Coastal-water hypoxia is increasing globally due to global warming and
urbanization, and the need to define management solutions to improve the
water quality of coastal ecosystems has become important. The lower tidal
Garonne River (TGR; southwestern France), characterized by the seasonal
presence of a turbidity maximum zone (TMZ) and urban water discharge, is
subject to episodic hypoxia events during low river flow periods in the
summer. Future climatic conditions (higher temperature and summer droughts) and
increasing urbanization could enhance hypoxia risks near the city of
Bordeaux in the coming decades. A 3-D model of dissolved oxygen (DO) that
couples hydrodynamics, sediment transport and biogeochemical processes was
used to assess the efficiency of different management solutions for
oxygenation of the TGR during summer low-discharge periods. We ran different
scenarios of reductions in urban sewage overflows, displacement of urban
discharges downstream from Bordeaux and/or temporary river flow support
during the summer period. The model shows that each option mitigates hypoxia,
but with variable efficiency over time and space. Sewage overflow reduction
improves DO levels only locally near the city of Bordeaux. Downstream
relocation of wastewater discharges allows for better oxygenation levels in
the lower TGR. The support of low river flow limits the upstream TMZ
propagation and dilutes the TGR water with well-oxygenated river water.
Scenarios combining wastewater network management and low-water
replenishment indicate an improvement in water quality over the entire TGR.
These modelling outcomes constitute important tools for local water
authorities to develop the most appropriate strategies to limit hypoxia in
the TGR. A 3-D model shows different efficiencies of management actions to limit hypoxia. Downstream relocation of wastewater discharge totally mitigates hypoxia. Sewage overflow reduction improves DO levels but only locally. Water replenishment improves DO in the upper estuary.
Hypoxia refers to low dissolved-oxygen (DO) conditions when concentrations
fall below 2 mg L
In an urban tidal river, the first obvious action to mitigate hypoxia is to
improve the urban wastewater network and treatment and to reduce the input
of organic matter and nutrients to the estuary. In several European
estuaries suffering from urban input, water quality improvement was
achieved by the installation and renovation of a wastewater treatment plant
(WWTP) in the Thames Estuary in the 1980s (Andrews and Rickard, 1980;
Tinsley, 1998) and the construction of a WWTP in the Seine River in the
1990s (Billen et al., 2001). In the Scheldt Estuary, sewage network
improvement reduced N, P and Si loads by 5.4 %, 1.3 % and 1 %,
respectively, and two WWTPs have been implemented for the city of Brussels
since 2000 (Billen et al., 2005; Soetaert et al., 2006; Vanderborght et al.,
2007). Sewage network systems in Europe usually combine both urban sewage
and stormwater collection. During heavy rain and storm events, the capacity
of the urban wastewater network is generally insufficient for treating all
effluents, inducing deoxygenation events due to untreated wastewater release
from sewage overflow (SO; Even et al., 2007). In the 2000s, the
Environmental Protection Agency promoted a strategy to monitor urban
drainage networks in real time to regulate flow and avoid the overflow of
untreated wastewater (EPA, 2006; Gonwa and Novotny, 1993). This control was developed in
several cities in the USA (Gonwa and Novotny, 1993) as well as in Québec (Pleau et al., 2005) and
Tokyo (Maeda et al., 2005). An additional management solution was tested in
the Thames Estuary: the construction of a 24 km long sewer network under the
riverbed that allows the transit of urban wastewater to the WWTP located
downstream (Thames Tideway Tunnel:
In macrotidal estuaries, the lowest DO concentrations occur during the lowest river flow (Lanoux et al., 2013; Talke et al., 2009; Zhang et al., 2015). A second possible action could therefore be to modify river discharges and to reduce water flushing time to promote the dilution by well-oxygenated water and/or the seaward dispersion of oxygen-consuming matter (Lajaunie-Salla et al., 2018). This implies providing water replenishment above critical levels by limiting water abstraction for irrigation in the watershed or by modulating water release from dams when hypoxia is present (Schmidt et al., 2017).
To optimize preventive management strategy, the efficiency of the potential solutions needs to be evaluated. Therefore, numerical modelling is an efficient tool to quantitatively assess hypoxia mitigation by management scenarios. Moreover, models provide guidelines for setting objectives to maintain good water quality in coastal environments (Kemp et al., 2009; Skerratt et al., 2013).
A recently developed 3-D coupled hydro-sedimentary–biogeochemical DO model simulated possible scenarios for the coming decades, suggesting a future spatial and temporal extension of summer hypoxia in the tidal Garonne River (TGR; SW France), an urban, turbid tidal river (Lajaunie-Salla et al., 2018). Until now in the TGR, only a few hypoxia events have been reported, for example, during summer 2006 (Lanoux et al., 2013). Previous work highlighted that these low DO levels are due to the combination of the presence of the TMZ, high water temperature, drought periods and urban effluent input (Lajaunie-Salla et al., 2017; Lanoux et al., 2013; Schmidt et al., 2017). Such a perspective of permanent summer hypoxia in the lower TGR implies the need to develop management strategies to protect the ecosystem. The aim of the present work was to assess the efficiency of possible management solutions to limit future hypoxia risk in the tidal Garonne River. For this purpose, we applied the aforementioned DO model in order to simulate scenarios based on two main management actions: optimization of the urban wastewater network and freshwater replenishment during low-water periods.
The Garonne River, located in southwestern France, is the main tributary of
the Gironde Estuary, which is formed by its confluence with the Dordogne
River and flows toward the Atlantic Ocean (Fig. 1). This macrotidal
fluvio-estuarine system is characterized by the presence of a TMZ, where
suspended sediment concentrations in surface water are
The Gironde–Garonne–Dordogne estuary, including the tidal Garonne River in southwestern France (Inset B). “KP” denotes the distances in kilometres from the city centre of Bordeaux; the control grid cell at Bordeaux is at KP4, and Portets is at KP20. Inset A indicates position of the sewage overflows (purple triangles) and of the two wastewater treatment plants (green squares). The area in orange represents the area of Bordeaux for which the biogeochemical fluxes were calculated.
The annual mean Garonne River flow is 680 m
The large city of Bordeaux is located at the border of the tidal Garonne
River, 25 km upstream of the confluence (Bec d'Ambès; Fig. 1). The
sewage system of the metropolis drains an urban area of 578 km
The Bordeaux metropolis has already taken several actions to improve the
urban wastewater network. In 2011, the WWTP Louis Fargue was resized and
upgraded to the treatment effectiveness of the WWTP Clos de Hilde. In
addition, since 2013, real-time control of the urban drainage network was
developed to reduce urban effluents during rainy weather (Andréa et al.,
2013). This system decreased the volume of untreated wastewater released by
30 % in 2013 and by 40 % in 2014 and 2015 (Robitaille et al., 2016),
improving the overall net purification efficiency to
The SiAM-3D model, which couples hydrodynamics, suspended sediment transport
and biogeochemical processes (Lajaunie-Salla et al., 2017), was used to test
the efficiency of possible management solutions. The model was implemented
for the Gironde Estuary from the 200 m isobath on the continental shelf to
the upstream limits of the tidal propagation on both rivers (Sottolichio et al., 2000). The mesh of
the model is an irregular grid, with a finer resolution in the estuary (200 m
The transport model solves the advection–dispersion equations for dissolved
and particulate variables, i.e. suspended sediment, salinity and
biogeochemical variables. The biogeochemical model extensively resolves the
processes that produce and consume oxygen in the water column, taking different types of dissolved and particulate organic matter into
account:
degradation of organic matter (mineralization of organic carbon and
ammonification using the
The model was compared with data available for the TGR and tested on the
basis of three criteria: (i) the ability to reproduce the observed DO
variability at a seasonal scale, (ii) the ability to reproduce the
spring-neap tidal cycle and (iii) a statistical evaluation based on the
Willmott skill score (WSS; Willmott, 1982). In brief, the
model performed well (WSS
Time series of Garonne River (black) and Dordogne River (grey) flow of the
reference simulation (
In this work, we want to demonstrate the advantage and/or effectiveness of
urban water networks and treatment processes for limiting hypoxia events
during critical conditions. The reference simulation is based on the real
conditions of 2006, which was a critical year from the point of view of
river discharge, temperature and hypoxia. A 21 d heat wave occurred, and
the summer water temperature reached a maximum of 29.5
Several scenarios have been designed to assess the efficiency of the retained management strategies to improve the DO levels of the tidal Garonne River (Table 1): optimization of the urban wastewater network and water replenishment during low-water periods.
Forcing of the different scenarios simulated with the
model (
Minimum simulated DO (in % saturation and in
mg L
Two main actions of wastewater management were simulated (Table 2).
low-intensity and long-term support (LTS) from 15 July by 10, 20 and 30 m intense and short-term support (STS) as an emergency solution by 100, 200 and 400 m LTS of 10 m LTS of 10 m
For the support of low river flow during the driest season, two actions were
tested according to the maximum stored water volume in the dams (58 hm
Finally, two scenarios that coupled wastewater management actions and the
support of low river flow were simulated (Table 2).
The 16 scenarios were run over 10 months, from 1 January to 31 October. To
evaluate the improvement of DO level, three indicators were used: (i) the
minimum DO value (DO
Differences (in %) of biogeochemical process rates impacting DO between the scenarios and reference simulations during summer in Bordeaux and Portets (WW: wastewater; WS: watershed).
The simulations of sewage overflow reduction do not show an increase in
DO
In these simulations, sudden wastewater release events from SO (late June) did not occur simultaneously with the maximum temperature (i.e. late July). In such a case, a more critical hypoxia event would have occurred. However, the modelling results show that the improvement of SO management contributes to improving the DO level only locally and temporarily in the vicinity of the city of Bordeaux.
In the case of a relocation of urban effluent discharge at KP15, only 4 d
of hypoxia were simulated with a minimum of 1.8 mg L
Snapshot of the vertical transect of simulated DO
saturation along the tidal Garonne river for the scenarios with urban
effluent discharge points in Bordeaux
A downstream relocation (KP15 or KP25) significantly decreases total DO consumption in the lower TGR by 33 % and 47 %, respectively: the mineralization of urban matter is reduced by 65 % and 95 %, and the nitrification is reduced by 47 % and 69 %, respectively (Table 3). At Portets, even if the total DO consumption decreases only by 8 %, the degradation of urban matter decreases strongly by 76 % and 94 % and the nitrification is reduced by 17 % (KP15) and 20 % (KP25; Table 3). In fact, the mineralization of urban matter occurs downstream of TGR, with less impact on the DO in this area, thanks to the dilution effect with estuarine water. Finally, at Bordeaux, the contribution of urban effluents to the DO consumption decreases from 27 % to 2 %, and nitrification decreases from 20 % to 10 % (Fig. 3d).
The discharge of the wastewater downstream from the city centre considerably improves the water quality in the vicinity of Bordeaux. However, hypoxia persists in Portets (30 hypoxic days; Table 2 and Fig. 3) because in the upper TGR, hypoxia is mainly due to temperature, very high turbidity and low-water renewal.
The simulations of low-intensity and long-term support of water flow show an
increase in the DO
Significant effects of maintaining summer river discharge in the area of Bordeaux are reflected by the decrease in nitrification processes and the increase in mineralization of matter coming from the watershed (Table 3). At Portets, nitrification and mineralization of organic matter are decreased due to the diluted input of urban water upstream (Table 3).
These simulations show that low-intensity and long-term support of river
flow considerably reduces hypoxia events in the upper TGR but not
enough to significantly influence Bordeaux water. The average time to
renew half of the water volume in Bordeaux is 22 and 67 d in the cases of
river flows increased by 10 and 30 m
Snapshot of the vertical transect of simulated DO
concentration (in % saturation) along the tidal Garonne river for the scenarios of
reference
Time series of river flow (
Intense and short-term support of freshwater allows low-oxygenated water
to be pushed downstream and induces a strong dilution of estuarine water
with well-oxygenated fluvial water due to the large amount of water supply
(100, 200 and 400 m
The total oxygen consumption decreases with STS only at Portets (Table 3). At Bordeaux, the decrease in nitrification is counterbalanced by an increase in river organic matter mineralization (Table 3). The intense short-term support moves the TMZ downstream to Portets, reducing organic matter mineralization in the area of Portets (Table 3 and Fig. 4).
The intense short-term support of freshwater (400 m
Summary of management solution efficiency and recommendations (WW: wastewater; WS: watershed). The level of efficiency of each solution is detailed for the lower and upper TGR according to the scale: low (+), moderate (++) and high (+++).
These different simulated scenarios allow us to quantitatively estimate the
efficiency of different management options to reduce hypoxia in the TGR. The
two management solutions have locally different impacts on DO (Table 4):
optimization of the urban wastewater network reduces hypoxia in the lower
TGR, whereas water replenishment during low-water periods enhances DO levels
in the upper TGR. The improvement of the wastewater network by a reduction
in labile organic matter input reduces oxygen consumption in Bordeaux
water. The alternative, consisting of discharging urban effluents
downstream of the lower TGR, has the advantage of diluting wastewater with
the Gironde water and favouring their dispersion downstream in the wider
sections of the estuary. In addition, taking the increasing
gradient of temperature landward into account (Sabine Schmidt, personal data, 2016), wastewater
effluents would be discharged in cooler water (approximately 1–2
Spatiotemporal evolution of daily average surface DO
(saturation in %) along the tidal Garonne River section for the scenarios
of reference
Regarding the projected population growth of the city of Bordeaux (1 million inhabitants will be reached in 2030, i.e.
A 3-D biogeochemical model for the tidal Garonne River coupling hydrodynamics and sediment transport was applied to assess the efficiency of different management solutions to improve the DO level in water. This study tested different scenarios of management solutions that can be implemented by local water authorities. Whereas a reduction in SO flows contributes only to improving DO levels locally and temporarily, the downstream relocation of WWTP outfalls totally mitigates hypoxia in the TGR and seems to be the most efficient management solution despite being difficult to implement in practice. The support of low river flow limits the propagation of the TMZ upstream of the TGR and dilutes the estuarine water with fresh oxygenated water. Low-intensity support over the summer maintains a good oxygen level of water during the entire drought period and prevents hypoxia in the upper TGR. In contrast, intense support of low water flow for 3 d improves the oxygen levels along the entire TGR quickly and considerably but only for a few weeks. The improvement in the urban effluent network and the support of low-water periods from dams or irrigation reduction are complementary. They contribute to reoxygenating the river water near the city of Bordeaux and upstream of the tidal Garonne River. The biogeochemical numerical model helps in guiding the management policy of urban effluents and watersheds to limit and mitigate hypoxia events.
References and/or websites of in situ data that we used for this work are mentioned throughout the text. The outputs of the different simulations are not stored in a publicly accessible database due to their large size. They are however available from the corresponding author upon request.
KLS designed the model, analysed the data and wrote the paper. AS, GA and SS contributed to the design and implementation of the research, to the analysis of the results, and to the writing of the paper. XL and GB supervised the findings of this work.
The authors declare that they have no conflict of interest.
This study was funded by the Aquitaine region (DIAGIR project) and LyRE (SUEZ research centre), who cosponsored a PhD grant to Katixa Lajaunie-Salla, and was supported by the Cluster of Excellence COTE at the Université de Bordeaux (ANR-10-LABX-45). The authors are grateful to the MAGEST network for the availability of data and to the SGAC and Bordeaux Métropole for providing urban effluent data and for fruitful discussions. This work was supported by the Avakas cluster resources of the Mésocentre de Calcul Intensif Aquitain (MCIA) of the Université de Bordeaux.
This study was funded by the Aquitaine region (DIAGIR project) and LyRE (SUEZ research centre).
This paper was edited by Mauricio Gonzalez and reviewed by three anonymous referees.