Editing Seine (section)

==Water quality==
Due to concentrated levels of industry, agriculture and urban populations of Paris and its surroundings, the Seine-Normandy watershed experiences the highest human impacts of any hydrographic basin in France. Compared to most other large European rivers, the ability of the Seine to dilute urban sewage and farmland runoff is very low. Low oxygen levels, high concentrations of ammonia, nitrites and faecal bacteria, extending from Paris to the estuary, have been issues for over a century. The advent of nitrogenous fertilizers in the 1960s marked an upturn in agricultural pollution due to land use changes that had previously scaled with population growth. Heavy industries near Paris and along the Oise River discharged virtually untreated wastewaters from the turn of the 19th century, causing concentrations of toxins in the river that were ignored until the late 1980s. Major French laws to address water quality were passed in 1898, 1964, 1996, and 2006.<ref name="Flipo Lestel Labadie Meybeck 2020">{{cite book | last1=Flipo | first1=Nicolas | last2=Lestel | first2=Laurence | last3=Labadie | first3=Pierre | last4=Meybeck | first4=Michel | last5=Garnier | first5=Josette | title=The Handbook of Environmental Chemistry | chapter=Trajectories of the Seine River Basin | publisher=Springer International Publishing | publication-place=Cham | year=2020 | isbn=978-3-030-54259-7 | issn=1867-979X | doi=10.1007/698_2019_437 | pages=1–28}}</ref>

At the beginning of the 20th century, most domestic sewage was [[reuse of excreta|used as fertilizer]] for nearby croplands. As populations grew, the agricultural capacity to absorb those wastewaters was exceeded. Large-scale construction of waste water treatment plants (WWTPs) began in 1940 to meet demand; however, by 1970, about 60% of urban sewage was allowed to flow into the river untreated. The resulting oxygen depletion reduced the number of fish species to three. Measures taken in the early 2000s due to the [[Water Framework Directive]] led to significant reductions of organic carbon, phosphorus and ammonium, which in turn decreased the occurrence and severity of phytoplankton blooms. Continued WWTP construction and new treatment methods improved environmental conditions.<ref name="Garnier Marescaux Guillon Vilmin 2020">{{cite book | last1=Garnier | first1=J. | last2=Marescaux | first2=A. | last3=Guillon | first3=S. | last4=Vilmin | first4=L. | last5=Rocher | first5=V. | last6=Billen | first6=G. | last7=Thieu | first7=V. | last8=Silvestre | first8=M. | last9=Passy | first9=P. | last10=Raimonet | first10=M. | last11=Groleau | first11=A. | last12=Théry | first12=S. | last13=Tallec | first13=G. | last14=Flipo | first14=N. | title=The Handbook of Environmental Chemistry | chapter=Ecological Functioning of the Seine River: From Long-Term Modelling Approaches to High-Frequency Data Analysis | publisher=Springer International Publishing | publication-place=Cham | year=2020 | isbn=978-3-030-54259-7 | issn=1867-979X | doi=10.1007/698_2019_379 | pages=189–216}}</ref> In 2009, it was announced that [[Atlantic salmon]] had returned to the Seine.<ref>{{cite web |url=http://www.rfi.fr/actuen/articles/116/article_4724.asp |title=Radio France Internationale – Atlantic salmon return to river Seine |publisher=Rfi.fr |access-date=18 May 2011 |url-status=live |archive-url=https://web.archive.org/web/20110217143719/http://www.rfi.fr/actuen/articles/116/article_4724.asp |archive-date=17 February 2011}}</ref> By the early 2020s, the number of fish species near Paris had rebounded to 32.<ref name="Garnier Marescaux Guillon Vilmin 2020" />

Periodically the [[Paris sewers|sewage systems of Paris]] experience a failure known as [[sanitary sewer overflow]], often in periods of high rainfall. Under these conditions, untreated residential and industrial [[sewage]] is discharged into the Seine to prevent [[backflow]]. This is due in large part to Paris' "single system" drainage scheme dating from the 19th century, which combines street runoff and sewage.<ref>Martin Seidl, ''The fate of [[Organic material|organic]] matter in river Seine after a combined sewer overflow'', ENPC – University Paris Val de Marne Paris XII (France), 1997, 181 pp.</ref><ref name="Schofield 2023 f780">{{cite web | last=Schofield | first=Hugh | title=Paris to bring back swimming in Seine after 100 years | website=BBC News | date=25 July 2023 | url=https://www.bbc.com/news/world-europe-66238618 | archive-url=https://web.archive.org/web/20230727081452/https://www.bbc.com/news/world-europe-66238618 | archive-date=27 July 2023 | url-status=live | access-date=27 July 2023}}</ref> The resulting [[oxygen]] deficit is principally caused by [[allochthonous]] bacteria larger than one micrometre in size. The specific activity of these sewage bacteria is typically three to four times greater than that of the autochthonous (background) bacterial population. Heavy metal concentrations in the Seine are relatively high.<ref>J.F.Chiffoleau. 2007. ''Metal contamination. the Seine-Aval scientific programme''. Quae. 40 pages</ref> The pH level of the Seine at Pont Neuf has been measured to be 8.46. Despite this, the water quality has improved significantly over what several historians at various times in the past called an "open sewer".<ref>{{cite book |last=Hogan |first=C. Michael |title=Water quality of fresh water bodies in France |publisher=Luminna Press |location=Aberdeen |year=2006}}</ref>

In 2018, a €1.4 billion ($1.55 billion) cleanup programme called the "Swimming Plan" was launched with the aim of making the river safe to use for the [[2024 Summer Olympics]]. The project included constructing a basin to store rainwater, which would then be slowly released into the sewer system, preventing overflow. Plans also call for several public swimming areas to be made available by 2025, ending a ban instituted in 1923 due to the polluted water.<ref name="Jack Guy 2023 g211">{{cite web | first1=Jack |last1=Guy | first2=Dalal |last2=Mawad |first3=Oliver |last3=Briscoe | title=Paris to bring back swimming in River Seine after 100 years | website=[[CNN]] | date=26 July 2023 | url=https://www.cnn.com/2023/07/26/sport/paris-seine-river-swimming-spt-scli-intl/index.html | archive-url=https://web.archive.org/web/20230727040222/https://www.cnn.com/2023/07/26/sport/paris-seine-river-swimming-spt-scli-intl/index.html | archive-date=27 July 2023 | url-status=live | access-date=27 July 2023}}</ref> These efforts have produced mixed results, as ''[[Escherichia coli|E. coli]]'' levels have often been found to be far higher than what is safe to swim in, though this could depend on the season.<ref>{{Cite news |last=Hartley |first=Noemie Bisserbe and Eve |title=Herculean Feat in Paris Olympics: Make the Seine Safe to Swim |url=https://www.wsj.com/world/olympics-paris-swimming-seine-sewage-944b33fa |access-date=2024-04-18 |work=WSJ |language=en-US}}</ref><ref name="Guy" /> At the same time, the fish population in the river has surged, from just two species to over 30.<ref name="Guy">{{Cite web |last1=Guy |first1=Jack |last2=Mawad |first2=Dalal |last3=Briscoe |first3=Oliver |date=2023-07-26 |title=Paris to bring back swimming in River Seine after 100 years |url=https://www.cnn.com/2023/07/26/sport/paris-seine-river-swimming-spt-scli-intl/index.html |access-date=2024-04-18 |website=CNN |language=en}}</ref>  To demonstrate the river's improved cleanliness, Mayor [[Anne Hidalgo]] and President [[Emmanuel Macron]] both pledged to take a swim in the waters,<ref>{{Cite news |last=Bush |first=Evan |date=2024-06-21 |title=Levels of contamination in the Seine remain unsafe for Paris Olympians, report says |url=https://www.nbcnews.com/science/environment/paris-olympics-river-seine-contamination-still-unsafe-rcna158381 |access-date=2024-06-23 |work=NBC News |language=en}}</ref> and Hidalgo did so on July 17, 2024.<ref>{{Cite web |last=Nouvian |first=Tom |date=2024-07-17 |title=Paris mayor dips into the Seine River to showcase its improved cleanliness before Olympic events |url=https://apnews.com/article/paris-olympics-river-seine-swim-clean-pollution-b1e7330d778babc070bc7348a8c998f3 |access-date=2024-08-18 |website=AP News |language=en}}</ref>

During the Summer Olympics, the date of the triathlon was postponed due to water quality issues,<ref>{{Cite news|url=https://thenationaldesk.com/news/from-the-desk/paris-olympics-postpone-mens-triathlon-due-to-poor-seine-river-water-quality-swimming-polluted-waterway-cleanup-effort-e-coli-bacteria-marathon-medical-team-mayor-anne-hidalgo|title=Paris Olympics postpone men's triathlon due to poor Seine River water quality|first1=Pat|last1=Graham|first2=Kate|last2=Brumback|date=2024-07-30|work=The National Desk|agency=Associated Press}}</ref> as the earlier rainstorm during the opening ceremony had driven some untreated rainwater back into the Seine.<ref>{{Cite magazine |last=Massy-Beresford |first=Helen |title=No, the Seine Cleanup Wasn't a Failure |url=https://www.wired.com/story/no-the-seine-cleanup-wasnt-a-failure/ |access-date=2024-08-14 |magazine=Wired |language=en-US |issn=1059-1028}}</ref> However, the triathlon proceeded the following day, after testing found the water quality to be sufficient for swimming.<ref>{{cite web | url=https://www.france24.com/en/europe/20240731-paris-olympics-triathlon-seine | title=Women's triathlon begins at Paris Games after tests confirm Seine water quality | date=31 July 2024 }}</ref>