Improving estuarine net flux estimates for dissolved cadmium export at the annual timescale: Application to the Gironde Estuary

Dissolved Cd (Cd-D) concentrations along the salinity gradient were measured in surface water of the Gironde Estuary during 15 cruises (2001-2007), covering a wide range of contrasting situations in terms of hydrology, turbidity and season. During all situations dissolved Cd concentrations displayed maximum values in the mid-salinity range, reflecting Cd addition by chloride-induced desorption and complexation. The daily net Cd-D fluxes from the Gironde Estuary to the coastal ocean were estimated using Boyle's method. Extrapolating Cd-D concentrations in the high salinity range to the freshwater end member using a theoretical dilution line produced 15 theoretical Cd concentrations (CdB), each representative of one distinct situation. The obtained CdB concentrations were relatively similar (similar to 201 +/- 28 ng L-1) when freshwater discharge Q was >500 m(3) s(-1) (508 <= Q <= 2600 m(3) s(-1)), but were highly variable (340 +/- 80 ng L-1; 247-490 ng L-1) for low discharge situations (169 <= Q <= 368 m(3) s(-1)). The respective daily Cd-D net fluxes were 5-39 kg day(-1), mainly depending on freshwater discharge. As this observation invalidates the existing method of estimating annual Cd-D net fluxes, we proposed an empirical model, using representative CdB values and daily freshwater discharges for the 2001-2007 period. Subsequent integration produced reliable Cd-D net flux estimates for the Gironde Estuary at the annual timescale that ranged between 3.8-5.0 t a(-1) in 2005 and 6.0-7.2 t a(-1) in 2004, depending on freshwater discharge. Comparing Cd-D net fluxes with the incoming CdD fluxes suggested that the annual net Cd-D addition in the Gironde Estuary ranged from 3.5 to 6.7 t a(-1), without any clear temporal trend during the past seven years. The annual Cd-D net fluxes did not show a clearly decreasing trend in spite of an overall decrease by a factor similar to 6 in Cd gross fluxes during the past decade. Furthermore, in six years out of seven (except 2003), the annual Cd-D net fluxes even exceeded river borne total (dissolved + particulate) gross Cd fluxes into the estuary. These observations were attributed to progressive Cd desorption from both suspended particles and bottom sediment during various sedimentation-resuspension cycles induced by tidal currents and/or continuous dredging (navigation channel) and diverse intra-estuarine sources (wet deposition, urban sources, and agriculture). Provided that gross fluxes remain stable over time, dissolved Cd exportation from the Gironde Estuary to the coastal ocean may remain at the present level for the coming decade and the estuarine sedimentary Cd stock is forecast to decrease slowly. (C) 2009 Elsevier Ltd. All rights reserved.