flux carried downstream of the study area outlet (F21). Several hypotheses can be
considered to explain this gap. Firstly, PAH fluxes from the urban areas could have
been underestimated. More specifically, the change in spatial scale constitutes a
major source of uncertainty because most of the calculation parameters were poorly
known at the entire basin scale. Secondly, and as previously described, an enrichment process that would occur during erosion could drastically increase the PAH
flux from soil to rivers. Thus, an average enrichment by a factor 3 [57] would
balance the PAH inputs and outputs. Finally, the mass balance could be right and the
Seine River basin may currently be undergoing a decontamination phase involving
the remobilisation of ancient and more contaminated sediment. This process can
explain the short-term difference between pollutant inputs and outputs, as was
already suggested for metals [69].
5 Conclusions and Perspectives
For the first time in the literature, a mass balance at a large spatial scale, i.e. the upper
basin of the Seine River, was attempted for PAHs. This approach was enlightening
in terms of both the environmental fate of PAHs and the priority research areas to
complete these investigations.
Except for PAH emissions, all the estimated fluxes had consistent orders of
magnitude. Although mistakes in the flux estimations could not be excluded, the
difference between the amounts of PAHs emitted to the atmosphere and deposited
from it suggests that physico-chemical processes still need to be investigated. In
particular, the fate of PAHs in the atmosphere is poorly known. A number of studies
have focused on the process responsible for chemical oxidation and photo-oxidation
of atmospheric PAHs, but the phenomenon has never been quantified at a large
spatial scale. Such an investigation would be challenging but would provide
significant knowledge on a process that may drastically limit environmental PAH
contamination. From a public perspective, it would be advantageous to quantify
the deposition of the compounds arising from these degradation processes because
they have been shown to constitute a health issue.
At the Paris conurbation scale, the sewer system inputs and outputs closely
matched given the well-established wastewater and sludge mass balances. The result
showed the importance of runoff and wastewater management to deal with PAH
fluxes. In particular, the treatment processes implemented at the WWTPs have
proven their efficiency in PAH removal, thereby drastically limiting the flux being
released through WWTP effluents. However, two major sources of uncertainty
remained. Firstly, the PAH fluxes related to industrial discharge to the sewers and
rivers were poorly known. Since neither the wastewater volume nor the PAH
concentrations were quantified, considerable work would be required to overcome
this issue despite the current deindustrialisation of the basin. Because various
industries are reported be responsible for different effluent contaminations, a specific
inventory would be necessary. Secondly, more data on the PAH concentrations
182
D. Gateuille et al.
considered to explain this gap. Firstly, PAH fluxes from the urban areas could have
been underestimated. More specifically, the change in spatial scale constitutes a
major source of uncertainty because most of the calculation parameters were poorly
known at the entire basin scale. Secondly, and as previously described, an enrichment process that would occur during erosion could drastically increase the PAH
flux from soil to rivers. Thus, an average enrichment by a factor 3 [57] would
balance the PAH inputs and outputs. Finally, the mass balance could be right and the
Seine River basin may currently be undergoing a decontamination phase involving
the remobilisation of ancient and more contaminated sediment. This process can
explain the short-term difference between pollutant inputs and outputs, as was
already suggested for metals [69].
5 Conclusions and Perspectives
For the first time in the literature, a mass balance at a large spatial scale, i.e. the upper
basin of the Seine River, was attempted for PAHs. This approach was enlightening
in terms of both the environmental fate of PAHs and the priority research areas to
complete these investigations.
Except for PAH emissions, all the estimated fluxes had consistent orders of
magnitude. Although mistakes in the flux estimations could not be excluded, the
difference between the amounts of PAHs emitted to the atmosphere and deposited
from it suggests that physico-chemical processes still need to be investigated. In
particular, the fate of PAHs in the atmosphere is poorly known. A number of studies
have focused on the process responsible for chemical oxidation and photo-oxidation
of atmospheric PAHs, but the phenomenon has never been quantified at a large
spatial scale. Such an investigation would be challenging but would provide
significant knowledge on a process that may drastically limit environmental PAH
contamination. From a public perspective, it would be advantageous to quantify
the deposition of the compounds arising from these degradation processes because
they have been shown to constitute a health issue.
At the Paris conurbation scale, the sewer system inputs and outputs closely
matched given the well-established wastewater and sludge mass balances. The result
showed the importance of runoff and wastewater management to deal with PAH
fluxes. In particular, the treatment processes implemented at the WWTPs have
proven their efficiency in PAH removal, thereby drastically limiting the flux being
released through WWTP effluents. However, two major sources of uncertainty
remained. Firstly, the PAH fluxes related to industrial discharge to the sewers and
rivers were poorly known. Since neither the wastewater volume nor the PAH
concentrations were quantified, considerable work would be required to overcome
this issue despite the current deindustrialisation of the basin. Because various
industries are reported be responsible for different effluent contaminations, a specific
inventory would be necessary. Secondly, more data on the PAH concentrations
182
D. Gateuille et al.