bordeaux in particulate form and (ii) under low discharge conditions dissolved Sb species predominate, with supposed increased bioavailability to organisms in the estuary due to direct exposure.

Keywords: antimony, radioactivity, Gironde Estuary, salinity gradient, distribution coefficient, wild oysters