Dispersion scenarios of radioactive antimony in a macro-tidal continent-ocean transition system

Antimony (Sb) radionuclides (e.g., ¹²⁵Sb half-life of 2.76 y), are fission products of nuclear reactions released to the environment during nuclear power plant (NPP) accidental events and current operating fuel reprocessing. In coastal systems, ¹²⁵Sb shows high mobility and dispersion in the dissolved phase but its environmental biogeochemical behaviour in continent-ocean transition systems is still not fully understood [1]. Based on the widely accepted hypothesis of similar geochemical behaviour between radioactive and stable isotopes of the same element, this work quantified inherent concentrations of dissolved Sb (Sb_d, <0.2 µm mesh size) along the salinity and turbidity gradients of the Gironde Estuary (SW of France) covering contrasting hydrological conditions (i.e., intermediate freshwater discharge and drought) by direct analysis of estuarine and seawater samples with QQQ-ICP-MS (KED mode, iCAP TQ Thermo®). Dissolved Sb trends along the salinity gradient showed a non-conservative (additive) behaviour, ranging between 100-140 ng L^-1 in the freshwater endmember (i.e., matching known upstream concentrations [2]) to max. 440 ng L^-1 in mid-salinities during drought conditions, followed by decreasing values towards the marine endmember due to dilution (mixing) with seawater (i.e., ~200 ng L^-1). The specific mechanisms behind Sb desorption from the particle phase are unknown, potentially related to the interplay between biogeochemical processes and intra-estuarine residence times of water and suspended particles in macrotidal, hyperturbid estuaries, independent from the salinity gradient [3]. Daily gross Sb_d fluxes into the estuary (i.e., 10.4 kg d^-1 and 3.4 kg d^-1) and net estuarine coastal output (i.e., 27.0 kg d^-1 and 11.4 kg d^-1) for intermediate and drought conditions were calculated, respectively, following known methods [4]. Sorption experiments using isotopically labelled spikes of stable Sb exposed to water and particles from the Gironde Estuary simulating the salinity and turbidity gradients showed <2% sorption of added Sb in 24h [5], suggesting that potential liquid releases of ¹²⁵Sb from a NPP in the central Gironde Estuary may persist in the dissolved fraction. Dispersion scenarios of hypothetical ¹²⁵Sb discharges are expected to reflect water residence times, resulting in long-term intra-estuarine ¹²⁵Sb retention during draught (water residence times of 80 days) and highest concentrations of inherent Sb. In contrast, hypothetical ¹²⁵Sb releases during intermediate conditions (i.e., water residence times of 1-2 months) would result in faster exportation of ¹²⁵Sb to the coastal ocean, where enhanced dilution might probably limit the exposure levels of coastal organisms to ¹²⁵Sb but imply a wider dispersion following oceanic currents along the Atlantic coast, possibly reaching the oyster farms north of the estuary mouth. Bio-uptake of Sb radionuclides, related radiotoxicity and potential sorption onto suspended particles (e.g., after longer contact times) or plankton and the resulting reactivity/mobility need further investigation.

 

References:

[1] Periáñez R., Miró C.J. Radiol Prot, 2009, 29(2), 219.

[2] Gil-Díaz T., Schäfer J., et al. Environ Chem, 2018, 15(3), 121.

[3] van der Sloot H.A., Hoede D., et al. Estuar Coast Shelf S, 1985, 21, 633.

[4] Andreae M.O., Byrd J.T., et al. Envir Sci Tech, 1983, 17, 731.

[5] Gil-Díaz T., Schäfer J., et al. Appl Geochem, 2019, 108, 104386.