3.2.4 Soil Erosion
The flux of PAH transferred from topsoils to the river was estimated by matching
the land use map, the average PAH contents and erosion rates estimated across the
area [18]. Thus, the PAH erosion fluxes reached F15 ¼ 1,330 Æ 1,010 kg year
À1
(Σ15) for agricultural lands, F16 ¼ 34 Æ 10 kg year
À1 (Σ15) for forests and
F17 + F18 ¼ 30 Æ 9 kg year
À1 (Σ15) for urban and industrial areas. Consequently,
the overall erosion flux from soils was about 1,400 Æ 1,030 kg year
À1 (Σ15). As for
the soil stock estimation, a high mathematical uncertainty due to the heterogeneity
of the PAH content database for agricultural lands was observed. Moreover,
this erosion flux was estimated from PAH content in soils instead of eroded
particles because no other data were available. This method did not consider a
potential enrichment process during erosion due to particle sorting. Yet, previous
laboratory experiments reported PAH-enriched eroded particles by a factor up to 3
[57, 58]. However, this enrichment process has never been investigated at
the catchment scale, and it could not be taken into account in this estimation.
Consequently, this flux estimation constitutes a rough estimation, and it was
awarded a quality grade of 1. In addition, the PAH transfer to the deep soil horizons
and to groundwater was not quantified. However, because of the PAH retention in
topsoil horizon [52, 53], vertical flux of PAHs within the soils was ignored.
3.2.5 Sediment Dredging
To estimate the fluxes related to sediment dredging in the main stem of the Seine
River basin, the mass of collected material was multiplied by values of the PAH
contamination of sediment. The data of 95 samples were provided by the French
department of waterways, which is responsible for supervising dredging in the Seine
River basin. The flux of PAHs removed from the stems with the dredged material
amounted to F25a ¼ 660 Æ 70 kg year
À1 with 6% (F25b ¼ 40 Æ 4 kg year
À1 ) of it
being spread over agricultural areas. Since the volume of sediment, the percentage of
dry matter, the PAH content in the sediment and the purpose of the dredge material
were known for each operation, the flux estimation was awarded a grade of 4.
3.2.6 Storage in Reservoirs and Floodplains
To quantify the amount of PAHs (Fig. 4) annually stored in reservoirs (F23a),
on river beds (F23b) and over floodplains (F24), the sedimentation rates were
multiplied by the PAH content in suspended sediment (SS) randomly drawn in the
database. As the floodplain and the reservoirs were located upstream of the Paris
conurbation, the database was limited to SS samples collected in the uppermost parts
of the Seine River basin. Finally, 104 samples from four studies were available
[9, 11, 59, 60]. The annual flux of PAHs from the rivers towards the reservoirs and
Mass Balance of PAHs at the Scale of the Seine River Basin
177
The flux of PAH transferred from topsoils to the river was estimated by matching
the land use map, the average PAH contents and erosion rates estimated across the
area [18]. Thus, the PAH erosion fluxes reached F15 ¼ 1,330 Æ 1,010 kg year
À1
(Σ15) for agricultural lands, F16 ¼ 34 Æ 10 kg year
À1 (Σ15) for forests and
F17 + F18 ¼ 30 Æ 9 kg year
À1 (Σ15) for urban and industrial areas. Consequently,
the overall erosion flux from soils was about 1,400 Æ 1,030 kg year
À1 (Σ15). As for
the soil stock estimation, a high mathematical uncertainty due to the heterogeneity
of the PAH content database for agricultural lands was observed. Moreover,
this erosion flux was estimated from PAH content in soils instead of eroded
particles because no other data were available. This method did not consider a
potential enrichment process during erosion due to particle sorting. Yet, previous
laboratory experiments reported PAH-enriched eroded particles by a factor up to 3
[57, 58]. However, this enrichment process has never been investigated at
the catchment scale, and it could not be taken into account in this estimation.
Consequently, this flux estimation constitutes a rough estimation, and it was
awarded a quality grade of 1. In addition, the PAH transfer to the deep soil horizons
and to groundwater was not quantified. However, because of the PAH retention in
topsoil horizon [52, 53], vertical flux of PAHs within the soils was ignored.
3.2.5 Sediment Dredging
To estimate the fluxes related to sediment dredging in the main stem of the Seine
River basin, the mass of collected material was multiplied by values of the PAH
contamination of sediment. The data of 95 samples were provided by the French
department of waterways, which is responsible for supervising dredging in the Seine
River basin. The flux of PAHs removed from the stems with the dredged material
amounted to F25a ¼ 660 Æ 70 kg year
À1 with 6% (F25b ¼ 40 Æ 4 kg year
À1 ) of it
being spread over agricultural areas. Since the volume of sediment, the percentage of
dry matter, the PAH content in the sediment and the purpose of the dredge material
were known for each operation, the flux estimation was awarded a grade of 4.
3.2.6 Storage in Reservoirs and Floodplains
To quantify the amount of PAHs (Fig. 4) annually stored in reservoirs (F23a),
on river beds (F23b) and over floodplains (F24), the sedimentation rates were
multiplied by the PAH content in suspended sediment (SS) randomly drawn in the
database. As the floodplain and the reservoirs were located upstream of the Paris
conurbation, the database was limited to SS samples collected in the uppermost parts
of the Seine River basin. Finally, 104 samples from four studies were available
[9, 11, 59, 60]. The annual flux of PAHs from the rivers towards the reservoirs and
Mass Balance of PAHs at the Scale of the Seine River Basin
177