were collected bimonthly, more frequently at some stations. The 2014–2018 period
was selected to limit the effect of the strong temporal trends shown in Sect. 4.
The longitudinal trends result from a combination of factors whose effect cannot
all easily be separated. Inputs from wastewater discharges or tributaries can increase
or dilute the E. coli concentrations, while the natural elimination processes (grazing,
settling, etc.) progressively decrease the concentration. The impact of point discharges or tributaries can be compared to concentrations at the nearest downstream
station, but the combined impact of diffuse sources and elimination processes
between recognized inputs cannot be disentangled.
Estimates of the impact of WWTPs were computed from the median concentrations discussed in Sect. 5, from the nominal daily volumes treated by WWTPs and
river discharges at the low stage (monthly average with a 2-year return period, data
from the French national hydrological database). The expected concentration
increases after dilution in the river were 1,080, 130, 1,810 and 1,390 E. coli per
100 mL for the SAM, SEC, SAV, and MAV WWTPs, respectively.
The strong increase of median concentration and flux at Ivry is twice as high as
the discharge of the SAM WWTP only. This suggests the existence of additional
sources. During summer 2017, contamination levels were analyzed in the Seine and
Marne rivers during well-established dry weather periods [50]. Samples were taken
every kilometer, near each bank, and in the center of the flow. This revealed a
substantial increase of E. coli concentrations upstream from the Ivry station (2S), but
the source was situated on the left bank, whereas the SAM WWTP is on the right
bank. A combined sewer overflow, which also drains non-contaminated groundwater during dry weather, was identified as the wastewater discharging outlet. It was
concluded that a malfunctioning of the sewer network in the municipalities
connected to the network was probably the cause of the presence of wastewater in
the CSO during a dry weather period, but the problem could not be precisely
identified.
Downstream from Paris, a progressive increase of concentrations is observed,
from Suresnes (6S) to Conflans (11S). More than 50% of the concentration jump at
Conflans-Seine can be attributed to the SAV WWTP, while the expected impact of
the SEC WWTP is negligible. Like upstream from Paris, it is likely that much of the
observed increase downstream from Paris is not only due to WWTP discharges but
to malfunctions of the sewer network during dry weather, with a possible contribution of well-known major CSOs in this area and a possible delayed transport of FIBs.
Dilution by the Oise River would explain most of the decrease at Poissy (13S), but
the downstream decrease at Triel (14S) is mainly due to natural decay processes
affecting E. coli in fresh water. The input of the SEG WWTP is a very minor
contribution to the concentration at the Triel station.
The situation is different inside Paris and just downstream of it: E. coli concentrations tend to decrease from Tolbiac (4S) to Suresnes (6S). It is likely that better
control of the sewer network has been achieved in this area, where the network is
fully accessible, which may have facilitated the control of dry weather discharges.
The longitudinal profile in the Marne River (Fig. 10) is quite regular with a
decrease of concentrations starting at station 5 M, which can be explained by natural
Bathing Activities and Microbiological River Water Quality in the Paris. . .
341
was selected to limit the effect of the strong temporal trends shown in Sect. 4.
The longitudinal trends result from a combination of factors whose effect cannot
all easily be separated. Inputs from wastewater discharges or tributaries can increase
or dilute the E. coli concentrations, while the natural elimination processes (grazing,
settling, etc.) progressively decrease the concentration. The impact of point discharges or tributaries can be compared to concentrations at the nearest downstream
station, but the combined impact of diffuse sources and elimination processes
between recognized inputs cannot be disentangled.
Estimates of the impact of WWTPs were computed from the median concentrations discussed in Sect. 5, from the nominal daily volumes treated by WWTPs and
river discharges at the low stage (monthly average with a 2-year return period, data
from the French national hydrological database). The expected concentration
increases after dilution in the river were 1,080, 130, 1,810 and 1,390 E. coli per
100 mL for the SAM, SEC, SAV, and MAV WWTPs, respectively.
The strong increase of median concentration and flux at Ivry is twice as high as
the discharge of the SAM WWTP only. This suggests the existence of additional
sources. During summer 2017, contamination levels were analyzed in the Seine and
Marne rivers during well-established dry weather periods [50]. Samples were taken
every kilometer, near each bank, and in the center of the flow. This revealed a
substantial increase of E. coli concentrations upstream from the Ivry station (2S), but
the source was situated on the left bank, whereas the SAM WWTP is on the right
bank. A combined sewer overflow, which also drains non-contaminated groundwater during dry weather, was identified as the wastewater discharging outlet. It was
concluded that a malfunctioning of the sewer network in the municipalities
connected to the network was probably the cause of the presence of wastewater in
the CSO during a dry weather period, but the problem could not be precisely
identified.
Downstream from Paris, a progressive increase of concentrations is observed,
from Suresnes (6S) to Conflans (11S). More than 50% of the concentration jump at
Conflans-Seine can be attributed to the SAV WWTP, while the expected impact of
the SEC WWTP is negligible. Like upstream from Paris, it is likely that much of the
observed increase downstream from Paris is not only due to WWTP discharges but
to malfunctions of the sewer network during dry weather, with a possible contribution of well-known major CSOs in this area and a possible delayed transport of FIBs.
Dilution by the Oise River would explain most of the decrease at Poissy (13S), but
the downstream decrease at Triel (14S) is mainly due to natural decay processes
affecting E. coli in fresh water. The input of the SEG WWTP is a very minor
contribution to the concentration at the Triel station.
The situation is different inside Paris and just downstream of it: E. coli concentrations tend to decrease from Tolbiac (4S) to Suresnes (6S). It is likely that better
control of the sewer network has been achieved in this area, where the network is
fully accessible, which may have facilitated the control of dry weather discharges.
The longitudinal profile in the Marne River (Fig. 10) is quite regular with a
decrease of concentrations starting at station 5 M, which can be explained by natural
Bathing Activities and Microbiological River Water Quality in the Paris. . .
341