During the 1950s to the 1980s, water quality deteriorated very rapidly, and the
maximum annual geometric mean of concentrations (5 Â 10
5 E. coli/100 mL) was
reached in 1985. This accelerated quality degradation is related, on the one hand, to a
significant increase in the urban population upstream from Paris (the urban population quadrupled between the 1950s and the 1980s [32]) and, on the other hand, to a
slow implementation of domestic sewage treatment plants. After the mid-1980s,
there was a very rapid and significant improvement of the microbiological quality at
the Ivry sampling site. This is partly due to the commissioning of the Seine Amont
WWTP located 4 km upstream from Ivry. The geometric means of E. coli concentrations continued to decrease until the beginning of the 2000s (Fig. 5). This
improvement was first related to the widespread installation of new WWTPs and
to subsequent improvement of their treatment processes. Since 2000, the microbiological quality has been quite stable, with annual geometric means around 3–5 Â 10
3
E. coli/100 mL.
The evolution of the annual geometric mean of E. coli in the Seine River
downstream from Paris at Conflans is presented in Fig. 6. The Conflans sampling
station is located just upstream from the confluence of the Seine River with the Oise
River. This station is situated downstream from the outfall of the effluents of the
Seine Aval (SAV) WWTP, which today treats 70% of the wastewaters from the Paris
agglomeration. During the entire 1911–1980 period, the E. coli concentrations were
always significantly higher in Conflans compared to the concentrations measured in
Ivry. The difference between the two stations was maximum in the 1950s (with
annual geometric means sometimes two log units higher in Conflans than in Ivry).
Unfortunately, data are not available for the period between 1980 and 2000, which
probably displayed the highest contamination levels in Conflans. In recent years
(since 2000), the E. coli concentrations have continued to decrease, with geometric
means between 5 Â 10
3 and 10
4 E. coli/100 mL. This can be attributed to the major
improvements of the treatment processes in the SAV WWTP [33] and also to a
reduction in the volume of effluents discharged by combined sewer overflows
(CSOs) in the Seine River downstream from Paris (see the next section).
5 Sources of Fecal Contamination
When assessing the contamination of natural aquatic ecosystems by microorganisms
of fecal origin, it is usual to distinguish point sources from nonpoint sources, also
called diffuse sources. Microbial contaminants brought by surface runoff and soil
leaching constitute the diffuse sources, which also include contamination linked to
leakage from sanitation sewers and septic tanks and also direct defecation of certain
animals (e.g., cows, waterfowl) in surface waters. The outfalls of raw urban wastewater, domestic WWTP effluents, industrial effluents, and urban runoff waters
collected in separate sewer systems and CSOs constitute the point sources.
The inputs of fecal microorganisms via nonpoint sources are not easy to quantify.
To assess the contribution of nonpoint sources to river fecal contamination,
Bathing Activities and Microbiological River Water Quality in the Paris. . .
333
maximum annual geometric mean of concentrations (5 Â 10
5 E. coli/100 mL) was
reached in 1985. This accelerated quality degradation is related, on the one hand, to a
significant increase in the urban population upstream from Paris (the urban population quadrupled between the 1950s and the 1980s [32]) and, on the other hand, to a
slow implementation of domestic sewage treatment plants. After the mid-1980s,
there was a very rapid and significant improvement of the microbiological quality at
the Ivry sampling site. This is partly due to the commissioning of the Seine Amont
WWTP located 4 km upstream from Ivry. The geometric means of E. coli concentrations continued to decrease until the beginning of the 2000s (Fig. 5). This
improvement was first related to the widespread installation of new WWTPs and
to subsequent improvement of their treatment processes. Since 2000, the microbiological quality has been quite stable, with annual geometric means around 3–5 Â 10
3
E. coli/100 mL.
The evolution of the annual geometric mean of E. coli in the Seine River
downstream from Paris at Conflans is presented in Fig. 6. The Conflans sampling
station is located just upstream from the confluence of the Seine River with the Oise
River. This station is situated downstream from the outfall of the effluents of the
Seine Aval (SAV) WWTP, which today treats 70% of the wastewaters from the Paris
agglomeration. During the entire 1911–1980 period, the E. coli concentrations were
always significantly higher in Conflans compared to the concentrations measured in
Ivry. The difference between the two stations was maximum in the 1950s (with
annual geometric means sometimes two log units higher in Conflans than in Ivry).
Unfortunately, data are not available for the period between 1980 and 2000, which
probably displayed the highest contamination levels in Conflans. In recent years
(since 2000), the E. coli concentrations have continued to decrease, with geometric
means between 5 Â 10
3 and 10
4 E. coli/100 mL. This can be attributed to the major
improvements of the treatment processes in the SAV WWTP [33] and also to a
reduction in the volume of effluents discharged by combined sewer overflows
(CSOs) in the Seine River downstream from Paris (see the next section).
5 Sources of Fecal Contamination
When assessing the contamination of natural aquatic ecosystems by microorganisms
of fecal origin, it is usual to distinguish point sources from nonpoint sources, also
called diffuse sources. Microbial contaminants brought by surface runoff and soil
leaching constitute the diffuse sources, which also include contamination linked to
leakage from sanitation sewers and septic tanks and also direct defecation of certain
animals (e.g., cows, waterfowl) in surface waters. The outfalls of raw urban wastewater, domestic WWTP effluents, industrial effluents, and urban runoff waters
collected in separate sewer systems and CSOs constitute the point sources.
The inputs of fecal microorganisms via nonpoint sources are not easy to quantify.
To assess the contribution of nonpoint sources to river fecal contamination,
Bathing Activities and Microbiological River Water Quality in the Paris. . .
333