Behavior of arsenic and antimony in the surface freshwater reaches of a highly turbid estuary, the Gironde Estuary, France |
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| Authors: | Matthieu Masson Jrg Schfer Grard Blanc Aymeric Dabrin Sabine Castelle Gilbert Lavaux |
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| Institution: | aUniversité de Bordeaux, UMR CNRS 5805 EPOC, Av. des Facultés, 33405 Talence Cedex, France |
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| Abstract: | This study reports on the behavior of two redox-sensitive elements, As and Sb, along the turbidity gradient in the freshwater reaches of the turbid Gironde Estuary. During a 17-month survey, surface water and suspended particulate matter (SPM) were sampled monthly at six sites representing both fluvial branches of the Gironde Estuary. Additionally, two longitudinal high resolution profiles were sampled along the fluvial estuary of the Garonne Branch during two contrasted seasons, i.e. with and without the presence of the maximum turbidity zone (MTZ). Seasonal variability and spatial distribution of dissolved (<0.2 μm; <0.02 μm) and particulate As, Sb and Fe were measured and combined with SPM data to understand metalloid behavior in the estuarine freshwater turbidity gradient.At the two main fluvial entries of the Gironde Estuary, dissolved As and Sb concentrations showed strong (by a factor of 2–4) seasonal variations, that were only partly controlled by discharge-related dilution. Seasonal addition of dissolved As and Sb was attributed to the degradation of particulate As and Sb carrier phases in bottom sediment and/or in the adjacent aquifers, rather than release from SPM. In the surface freshwater reaches of the Gironde Estuary, Sb behaved conservatively under all hydrological conditions. In contrast, As was strongly reactive in the presence of the MTZ, with opposite behaviors in the two fluvial branches of the estuary: in the Garonne Branch As was removed from the dissolved phase, whereas in the Dordogne Branch As was added. Redistribution of As between the dissolved and the particulate phases along the turbidity gradient in estuarine freshwater only affected the <0.02 μm fraction, as the 0.02–0.2 μm fraction remained constant ( 300 ng L−1 in September 2005). Accordingly, As removal seemed to be decoupled from concomitant “colloidal” (0.02–0.2 μm) Fe flocculation in the turbidity gradient. The contrasting behavior of dissolved As in the fluvial estuaries of the Garonne and Dordogne Branches was attributed to sorption processes during equilibration of river-borne dissolved As with estuarine SPM forming the MTZ. This equilibrium, described by a distinct distribution coefficient Kd(As) 11,000 L kg−1 in the MTZ, resulted in either As release (desorption; Dordogne Branch) or removal (adsorption; Garonne Branch) in the respective fluvial estuaries. A mixing experiment under controlled laboratory conditions tended to support that equilibration between the dissolved phase and MTZ particles may induce both As release and removal in the estuarine freshwater reaches, with As distribution evolving towards a distinct Kd value for increasing SPM concentrations. The long-term survey allowed estimating annual (2004) dissolved fluxes of As and, for the first time Sb, at the main fluvial entries of the Gironde Estuary at 30.7 t a−1 and 3.2 t a−1 (Garonne River) and at 8.0 t a−1 and 2.3 t a−1 (Dordogne River), respectively. |
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