sewer system. In the mass balance analysis for urban areas, the fluxes of PAHs
released by industries constituted the main uncertainty. To refine these results,
further investigations on the PAH concentrations in industrial effluents are required.
A specific investigation on the WWTP incoming flux (F22e) showed that separate
sewers only accounted for 9% of annual PAH load to the WWTPs, while supplies
from the combined sewer system reached 15% and 76% during dry and wet weather
flows, respectively. It was assumed that the wet weather supply of PAHs was directly
related to sewer sediment remobilisation rather than runoff [67].
When estimated at the WWTP scale, the fluxes related to sludge and effluents,
respectively, amounted to 55% (Σ13) and 2% (Σ15) of the incoming flux, suggesting
that approximately 43% of PAHs were degraded or volatilised during the treatment
process. This proportion of PAHs in sludge was in agreement with previously
published results [68], but the amount of PAHs released through effluent discharge
was about 15 times lower. This result could be explained because the database was in
part collected at the Seine-Centre WWTP characterised by high suspended matter
removal efficiency, which may not be fully representative of the other WWTPs.
The overall discharge of PAHs into the Seine River system, including both
WWTP effluents and overflows from the sewer system (F22c + F22d + F22f + E22g)
within the Paris conurbation, reached 410 Æ 110 kg year
À1 . The main uncertainties
on this flux estimation lay in data related to the characterisation of industrial
and WWTP effluents. At the entire basin scale, the quality of this estimation was
degraded because of the lack of information on the wastewater volumes and on the
efficiency of the small-capacity WWTPs. This observation can be generalised for
all the environmental fluxes since data were scarce for the rural part of the basin.
A substantial study would be necessary to gather all the required information and
properly estimate the flux at this scale.
Agricultural lands, forests and urban areas accumulated 72%, 19% and 9%,
respectively, of the total atmospheric deposition. The forest filter effect (F11b)
totalled 20% of the direct deposition flux over forest areas. In comparison, the
PAH stocks in soils were distributed between agricultural lands (89%), urban areas
(7%) and forests (4%). The low values of the inputs to the stock ratio in soils suggest
that the PAH accumulation occurred over a long period of time and that biodegradation processes were not significant. Consequently, PAHs constitute a very persistent pollution within the River Seine basin.
The erosion-related flux was estimated at 1400 kg year
À1 , but the relationship
between the PAH contents in soil and in eroded particles could not be specifically
investigated. A potential enrichment process occurring during erosion could not
be quantified. Yet, a previous study has shown that the PAH content is much higher
in suspended sediment than in the surrounding soils even in the most remote
rural catchment [11]. The overall flux of PAHs stored within the river system
(F23a + F23b) was estimated at 760 Æ 70 kg year
À1 , and it reached about 28% of
the PAH flux carried downstream of the study area outlet (F21).
Overall, the soil erosion was the main source of PAHs in the Seine River, while
urban release constituted a significant but smaller source. The PAH inputs to the
Seine River (F15 + F16 + F17 + F18+ F22 # ) were about twofold lower than the PAH
Mass Balance of PAHs at the Scale of the Seine River Basin
181
released by industries constituted the main uncertainty. To refine these results,
further investigations on the PAH concentrations in industrial effluents are required.
A specific investigation on the WWTP incoming flux (F22e) showed that separate
sewers only accounted for 9% of annual PAH load to the WWTPs, while supplies
from the combined sewer system reached 15% and 76% during dry and wet weather
flows, respectively. It was assumed that the wet weather supply of PAHs was directly
related to sewer sediment remobilisation rather than runoff [67].
When estimated at the WWTP scale, the fluxes related to sludge and effluents,
respectively, amounted to 55% (Σ13) and 2% (Σ15) of the incoming flux, suggesting
that approximately 43% of PAHs were degraded or volatilised during the treatment
process. This proportion of PAHs in sludge was in agreement with previously
published results [68], but the amount of PAHs released through effluent discharge
was about 15 times lower. This result could be explained because the database was in
part collected at the Seine-Centre WWTP characterised by high suspended matter
removal efficiency, which may not be fully representative of the other WWTPs.
The overall discharge of PAHs into the Seine River system, including both
WWTP effluents and overflows from the sewer system (F22c + F22d + F22f + E22g)
within the Paris conurbation, reached 410 Æ 110 kg year
À1 . The main uncertainties
on this flux estimation lay in data related to the characterisation of industrial
and WWTP effluents. At the entire basin scale, the quality of this estimation was
degraded because of the lack of information on the wastewater volumes and on the
efficiency of the small-capacity WWTPs. This observation can be generalised for
all the environmental fluxes since data were scarce for the rural part of the basin.
A substantial study would be necessary to gather all the required information and
properly estimate the flux at this scale.
Agricultural lands, forests and urban areas accumulated 72%, 19% and 9%,
respectively, of the total atmospheric deposition. The forest filter effect (F11b)
totalled 20% of the direct deposition flux over forest areas. In comparison, the
PAH stocks in soils were distributed between agricultural lands (89%), urban areas
(7%) and forests (4%). The low values of the inputs to the stock ratio in soils suggest
that the PAH accumulation occurred over a long period of time and that biodegradation processes were not significant. Consequently, PAHs constitute a very persistent pollution within the River Seine basin.
The erosion-related flux was estimated at 1400 kg year
À1 , but the relationship
between the PAH contents in soil and in eroded particles could not be specifically
investigated. A potential enrichment process occurring during erosion could not
be quantified. Yet, a previous study has shown that the PAH content is much higher
in suspended sediment than in the surrounding soils even in the most remote
rural catchment [11]. The overall flux of PAHs stored within the river system
(F23a + F23b) was estimated at 760 Æ 70 kg year
À1 , and it reached about 28% of
the PAH flux carried downstream of the study area outlet (F21).
Overall, the soil erosion was the main source of PAHs in the Seine River, while
urban release constituted a significant but smaller source. The PAH inputs to the
Seine River (F15 + F16 + F17 + F18+ F22 # ) were about twofold lower than the PAH
Mass Balance of PAHs at the Scale of the Seine River Basin
181