Bernard Boutier

Influence of the Gironde estuary outputs on cadmium concentrations in the waters: consequences on the Marennes-Oléron bay (France)

The Gironde, an estuary in Southwestern France, has been highly contaminated by cadmium for many years, and the results of the French mussel watch suggest that this contamination is not restricted to the estuary, but could be responsible for relatively high levels of Cd in the oysters from the Marennes-Oleron bay located nearby. This hypothesis has been evaluated through three cruises carried out in the Gironde estuary and plume, the Marennes-Oleron bay, and the Charente estuary, a river of minor importance that influences directly this bay. The field results from the Pertuis cruise (May 1991) and two others conducted in January and December of the same year show that the Gironde is an important source of dissolved cadmium for the bordering continental shelf (11 t year–1), while the Charente inputs are far lower (60 kg year–1). Dissolved Cd concentrations on the shelf vary according to the salinity, ranging from 0.76 nmol L–1 close to the Gironde to 0.15 nmol L–1 far from the shore. The field data as well as simulations from a 3D hydrodynamic model show that the Gironde plume reaches the Antioche Straits north of the Marennes-Oleron bay, and that the Charente plume is diluted in the bay by marine waters strongly influenced by the Gironde. This study confirms that the Gironde estuary is an important source of cadmium for the Marennes-Oleron bay.

Cadmium in the gironde fluvioestuarine system: Behaviour and flow

Abstract Major cadmium contamination of molluscs was discovered in the Gironde estuary. Studies were then undertaken on biota and on particulate and dissolved phases. Biota was studied by monitoring populations of oysters and mussels transplanted from uncontaminated areas into the Gironde. Studies on particulate and dissolved phases were carried out within the estuary, on the inner shelf, in Marennes-Oleron Bay, and in the drainage basin of the Gironde. It appears that: (i) the estuarine particulate stock, with its long residence time, plays a part in particulate-dissolved transformation, and thus has an effect on chemical conditions in the lower estuary and the coastal zone; (ii) the dissolved cadmium concentrations downstream can explain the concentrations measured in the oysters; (iii) the contamination is recent, and originates from the drainage basin; (iv) a geochemical cadmium budget in the Gironde has been formulated and is discussed.

Modelling of pollutant behaviour in estuaries: Application to cadmium in the Loire estuary

A one-dimensional multivariate mathematical model was used to study the behaviour of cadmium in a macrotidal estuary. Dissolved and particulate Cd were computed using a complexation model to solve the thermodynamic equations governing balances between free ion Cd2+ and various ligands (chloride, sulfate and hydroxide ions). Suspended matter is also considered as a ligand, characterised by an adsorption constant linking free ion Cd2+ activity to concentrations of sorbed cadmium on suspended matter. The models sensitivity study, which was applied to the Loire estuary, shows that results can be highly dependent on: (1) the adsorption constant; (2) the initial conditions determining cadmium concentrations in particulate matter at the start of each simulation; (3) the longitudinal distribution of salinity and suspended matter. The above data is often badly evaluated, introducing many uncertainties as to the results and their interpretation. The number of degrees of freedom becomes too high to permit a proper validation. However, this type of approach does allow for a rapid evaluation of the variables controlling the evolution of a given pollutant in an estuary. The results obtained show that the application and validation of a model simulating pollutant behaviour require: (1) validation of the distribution of master variables (salinity and suspended matter (S.M.) in the case of Cd); (2) relatively precise evaluation of: either the initial conditions if the simulation time scale is short, or of boundary conditions and various inputs into the estuary if the model is capable of simulating the systems evolution over a long period of time (several months).

PARTICULATE CD AND HG FLUXES TO THE SEDIMENT IN THE MARENNES-OLERON BAY : ORIGIN AND EVOLUTION

Abstract The location of one of the largest oyster centres in France in the Marennes‐Oleron Bay instigated the study of two toxic metals: cadmium and mercury, most likely originated from the Charente Estuary. Analyses of particulate metallic inputs in sediments (60 kg Cd and 35 kg Hg/yr) and radiochronological data showed that outputs from the Charente river (20 kg Cd and 12 kg Hg/yr) alone could not account for the high metallic contents recorded in the bay. This metallic input could be due to the presence of an exterior source to the bay.

Retrospective study of metal contamination time trends in the French part of the Bay of Biscay

Two cores were sampled in the Bay of Biscay: one a few miles off the Gironde estuary (pointe de la Coubre), the other near the Spanish border (Capbreton canyon) to study past variations in sediment contamination. Radiochronology (210Pb, 137Cs) and total metal determinations by ICP-MS allowed the study of time trends for metal concentrations in sediment. Off the Gironde, during the time interval recorded by the core (1958–1999), all metal concentrations remained less than OSPAR background values. Some variations occurred along the Cd and Zn profiles, which may correspond to a very attenuated signal from the Gironde outputs. This suggests that this area was not submitted to any significant anthropogenic input of metals between 1958 and 1999. In the canyon of Capbreton, the undisturbed part of the core recorded the influence of surface deposition between 1977 and 1999. From as early as 1977 high lead and mercury concentrations were noticed. Mercury concentrations increased until 1992, then decreased until 1999. This marks a drop of inputs in 1992, but the final concentration remained elevated. Lead had important concentrations (more than twice the OSPAR reference) all along the core and began to decrease only in the upper level of the unmixed layer. This shows a late (near 1998) drop of lead inputs. Other metals (Cd, Cu, Zn, Ni, Cr) stayed at a much lower level: less than the OSPAR reference for Cu, Cr and Ni or not far from the OSPAR reference for Cd and Zn.