sedimentary aquifers, and, for the middle and lower reaches of the Seine and Marne
rivers, by large reservoirs that were built between 1931 and 1990 (Fig. 1) [35]. These
reservoirs, totalling 841 Mm
3 , regulate floods and can provide more than 50% of the
river flow in summer, thus contributing to dilute wastewater inputs of the Paris
conurbation (i.e. 25 m
3 s
À1 discharge downstream of Paris). Water management is
therefore a key factor of the current Seine River discharge profile [32]. It should be
noted that the relative natural dilution power of the Seine River in relation to Parisian
wastewaters is very limited, making the Seine River sensitive to point source
pollution compared with most large European rivers.
The PIREN-Seine programme focuses mainly on the Seine River basin upstream
of its estuary, the latter also being studied by the long-term programme, Seine-Aval,
which started in 1995 [36]. The main experimental sites studied within the Seine
River basin and the coring sites presented in Ayrault et al. [26] are shown in Fig. 1
and will be discussed in further detail in the following chapters.
2.2 Evolution of the Basin Population
Paris, the largest megacity in Europe, has developed on the Seine River in the central
part of the basin, near the confluence of the Marne River and upstream of the
confluence with the Oise River. This location once represented a favourable factor
for the long-distance transport of food, timber, and construction materials. The
urbanized part of the basin has been growing over the past two centuries. Paris
megacity increased from 75 km
2 in the 1850s (Paris area) to 2,850 km
2 today [37]. In
2015, it was home to 12.4 million (M) of the 16.7 M inhabitants (inhab.) within the
basin. The population density of the Seine basin in 1901, 1954, and 2016 is provided
in Fig. 3, showing the increasing population in the Ile-de-France region, around
Paris, and downstream along the lower Seine. The population density near urban
tributaries in the Paris area ranges between 1,000 and 5,000 inhab km
À2 , while it
is much lower (<20 inhab km
À2 ) in the upstream areas, with an average of
250 inhab km
À2 for the whole basin.
The urban pollution generated by the city of Paris, and later by the Parisian
conurbation, which impacts the lower Seine and the estuary has been a major
concern of the authorities for over a century [37–39]. Water quality has been
severely degraded in terms of oxygen levels, ammonia, and nitrite concentrations,
as well as the occurrence of faecal bacteria over a section of river extending from
100 to 250 km downstream from Paris [40]. In the 1870s, the collected wastewater
started to be spread over sewage farms near Paris. Wastewater treatment plants
(WWTPs) were then gradually built but insufficiently to treat the volume of wastewater generated by the Paris conurbation [41]. One of the sewage farms at Achères,
located at 60 km downstream from Paris, was converted into the Seine-Aval WWTP
between 1930 and the 1980s, treating up to 8 M inhab. equivalents in the 1970s
[35]. The lag between sewage collection and its adequate treatment was only bridged
in the 1990s [42].
Trajectories of the Seine River Basin
7
rivers, by large reservoirs that were built between 1931 and 1990 (Fig. 1) [35]. These
reservoirs, totalling 841 Mm
3 , regulate floods and can provide more than 50% of the
river flow in summer, thus contributing to dilute wastewater inputs of the Paris
conurbation (i.e. 25 m
3 s
À1 discharge downstream of Paris). Water management is
therefore a key factor of the current Seine River discharge profile [32]. It should be
noted that the relative natural dilution power of the Seine River in relation to Parisian
wastewaters is very limited, making the Seine River sensitive to point source
pollution compared with most large European rivers.
The PIREN-Seine programme focuses mainly on the Seine River basin upstream
of its estuary, the latter also being studied by the long-term programme, Seine-Aval,
which started in 1995 [36]. The main experimental sites studied within the Seine
River basin and the coring sites presented in Ayrault et al. [26] are shown in Fig. 1
and will be discussed in further detail in the following chapters.
2.2 Evolution of the Basin Population
Paris, the largest megacity in Europe, has developed on the Seine River in the central
part of the basin, near the confluence of the Marne River and upstream of the
confluence with the Oise River. This location once represented a favourable factor
for the long-distance transport of food, timber, and construction materials. The
urbanized part of the basin has been growing over the past two centuries. Paris
megacity increased from 75 km
2 in the 1850s (Paris area) to 2,850 km
2 today [37]. In
2015, it was home to 12.4 million (M) of the 16.7 M inhabitants (inhab.) within the
basin. The population density of the Seine basin in 1901, 1954, and 2016 is provided
in Fig. 3, showing the increasing population in the Ile-de-France region, around
Paris, and downstream along the lower Seine. The population density near urban
tributaries in the Paris area ranges between 1,000 and 5,000 inhab km
À2 , while it
is much lower (<20 inhab km
À2 ) in the upstream areas, with an average of
250 inhab km
À2 for the whole basin.
The urban pollution generated by the city of Paris, and later by the Parisian
conurbation, which impacts the lower Seine and the estuary has been a major
concern of the authorities for over a century [37–39]. Water quality has been
severely degraded in terms of oxygen levels, ammonia, and nitrite concentrations,
as well as the occurrence of faecal bacteria over a section of river extending from
100 to 250 km downstream from Paris [40]. In the 1870s, the collected wastewater
started to be spread over sewage farms near Paris. Wastewater treatment plants
(WWTPs) were then gradually built but insufficiently to treat the volume of wastewater generated by the Paris conurbation [41]. One of the sewage farms at Achères,
located at 60 km downstream from Paris, was converted into the Seine-Aval WWTP
between 1930 and the 1980s, treating up to 8 M inhab. equivalents in the 1970s
[35]. The lag between sewage collection and its adequate treatment was only bridged
in the 1990s [42].
Trajectories of the Seine River Basin
7