also to greater vehicle use. Accordingly, it was assumed that the urban runoff
flux at the entire basin scale could be estimated proportionally to the total population
but that the runoff specifically related to the road runoff outside of urban areas
should be quantified independently (detailed below). Finally, at the basin scale
investigated, the urban releases were F22c # ¼ 170 Æ 20 kg year
À1 (Σ15) and
F22d # ¼ 70 Æ 10 kg year
À1 (Σ15) for separate and combined sewer discharge,
respectively. The direct discharges of PAHs from industries to rivers were estimated
at E22g # ¼ 310 Æ 90 kg year
À1 (Σ15). Finally, the PAH flux within the WWTP
effluents reached F22f # ¼ 30 Æ 6 kg year
À1 (Σ15). Thus, the amount of PAHs
released from the urban areas to the river system totalled F22 # ¼ 580 Æ 140 kg year
À1
(Σ15).
4 Discussion
The estimated fluxes at both area scales investigated are summarised in Fig. 4.
The PAH flux to the atmosphere was dominated by household heating and road
traffic (E5). The atmospheric deposition estimated at the Paris conurbation scale
(F12) reached 2% of the emission quantification based on economic data (E5 + E6).
At the entire basin scale, the atmospheric fallout (F10 + F11a + F11b + F12#)
reached 8 tons year
À1 and accounted for only 6.5% of the emissions. This disparity
between emissions and atmospheric deposition was unexpected, and it may stem
from the difference in the calculation methods, where the estimation of emissions
from economic data required the use of emission factors that may be poorly defined,
and the measurement of atmospheric deposition may be biased due to underestimation of the gaseous exchanges [64]. In addition, environmental processes such as
long-range transportation outside of the investigated area or photo-oxidation
resulting in PAH degradation in the atmosphere [65, 66] could also partly explain
that the deposition flux is far lower than the emissions flux.
The flux related to urban runoff across the investigated area (F12e) was similar to
atmospheric deposition. When considering only the impervious area, the fallout
flux only accounted for 22% of the estimated runoff flux, thereby suggesting that
remobilisation of pollutants deposited on roofs and roads was the main process of
runoff contamination. This result was consistent with previously published work
[10] depicting runoff flux as four times higher than deposition flux for a small
residential catchment.
The mass balance of PAHs at the Paris conurbation sewer system scale was
very consistent. Indeed, the sum of incoming PAH fluxes (F22a + F12e + F12f + E22h)
to the sewer system was 1,350 Æ 310 kg year
À1 , while the sewer outflows
(F22c + F22d + F22i + F22e) amounted to 1,360 Æ 20 kg year
À1 (Fig. 2). This
approach was made possible by the SIAAP’s thorough knowledge of the collected
and treated wastewater volumes across the Paris conurbation. The result showed
that the three main PAH sources, i.e. the domestic and industrial wastewater and
the runoff on impervious surfaces, contributed equally to the PAH supplies to the
180
D. Gateuille et al.
flux at the entire basin scale could be estimated proportionally to the total population
but that the runoff specifically related to the road runoff outside of urban areas
should be quantified independently (detailed below). Finally, at the basin scale
investigated, the urban releases were F22c # ¼ 170 Æ 20 kg year
À1 (Σ15) and
F22d # ¼ 70 Æ 10 kg year
À1 (Σ15) for separate and combined sewer discharge,
respectively. The direct discharges of PAHs from industries to rivers were estimated
at E22g # ¼ 310 Æ 90 kg year
À1 (Σ15). Finally, the PAH flux within the WWTP
effluents reached F22f # ¼ 30 Æ 6 kg year
À1 (Σ15). Thus, the amount of PAHs
released from the urban areas to the river system totalled F22 # ¼ 580 Æ 140 kg year
À1
(Σ15).
4 Discussion
The estimated fluxes at both area scales investigated are summarised in Fig. 4.
The PAH flux to the atmosphere was dominated by household heating and road
traffic (E5). The atmospheric deposition estimated at the Paris conurbation scale
(F12) reached 2% of the emission quantification based on economic data (E5 + E6).
At the entire basin scale, the atmospheric fallout (F10 + F11a + F11b + F12#)
reached 8 tons year
À1 and accounted for only 6.5% of the emissions. This disparity
between emissions and atmospheric deposition was unexpected, and it may stem
from the difference in the calculation methods, where the estimation of emissions
from economic data required the use of emission factors that may be poorly defined,
and the measurement of atmospheric deposition may be biased due to underestimation of the gaseous exchanges [64]. In addition, environmental processes such as
long-range transportation outside of the investigated area or photo-oxidation
resulting in PAH degradation in the atmosphere [65, 66] could also partly explain
that the deposition flux is far lower than the emissions flux.
The flux related to urban runoff across the investigated area (F12e) was similar to
atmospheric deposition. When considering only the impervious area, the fallout
flux only accounted for 22% of the estimated runoff flux, thereby suggesting that
remobilisation of pollutants deposited on roofs and roads was the main process of
runoff contamination. This result was consistent with previously published work
[10] depicting runoff flux as four times higher than deposition flux for a small
residential catchment.
The mass balance of PAHs at the Paris conurbation sewer system scale was
very consistent. Indeed, the sum of incoming PAH fluxes (F22a + F12e + F12f + E22h)
to the sewer system was 1,350 Æ 310 kg year
À1 , while the sewer outflows
(F22c + F22d + F22i + F22e) amounted to 1,360 Æ 20 kg year
À1 (Fig. 2). This
approach was made possible by the SIAAP’s thorough knowledge of the collected
and treated wastewater volumes across the Paris conurbation. The result showed
that the three main PAH sources, i.e. the domestic and industrial wastewater and
the runoff on impervious surfaces, contributed equally to the PAH supplies to the
180
D. Gateuille et al.