Tracking the Turbidity Maximum Zone in the Gironde estuary (SW France) based on continuous monitoring and radionuclides

Transcription

1 Tracking the Turbidity Maximum Zone in the Gironde estuary (SW France) based on continuous monitoring and radionuclides Sabine Schmidt, A. Sottolichio, H. Derriennic, G. Chabaud, P. Lebleu, H.-K. Saari, P. Castaing UMR 585 EPOC, CNRS Univ. Bordeaux

2 The Gironde estuary (S-W France) the largest estuary of the European Atlantic coast 17 km long 625 km 2 surface area mean discharge : 1 m 3 s -1 two main tributaries : Garonne / Dordogne draining a watershed area of 71 km 2 transitional waters important as nursery for juvenile and a pathway for migratory fishes (eel, salmon, sturgeon)

3 The Gironde estuary (S-W France) a macrotidal estuary asymmetrical propagation of tidal wave + length of the estuary long residence times of water (2 86 days) fine particles ( > 1 year) depending of river discharges a highly concentrated turbidity maximum zone (TMZ) particle concentration > 1 g L -1 in surface waters

4 The Gironde estuary (S-W France) basic mechanisms of the TMZ formation and dynamics well known (Allen and Castaing 1973; Castaing and Allen, 1981) PK1 the TMZ moves along the estuary axis according to the water discharge PK8 PK6 PK4 PK2 Bordeaux PK PK-7

5 The Gironde estuary (S-W France) basic mechanisms of the TMZ formation and dynamics well known (Allen and Castaing 1973; Castaing and Allen, 1981) PK1 Flood the TMZ moves along the estuary axis according to the water discharge PK8 PK6 PK4 PK2 Bordeaux PK PK-7 Estuarine particulate material is flushed in the Bay of Biscay under extreme hydrological conditions, including: series of floods, high water discharges (> 3, m 3 s -1 ) and spring tides

6 The Gironde estuary (S-W France) basic mechanisms of the TMZ formation and dynamics well known (Allen and Castaing 1973; Castaing and Allen, 1981) PK1 the TMZ moves along the estuary axis according to the water discharge PK8 PK6 PK4 PK2 Bordeaux PK Low water PK-7

7 The Gironde estuary (S-W France) basic mechanisms of the TMZ formation and dynamics well known (Allen and Castaing 1973; Castaing and Allen, 1981) PK1 Flood the TMZ moves along the estuary axis according to the water discharge PK8 PK6 PK4 PK2 Bordeaux PK Low water PK-7 but a lack of knowledge about: - the TMZ occurrence in the fluvial section - seasonal trends of TMZ in response to long-term changes on hydrology.

8 The Gironde estuary (S-W France) changes Over the last decades, there is a marked decrease of the annual mean discharge of the Gironde: 15 m 3 s Annual mean discharge of the Gironde

9 The Gironde estuary (S-W France) changes Over the last decades, there is a marked decrease of the annual mean discharge of the Gironde: 15 m 3 s Annual mean discharge of the Gironde The Garonne is the main tributary of the Gironde. There are also evidences of recent changes in Garonne discharge: an increase in the duration of low-water periods a decrease in flood intensity. Number of days the Garonne discharge, during the period , is: number of days yr number of days yr -1 3 < 1 m 3 s m 3 s -1 number of days yr (biennal flood) number of days yr -1 3

10 The Gironde estuary (S-W France) changes Over the last decades, there is a marked decrease of the annual mean discharge of the Gironde: 15 m 3 s Annual mean discharge of the Gironde The Garonne is the main tributary of the Gironde. There are also evidences of recent changes in Garonne discharge: an increase in the duration of low-water periods a decrease in flood intensity. Number of days the Garonne discharge, during the period , is: number of days yr number of days yr -1 3 < 1 m 3 s m 3 s -1 number of days yr (biennal flood) number of days yr -1 3 Are these changes likely to affect notably the presence of the turbidity maximum zone in the tidal river, at a distance > 1 km of the mouth of the Gironde estuary?

11 The Gironde estuary (S-W France) changes highly impacted by human activities: > 6 hydropower dams in the upper watershed land use, agriculture heavy metal pollution (Cd, former mining and smelting activities upstream) prohibiting shellfish production hypoxia events close to Bordeaux in summer during low water periods increasing occurrence of TMZ in the fluvial section changes in dredging

12 The Gironde estuary (S-W France) changes highly impacted by human activities: > 6 hydropower dams in the upper watershed land use, agriculture heavy metal pollution (Cd, former mining and smelting activities upstream) prohibiting shellfish production hypoxia events close to Bordeaux in summer during low water periods increasing occurrence of TMZ in the fluvial section changes in dredging growing interests in understanding the mechanisms controlling the fate of particles and of associated contaminants in this watershed in evaluating the long-term evolution of particulate export to the coastal ocean.

13 Different strategies to investigate the Gironde estuary to improve the knowledge of the estuary functioning, encompassing the aspects of hydrology, sediment dynamics and biogeochemistry. - Long-term dataset: a automatic continuous monitoring network (MAGEST) has been implemented in the Gironde estuary in 25; - Use of radionuclides to estimate particle residence time

14 The MAGEST network: 4 automated stations that measure each 1 mn: - temperature - salinity - turbidity - dissolved oxygen in surface waters In the central estuary (1) In the fluvial sections (3)

15 The MAGEST network: data transferred each 6 hours to a website of the University of Bordeaux

16 Data at different timescales: semi-diurnal tidal cycles Tidal cycles Etcheber et al (211). Monitoring water quality in estuarine environments. Hydrol. Earth Syst. Sci.

17 Data at different timescales: neap-spring cycles Lanoux et al, (213). Factors contributing to hypoxia in a highly turbid, macrotidal estuary (the Gironde, France). Environ. Sci.: Processes Impacts, 213, 15,

18 Data at different timescales: neap-spring cycles Lanoux et al, (213). Factors contributing to hypoxia in a highly turbid, macrotidal estuary (the Gironde, France). Environ. Sci.: Processes Impacts, 213, 15,

19 Data at different timescales: seasonal changes (daily-mean) m 3 s -1 2 Discharge Garonne Dordogne 1 1/1/213 1/4/213 1/7/213 1/1/213 1/1/ Pauillac Libourne Bordeaux Turbidity NTU /1/213 1/4/213 1/7/213 1/1/213 1/1/214 Schmidt (213). MAGEST newsletter.

20 Radionuclides as tracers of particle dynamics in aquatic system Simplified cycle of 7 Be and 21 Pb 7 Be (53 days) and 21 Pb (T 1/2 = 22.3 years) produced in the atmosphere Adsorption and erosion λ λ λ short half-life strongly bound to particulate matter appropriate tracers to study shortterm particle transport

21 Particulate 7 Be and 21 Pb xs against salinity Monthly survey in All data (surface, bottom waters) 4 7 Be (mbq g -1 ) Pb xs (mbq g -1 ) Salinity PK3 PK5 PK67 PK87

22 Particule transfer along the fluvio-estuarine system of the Gironde 7 Be/ 21 Pb xs 14, 1,5 Low tide surface fluvial estuary Min-Max mean central 7,7 3,5, LR POR LIB PK3 PK11 PK5 PK87 PK Distance from Bordeaux (km)

23 Model to derive age or residence time of particles from 7 Be / 21 Pb xs Activity Ratio (AR) (Matisoff et al, 25) Changes in AR = - Age of suspended particles - Mixing of «young» 7 Be particle with old ( 7 Bedeficient) resuspended particles * * * * * 7 Be/ 21 Pb =14 7 Be/ 21 Pb xs 5 Age (days) % old particles fraction % 7 Be/ 21 Pb particules 7 Be/ 21 Pb reference

24 Particule transfer along the fluvio-estuarine system of the Gironde * * * * * 7 Be/ 21 Pb = 14 5 Low tide surface Min-Max mean 7 Be/ 21 Pb xs Age 4 (days) LR ( ) POR fluvial LIB central estuary PK67 PK5 PK3 PK87 PK11 Particle age days Distance from Bordeaux (km)

25 Occurrence of TMZ upstream and river discharge: 4 m 3 s -1 TMZ River discharge (daily-mean) and turbidity (dayly mean) at Bordeaux for two contrasted hydrological situtations Jan Avril Jan Avril NTU Jan Avril Jan Avril

26 Occurrence of TMZ upstream and river discharge: TMZ NTU Water level (m) River discharge (daily-mean) and turbidity (daily mean) at Bordeaux for two contrasted hydrological situations Record of a large flood in June 2, decembre 1981 m 3 s mars NTU Jan Avril Jan Avril june 5 june Bordeaux Turbidity Raw data 1 Jan Avril Jan Avril 1 june 5 june

27 NTU 1k TMZ Turbidity trend in the Gironde estuary and seasonal change in TMZ position Monthly average turbidity (quartiles) at the four stations of the MAGEST network for the period from January 25 to October 213 the Dordogne Tidal River Libourne 1% 5k 1k Central estuary Pauillac 75% 5% % Jan Apr Jul Oct NTU 5k Jan Apr Jul Oct NTU 1k 5k Bordeaux the Garonne Tidal River NTU 1k 5k Portets Jan Apr Jul Oct Jan Apr Jul Oct 52 km 1 km 12 km distance from the mouth of the Gironde estuary

28 NTU SEDNET November 213, Lisbon Portugal TMZ Occurrence of TMZ upstream and river discharge: Turbidity against discharge for the period from January 25 to October 213 the Dordogne Tidal River 1 Libourne 1 1 Central estuary 1 1 NTU Daily-discharge (m 3 s -1 ) 1 1 the Garonne Tidal River 1 1 NTU 1 NTU 1 Pauillac Daily-discharge (m 3 s -1 ) Flood erosion Bordeaux Portets Daily-discharge (m 3 s -1 ) Daily-discharge (m 3 s -1 ) 52 km 1 km 12 km distance from the mouth of the Gironde estuary

29 NTU SEDNET November 213, Lisbon Portugal TMZ Occurrence of TMZ upstream and river discharge: Turbidity against discharge for the period from January 25 to October 213 the Dordogne Tidal River 1 Libourne 1 NTU 1 1 Central estuary The threshold to move TMZ downstream ranges between 1 and 2 m 3 s Daily-discharge (m 3 s -1 ) 1 1 the Garonne Tidal River 1 1 NTU 1 NTU 1 Pauillac Daily-discharge (m 3 s -1 ) Flood erosion Bordeaux Portets Daily-discharge (m 3 s -1 ) Daily-discharge (m 3 s -1 ) 52 km 1 km 12 km distance from the mouth of the Gironde estuary

30 In conclusion: In the Gironde estuary, continuous monitoring and radionuclides confirm that TMZ and particle residence times are controlled by river discharge. The continuous monitoring had allowed to precise the fluvial discharge levels that control the position of the TMZ. This results demonstrates that in a context of fluvial discharge decrease, the TMZ will stay longer in the fluvial section.

31 In conclusion: These results are explained and discussed with local public and private agencies to assist the strategic choices for the management of the estuary (newsletter, annual report, meeting, data transmission..)

32 Development of the network VERDON (214) Automated stations Specific cruises PAUILLAC (juin 24 --) BORDEAUX (mars 25 -) LIBOURNE (nov. 24 -) PORTETS (nov. 24-janvier 212) CADILLAC (août 213)

33 Thanks for your attention

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36 Evolution de l âge des particules dans la ZTM (PK3) Âge relatif des particules de PK3 surface et fond à marée basse MES 7 Be/ 21 Pb xs * * * * * Référence : particules les plus jeunes à ce site Max 7 Be/ 21 Pb xs 5 5 PK3 Basse mer Surface Fond Débit (m 3 s -1 ) Âge Relatif (jours) J M M J S N J M M J S N J M M J S N J M M J S N

37 Age des particules et cycle marée * * * * * 7 Be/ 21 Pb =14 4 Temps (jours) Surface Fond AR ( 234 Th xs / 21 Pb xs ) Surface Fond 8 7 Be/ 21 Pb xs Âge (jours) 3 6 AR ( 234 Th xs / 21 Pb xs ) 2 4 Mi-Flot Pleine mer Mi-jusant Basse mer 1 2 Site : MES (mg l -1 ) : Salinité (psu) : PK PK PK PK

38 En résumé sur le transfert particulaire le long du continuum fluvio-estuarien de la Gironde, basé sur le bilan des radioéléments Bordeaux Lot Fluvial section Âge des particules : Rétention : Estuaire (ZTM) Âge des particules : Low waters Flood 5-1 j 1-15 j 4-8 j j particules sont 2-4 j évacués Particules remontent dans les rivières

39 En résumé sur le transfert particulaire le long du continuum fluvio-estuarien de la Gironde, basé sur le bilan des radioéléments Figure 8 High river discharge Low marine intrusion.9 GBq d -1 79% Atmospheric fallout.1 GBq d -1 9% River P TMZ oceanic water K d D.13 GBq d -1 12% Low river discharge High marine intrusion Atmospheric fallout.2 GBq d -1 2%.18 GBq d -1 21% River t t t TMZ P D oceanic water.66 GBq d -1 77% Up- Central- Down-estuary