title: Estimation of the water quality of a large urbanized river as defined by the European WFD: what is the optimal sampling frequency?
abstract: Assessment of the quality of freshwater bodies is essential to determine the impact of human activities on water resources. The water quality status is estimated by comparing indicators with standard thresholds. Indicators are usually statistical criteria that are calculated on discrete measurements of water quality variables. If the time step of the measured time series is not sufficient to fully capture the variable's variability, the deduced indicator may not reflect the system's functioning. The goal of the present work is to assess, through a hydro-biogeochemical modeling approach, the optimal sampling frequency for an accurate estimation of 6 water quality indicators defined by the European Water Framework Directive (WFD) in a large human-impacted river, which receives large urban effluents (the Seine River across the Paris urban area). The optimal frequency depends on the sampling location and on the monitored variable. For fast varying compounds that originate from urban effluents, such as PO[Formula: see text], NH[Formula: see text] and NO[Formula: see text], a sampling time step of one week or less is necessary. To be able to reflect the highly transient character of bloom events, chl a concentrations also require a short monitoring time step. On the contrary, for variables that exert high seasonal variability, as NO[Formula: see text] and O2, monthly sampling can be sufficient for an accurate estimation of WFD indicators in locations far enough from major effluents. Integrative water quality variables, such as O2, can be highly sensitive to hydrological conditions. It would therefore be relevant to assess the quality of water bodies at a seasonal scale rather than at annual or pluri-annual scales. This study points out the possibility to develop smarter monitoring systems by coupling both time adaptative automated monitoring networks and modeling tools used as spatio-temporal interpolators.
authors.affiliation: Geosciences Department, MINES ParisTech, PSL Research University, 35 rue Saint Honoré, 77305, Fontainebleau, France. l.m.vilmin@uu.nl.|Department of Earth Sciences - Geochemistry, Faculty of Geosciences, Utrecht University, P.O. Box 80021, 3508TA, Utrecht, The Netherlands. l.m.vilmin@uu.nl.
authors.forename: Lauriane
authors.initials: L
authors.lastname: Vilmin
authors.affiliation: Geosciences Department, MINES ParisTech, PSL Research University, 35 rue Saint Honoré, 77305, Fontainebleau, France.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Flipo
authors.affiliation: Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, 75005, Paris, France.|Stream Biofilm and Ecosystem Research Laboratory, École Polytechnique Fédérale de Lausanne (EPFL), Station 2, CH-1015, Lausanne, Switzerland.
authors.forename: Nicolas
authors.initials: N
authors.lastname: Escoffier
authors.affiliation: Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ Paris Diderot, UMR 7154 CNRS, 75005, Paris, France.
authors.forename: Alexis
authors.initials: A
authors.lastname: Groleau
mesh_terms: D064751:Ammonium Compounds / Q000032:analysis; D000077194:Chlorophyll A / Q000032:analysis; D004784:Environmental Monitoring / Q000379:methods* / Q000592:standards; D058735:Environmental Policy; D005602:France; D006801:Humans; D062070:Hydrology / Q000379:methods; D008962:Models, Theoretical; D009566:Nitrates / Q000032:analysis; D010100:Oxygen / Q000032:analysis; D010297:Paris; D010710:Phosphates / Q000032:analysis; D045483:Rivers / Q000737:chemistry*; D014507:Urbanization; D014874:Water Pollutants, Chemical / Q000032:analysis; D060753:Water Quality
chemical_list: D064751:Ammonium Compounds; D009566:Nitrates; D010710:Phosphates; D014874:Water Pollutants, Chemical; D010100:Oxygen; D000077194:Chlorophyll A
keywords: Chlorophyll a; European water framework directive; Hydro-biogeochemical modeling; Inorganic nitrogen; Optimal sampling frequency; Orthophosphate; Oxygen; River water quality assessment