stage. Cd analysis on river SPM during Seine River floods has shown that actually
the maximum content is observed at the receding stage, i.e. when particulates settle
in the floodplain [29], so that the sediment contamination is not underestimated and
hypothesis H 1 is satisfied. For the assessment of excess metal fluxes, the effect of
interannual variations of suspended loads (H 2 ) is minimised by taking a 2 to 5 cm
core slice, homogenised and then analysed (corresponding to an average period of
4–6 years, depending on the sedimentation rate), to which an average Seine River
interannual sediment load (700,000 t year
À1 ; [30]) has been attributed throughout the
core. In some cores of the Seine River (e.g. [14]), the Cs-137 peak maximum is very
high, an indication of the limited mobility of this element and the absence of postdepositional bioturbation (H 3 , H 4 ).
Finally, it must be remembered that sediment transport in fluvial systems results
in a mixing of particulates of various ages of mobilisation/remobilisation, ranging
from a few hours (remobilised bottom sediments) to a few hundred years and more
(erosion of floodplain banks). Thus, the comparison of contemporary river output of
polluted particles to the present-day circulation of material is an approach that is
mostly used to compare the life cycle of different chemicals (they are affected by the
same bias) or to assess the general trend of exported pollutants. These reservations
explain why modelling particulate pollutant transfer in the anthroposphere is so
difficult.
2.3 Circulation of Contaminants Within River Basins
The river’s excess fluxes of particulate contaminants are then compared to the
circulation of products and materials within the anthroposphere, i.e. the material
flow [31] such as imports and exports, transformation and use of products and
recycling (e.g. for metals). In the case of metals, one should also take into account
mining and smelting, which are not always present within the river basin intercepted
by the core. The anthroposphere also gradually stores contaminants in contaminated
soils, infrastructures, etc. In large basins, only a very small proportion of this
enormous cycling of material actually leaks into the aquatic environment. In small
urban streams, it may be greater. Such material flow analysis has been done for Paris
for the metal circulation [32, 33] and for the whole basin for specific elements such
as nitrogen and phosphorus [34]. Fluvial sediment archives are key elements in the
validation of contaminant circulation in river basins. They concern environmental
transfers between the major Earth system components – atmosphere, biosphere,
hydrosphere, soils and sediments – which intercept different diffuse and point
sources of contamination, which are considered as leaks to the river system: contaminated atmospheric fallout, agricultural runoff (sometimes impacted by reuse of
urban sewage sludge on cropland), sewage outfalls and leaks from waste dumps.
The ratio of the exported river pollutant fluxes to the circulation of the related
material within the intercepted basin generates the leakage ratio (Fig. 1, Step H), a
dimensionless factor, which is a performance indicator for regulation measures
Sedimentary Archives Reveal the Concealed History of Micropollutant. . .
275
the maximum content is observed at the receding stage, i.e. when particulates settle
in the floodplain [29], so that the sediment contamination is not underestimated and
hypothesis H 1 is satisfied. For the assessment of excess metal fluxes, the effect of
interannual variations of suspended loads (H 2 ) is minimised by taking a 2 to 5 cm
core slice, homogenised and then analysed (corresponding to an average period of
4–6 years, depending on the sedimentation rate), to which an average Seine River
interannual sediment load (700,000 t year
À1 ; [30]) has been attributed throughout the
core. In some cores of the Seine River (e.g. [14]), the Cs-137 peak maximum is very
high, an indication of the limited mobility of this element and the absence of postdepositional bioturbation (H 3 , H 4 ).
Finally, it must be remembered that sediment transport in fluvial systems results
in a mixing of particulates of various ages of mobilisation/remobilisation, ranging
from a few hours (remobilised bottom sediments) to a few hundred years and more
(erosion of floodplain banks). Thus, the comparison of contemporary river output of
polluted particles to the present-day circulation of material is an approach that is
mostly used to compare the life cycle of different chemicals (they are affected by the
same bias) or to assess the general trend of exported pollutants. These reservations
explain why modelling particulate pollutant transfer in the anthroposphere is so
difficult.
2.3 Circulation of Contaminants Within River Basins
The river’s excess fluxes of particulate contaminants are then compared to the
circulation of products and materials within the anthroposphere, i.e. the material
flow [31] such as imports and exports, transformation and use of products and
recycling (e.g. for metals). In the case of metals, one should also take into account
mining and smelting, which are not always present within the river basin intercepted
by the core. The anthroposphere also gradually stores contaminants in contaminated
soils, infrastructures, etc. In large basins, only a very small proportion of this
enormous cycling of material actually leaks into the aquatic environment. In small
urban streams, it may be greater. Such material flow analysis has been done for Paris
for the metal circulation [32, 33] and for the whole basin for specific elements such
as nitrogen and phosphorus [34]. Fluvial sediment archives are key elements in the
validation of contaminant circulation in river basins. They concern environmental
transfers between the major Earth system components – atmosphere, biosphere,
hydrosphere, soils and sediments – which intercept different diffuse and point
sources of contamination, which are considered as leaks to the river system: contaminated atmospheric fallout, agricultural runoff (sometimes impacted by reuse of
urban sewage sludge on cropland), sewage outfalls and leaks from waste dumps.
The ratio of the exported river pollutant fluxes to the circulation of the related
material within the intercepted basin generates the leakage ratio (Fig. 1, Step H), a
dimensionless factor, which is a performance indicator for regulation measures
Sedimentary Archives Reveal the Concealed History of Micropollutant. . .
275