Fluvial Transports of Mercury Pollution in the III River Basin (Northeastern France)
517
air temperature, low-water column depth and low-river suspended sediment
concentration could be favourable to methylation processes in the river itself,
decreasing the mercury content, particularly in the river bottom sediments. The
mercury methylation fluxes which could be roughly estimated for the III river are
in good agreement with the values literature. On the contrary, during high flows,
these processes are relatively limited and mercury contents in the river system
could be higher even if the dilution effect is more important.
Input-output budgets of mercury river fluxes from upstream to downstream in
the basin showed the different processes which control the particulate and solute
river transports of mercury pollution, such as mercury methylation in bottom
sediments particularly during low-water periods, deposition or remobilization of
suspended matter according to the hydrological season and the channel section.
A granulometric study of the bottom sediments shows that mercury
concentrations are higher in the finest particles (clays and silts <50 ~tm) for all
the stations, but the abundance of the different granulometric fractions must be
taken into account in the global mercury budget of the sediment. That means
that if coarse fractions (sand >50 ~lm) are abundant, the contribution of these
fractions to total mercury content in the sediment must be considered.
It could be shown also that river water salinity could play an important role in
the mercury sorption onto river sediments, particularly in the III river basin,
where industrial saline activity supplies high sodium chloride wastewater to the
Thur and III rivers. Our field measurements show that when the chloride
concentrations increase in the river water, the decrease in mercury content in the
solution is associated with an increase in mercury concentration in the
suspended matter and in river bottom sediments. Laboratory experiments
showed that when the salinity increases, mercury can be trapped, for example, by
manganese precipitates. This example illustrates the possibility of solute mercury
being adsorbed onto colloidal phases when salinity increases, as could be
observed in the field.
New laboratory experiments and other field sampling missions are planned to
confirm the different assumptions. It is also very important now to study
mercury speciation in the different phases and to determine the impact of this
mercury contamination on the groundwater and on the III river ecosystem by
studying, for example, mercury content in mosses and in fishes.
Acknowledgements This study was supported by the GIP HYDROSYSTEMES (Groupement d'Interet
Public) of BRGM-CEMAGREF-CNRS-IFREMER-INRA-ORSTOM-OIEau and the Programme Environnement, Vie et Societes (SEAH, Systemes Ecologiques et Actions de I'Homme) of CNRS. The
authors are grateful to the reviewers for their helptul comments on the first version of the manuscript.
References
AFEE (Association Fran~aise pour l'Etude des Eaux) (1975) Les micropolluants mineraux dans les
eaux superficielles continentales. no 1. Le mercure. Synthese bibliographique du Bureau National de
\'Information Scientifique et Technique (BNIST), Paris, p 103
517
air temperature, low-water column depth and low-river suspended sediment
concentration could be favourable to methylation processes in the river itself,
decreasing the mercury content, particularly in the river bottom sediments. The
mercury methylation fluxes which could be roughly estimated for the III river are
in good agreement with the values literature. On the contrary, during high flows,
these processes are relatively limited and mercury contents in the river system
could be higher even if the dilution effect is more important.
Input-output budgets of mercury river fluxes from upstream to downstream in
the basin showed the different processes which control the particulate and solute
river transports of mercury pollution, such as mercury methylation in bottom
sediments particularly during low-water periods, deposition or remobilization of
suspended matter according to the hydrological season and the channel section.
A granulometric study of the bottom sediments shows that mercury
concentrations are higher in the finest particles (clays and silts <50 ~tm) for all
the stations, but the abundance of the different granulometric fractions must be
taken into account in the global mercury budget of the sediment. That means
that if coarse fractions (sand >50 ~lm) are abundant, the contribution of these
fractions to total mercury content in the sediment must be considered.
It could be shown also that river water salinity could play an important role in
the mercury sorption onto river sediments, particularly in the III river basin,
where industrial saline activity supplies high sodium chloride wastewater to the
Thur and III rivers. Our field measurements show that when the chloride
concentrations increase in the river water, the decrease in mercury content in the
solution is associated with an increase in mercury concentration in the
suspended matter and in river bottom sediments. Laboratory experiments
showed that when the salinity increases, mercury can be trapped, for example, by
manganese precipitates. This example illustrates the possibility of solute mercury
being adsorbed onto colloidal phases when salinity increases, as could be
observed in the field.
New laboratory experiments and other field sampling missions are planned to
confirm the different assumptions. It is also very important now to study
mercury speciation in the different phases and to determine the impact of this
mercury contamination on the groundwater and on the III river ecosystem by
studying, for example, mercury content in mosses and in fishes.
Acknowledgements This study was supported by the GIP HYDROSYSTEMES (Groupement d'Interet
Public) of BRGM-CEMAGREF-CNRS-IFREMER-INRA-ORSTOM-OIEau and the Programme Environnement, Vie et Societes (SEAH, Systemes Ecologiques et Actions de I'Homme) of CNRS. The
authors are grateful to the reviewers for their helptul comments on the first version of the manuscript.
References
AFEE (Association Fran~aise pour l'Etude des Eaux) (1975) Les micropolluants mineraux dans les
eaux superficielles continentales. no 1. Le mercure. Synthese bibliographique du Bureau National de
\'Information Scientifique et Technique (BNIST), Paris, p 103