508
J.L. Probst et al.
km 2 per year). Even if these values are only rough estimations, they are in the
same order of magnitude as the literature values quoted above.
The longitudinal profiles of July 1996 are rather exceptional because the high
mercury concentrations which can be observed both in the suspended matter and
in the bottom sediments must be attributed to the highest solute mercury
concentrations (169 ng 1-') measured in the industrial wastewater in Vieux Thann
and to accidental pollution of the Thur from the wastewater treatment station in
Cernay.
The mercury concentrations measured in the different tributaries of the III
river after the confluence with the Thur river, i.e. the Lauch upstream Colmar,
the Fecht between Colmar and Selestat and the Giessen downstream from Selestat
are very low in the different phases and within the ranges of natural values.
4
Mercury River Fluxes in Particulate and Dissolved Phases
Using mercury content in the dissolved and particulate phases and river discharge
measurements at different gauging stations, it has been possible to calculate the
amount of mercury transported in solution and in suspension by the III river and
its tributaries. For each sampling period, one can follow the spatial evolution of
mercury fluxes going from the upstream part of the basin to downstream (Fig. 6a,
b). Globally for the whole basin, the particulate river fluxes of mercury appear to
be higher during the high-water period of February 1996 than during the low
flows ofJuly 1996 (Fig. 6a). This is mainly due to higher industrial mercury fluxes,
higher river-suspended sediment transports and important remobilization of
river bottom sediments. In contrast to particulate phases, the solute mercury river
fluxes are higher during low flows than during high-water periods (Fig. 6b). This
can be attributed to a higher solute mercury content in the industrial wastewater
and to methylation processes of bottom sediment mercury which increase the
mercury concentration in the solution. It is interesting to note in Fig. 6b that
solute mercury fluxes are rather important downstream from Oberhergheim
station, even if at this station the river discharge decreases to zero because of river
water infiltration to groundwater. This means that downstream from this station,
mercury in dissolved phases mainly originates from methylation processes of
mercury previously adsorbed on bottom sediments.
If one compares the shape of the spatial evolution of particulate mercury fluxes
during the two sampling periods (February and July 1996), it becomes evident
that for the high- water period the fluxes increase until Oberhergheim and
decrease in the lower part of the basin, whereas for the low flows it is the
contrary. In the upper part of the river, the increase during high flows is due to
remobilization of mercury-enriched bottom sediments while the decrease during
low flows can be attributed to sedimentation processes. In the downstream part,
after Oberhergheim station, the slope of the main channel decreases and the
sedimentation processes become active even during high-water periods, as could
J.L. Probst et al.
km 2 per year). Even if these values are only rough estimations, they are in the
same order of magnitude as the literature values quoted above.
The longitudinal profiles of July 1996 are rather exceptional because the high
mercury concentrations which can be observed both in the suspended matter and
in the bottom sediments must be attributed to the highest solute mercury
concentrations (169 ng 1-') measured in the industrial wastewater in Vieux Thann
and to accidental pollution of the Thur from the wastewater treatment station in
Cernay.
The mercury concentrations measured in the different tributaries of the III
river after the confluence with the Thur river, i.e. the Lauch upstream Colmar,
the Fecht between Colmar and Selestat and the Giessen downstream from Selestat
are very low in the different phases and within the ranges of natural values.
4
Mercury River Fluxes in Particulate and Dissolved Phases
Using mercury content in the dissolved and particulate phases and river discharge
measurements at different gauging stations, it has been possible to calculate the
amount of mercury transported in solution and in suspension by the III river and
its tributaries. For each sampling period, one can follow the spatial evolution of
mercury fluxes going from the upstream part of the basin to downstream (Fig. 6a,
b). Globally for the whole basin, the particulate river fluxes of mercury appear to
be higher during the high-water period of February 1996 than during the low
flows ofJuly 1996 (Fig. 6a). This is mainly due to higher industrial mercury fluxes,
higher river-suspended sediment transports and important remobilization of
river bottom sediments. In contrast to particulate phases, the solute mercury river
fluxes are higher during low flows than during high-water periods (Fig. 6b). This
can be attributed to a higher solute mercury content in the industrial wastewater
and to methylation processes of bottom sediment mercury which increase the
mercury concentration in the solution. It is interesting to note in Fig. 6b that
solute mercury fluxes are rather important downstream from Oberhergheim
station, even if at this station the river discharge decreases to zero because of river
water infiltration to groundwater. This means that downstream from this station,
mercury in dissolved phases mainly originates from methylation processes of
mercury previously adsorbed on bottom sediments.
If one compares the shape of the spatial evolution of particulate mercury fluxes
during the two sampling periods (February and July 1996), it becomes evident
that for the high- water period the fluxes increase until Oberhergheim and
decrease in the lower part of the basin, whereas for the low flows it is the
contrary. In the upper part of the river, the increase during high flows is due to
remobilization of mercury-enriched bottom sediments while the decrease during
low flows can be attributed to sedimentation processes. In the downstream part,
after Oberhergheim station, the slope of the main channel decreases and the
sedimentation processes become active even during high-water periods, as could