Fingerprinting the sources and quantifying the dynamics of suspended sediment during floods : an application to the Houay Xon River catchment in Laos

L’érosion des sols est intense dans les petits bassins versants montagneux en climat tropical. Lorsqu’ils atteignent les rivières, ces sédiments posent divers problèmes à l’aval. L’identification des sources de sédiments et la compréhension de leur dynamique au sein des bassins versants est donc cruciale pour proposer des mesures de lutte efficaces contre ces problèmes. Cette thèse vise à tester et à développer des méthodes de traçage qui consistent à comparer les propriétés physico-chimiques des sols à celles des sédiments transportés dans les rivières pour en définir les sources et pour en contraindre les temps de transfert. Le site d’étude est un bassin de 22,4 km² au Laos équipé d’un emboîtement de stations de suivi et soumis à un régime de mousson. Trois campagnes de prélèvements ont permis de collecter des échantillons de sols de surface, de ravines et de berges ainsi que des eaux de pluies, de ruissellement et de rivières, les matières en suspension (MES) associées et des laisses, au cours de 3 crues de début de mousson en mai 2012 et juin 2013. Les échantillons ont été analysés afin de déterminer leurs activités en radionucléides apportés au sol par les pluies de manière ponctuelle (¹³⁷Cs) ou continue (⁷Be, ²¹ºPb), la teneur et la composition de la matière organique (C organique, N, C/N, δ¹³C et δ¹⁵N), la granulométrie des MES et la composition de l’eau (conductivité électrique, δ¹⁸O). Plusieurs expériences méthodologiques ont permis de tester et valider les hypothèses qui sous-tendent l’utilisation du marquage en ⁷Be et en excès de ²¹ºPb des sédiments (adsorption rapide sur les particules de sol, rapport ⁷Be / ²¹ºPbxs des particules transportées par le ruissellement comparable à celui de la pluie correspondante). Durant la crue de mai 2012, la contribution des sédiments récemment érodés est élevée en début d’événement (25-35%), mais elle est ensuite diluée par la remobilisation de particules déposées sur le lit du cours d’eau ou stockées dans les zones de dépôt. La caractérisation combinée du carbone organique particulaire (COP) et de l’activité en ¹³⁷Cs des particules a mis en évidence la prédominance de l’érosion des sols de surface marqués par leur signature C₃ à l’amont, et l’augmentation de la contribution de l’érosion des berges (marquage C₄) dans la partie aval du bassin. Les valeurs les plus élevées des taux d’exportation de sédiments (43,3 Mg km-2) et de C (0,83 MgC km-2), du coefficient de ruissellement (11,7%) et du pourcentage d’eau de ruissellement dans l’écoulement total (78-100%) ont été observées au niveau de la station drainant en grande partie des versants sous teck. Les teneurs en C mesurées et les flux de COP calculés ici sont beaucoup plus élevés que ceux qui ont été estimés lors de travaux réalisés il y a 10 ans dans le même bassin versant, lorsque la surface couverte de teck y était beaucoup plus faible (2,5% en 2002-2003 contre 32% en 2012). Ainsi, à l’aval, le taux d’exportation spécifique de C lors d’une seule crue en 2012 s’est révélé être 2,6 fois plus important que celui observé au cours de l’année hydrologique 2002-2003 (34 crues érosives). Lors des deux crues de juin 2013, la variabilité spatio-temporelle des retombées de ⁷Be et ²¹ºPb a pu être quantifiée. Ces résultats indiquent qu’il est indispensable de mesurer le signal d’entrée du marquage ⁷Be /²¹ºPb de chaque événement et qu’il est préférable de réaliser un échantillonnage en plusieurs points du bassin plutôt que de le fractionner en un seul point au cours du temps. Durant ces crues, la composition des sédiments a pu être étudiée le long d’un continuum de dix stations emboitées (0,1 ha à 19,8 km²). Les contributions respectives des différentes sources de sédiments, et leur propagation à travers le bassin, ont pu être déterminées à l’aide des mêmes traceurs que lors de la crue de mai 2012. À l’avenir, la combinaison de traceurs proposée dans cette thèse pourrait être appliquée à d’autres évènements ou d’autres sites.Soil erosion is particularly intense in small mountainous tropical catchments. When supplied to the rivers, eroded sediment leads to numerous problems downstream (transfer of pollutants, increase of water turbidity, dam siltation…). Identifying sediment sources and understanding their dynamics across catchments constitutes a prerequisite to design and implement efficient measures to reduce these problems. This thesis aimed at testing and developing fingerprinting methods, by comparing physico-chemical properties of soils and riverine suspended sediment to define their origin and constrain their transfer across catchments. The study site is a 22.4 km² catchment in northern Laos equipped with nested monitoring stations and submitted to monsoon rainfalls. Three field campaigns were conducted to collect samples of surface soils, gullies, riverbanks, and rainwater, overland flow, river water, suspended matter and deposited sediment during three floods at the beginning of the monsoon in May 2012 and June 2013. Collected samples were analysed to determine fallout radionuclide activities (¹³⁷Cs, ⁷Be and ²¹ºPb), organic matter composition (organic carbon and nitrogen concentrations, C/N ratio, δ¹³C et δ¹⁵N), particle size distribution and water composition (electric conductivity and δ¹⁸O). Several methodological experiments were performed to check assumptions underlying the use of atmospheric 7Be and ²¹ºPb as tracers of sediment (quick adsorption of rainwater radionuclides to soil particles; similarity between /210Pbxs activity ratio measured in overland flow particles and corresponding rainfall signature). During the flood studied in May 2012, the contribution of recently eroded sediment was high at the beginning of the rising stage (25-35%), but was then diluted by sediment remobilization from the riverbed and deposition areas (swamps). The combined determination of particulate organic carbon (POC) and ¹³⁷Cs activity of sediment indicated the dominance of surface soil ( C₃ vegetation labelling) erosion in upstream parts and the increase of riverbank (labelled by C₄ plants) erosion contribution in the downstream part of the catchment. The highest runoff coefficient (11.7%), sediment specific yield (43.3 Mg km-2), total organic carbon specific yield (0.83 MgC km-2) and overland flow contribution (78–100%) were found at the station draining mainly the areas covered with teak plantations. Total organic carbon concentrations and POC specific yields calculated during this thesis are much higher than those obtained during a previous study carried out 10 years earlier in the same catchment, when teak plantations covered a much lower proportion of its area (2.5% in 2002-2003 vs 32% in 2012). At the downstream station, carbon specific yields for the studied flood in 2012 were twice higher than the annual ones observed in 2002-2003 (34 erosive events). During the two floods analysed in June 2013, the spatial and temporal variability of ⁷Be and ²¹ºPb wet deposition was investigated and quantified. The results indicate that the input signal of ⁷Be/²¹ºPb labelling associated with each storm should be determined and that spatially distributed collection of rainfall should be privileged to the sequential time-fractionated sampling at a single location. During these floods, the composition of exported suspended sediment could be studied along a continuum of ten nested stations draining areas between 0.1 ha and 19.8 km². Respective contributions of the different surface and subsurface sources of sediment and their propagation across the catchment, were successfully determined with the same tracers as during the May 2012 event. In future, the panel of parameters used in this thesis could be applied to characterise suspended matter behaviour during floods occurring later in the rainy season, or along a longer continuum of nested monitoring stations

Traçage des sources et quantification de la dynamique des sédiments en crue : application au bassin tropical montagneux de la Houay Xon au Laos

Soil erosion is particularly intense in small mountainous tropical catchments. When supplied to the rivers, eroded sediment leads to numerous problems downstream (transfer of pollutants, increase of water turbidity, dam siltation…). Identifying sediment sources and understanding their dynamics across catchments constitutes a prerequisite to design and implement efficient measures to reduce these problems. This thesis aimed at testing and developing fingerprinting methods, by comparing physico-chemical properties of soils and riverine suspended sediment to define their origin and constrain their transfer across catchments. The study site is a 22.4 km² catchment in northern Laos equipped with nested monitoring stations and submitted to monsoon rainfalls. Three field campaigns were conducted to collect samples of surface soils, gullies, riverbanks, and rainwater, overland flow, river water, suspended matter and deposited sediment during three floods at the beginning of the monsoon in May 2012 and June 2013. Collected samples were analysed to determine fallout radionuclide activities (¹³⁷Cs, ⁷Be and ²¹ºPb), organic matter composition (organic carbon and nitrogen concentrations, C/N ratio, δ¹³C et δ¹⁵N), particle size distribution and water composition (electric conductivity and δ¹⁸O). Several methodological experiments were performed to check assumptions underlying the use of atmospheric 7Be and ²¹ºPb as tracers of sediment (quick adsorption of rainwater radionuclides to soil particles; similarity between /210Pbxs activity ratio measured in overland flow particles and corresponding rainfall signature). During the flood studied in May 2012, the contribution of recently eroded sediment was high at the beginning of the rising stage (25-35%), but was then diluted by sediment remobilization from the riverbed and deposition areas (swamps). The combined determination of particulate organic carbon (POC) and ¹³⁷Cs activity of sediment indicated the dominance of surface soil ( C₃ vegetation labelling) erosion in upstream parts and the increase of riverbank (labelled by C₄ plants) erosion contribution in the downstream part of the catchment. The highest runoff coefficient (11.7%), sediment specific yield (43.3 Mg km-2), total organic carbon specific yield (0.83 MgC km-2) and overland flow contribution (78–100%) were found at the station draining mainly the areas covered with teak plantations. Total organic carbon concentrations and POC specific yields calculated during this thesis are much higher than those obtained during a previous study carried out 10 years earlier in the same catchment, when teak plantations covered a much lower proportion of its area (2.5% in 2002-2003 vs 32% in 2012). At the downstream station, carbon specific yields for the studied flood in 2012 were twice higher than the annual ones observed in 2002-2003 (34 erosive events). During the two floods analysed in June 2013, the spatial and temporal variability of ⁷Be and ²¹ºPb wet deposition was investigated and quantified. The results indicate that the input signal of ⁷Be/²¹ºPb labelling associated with each storm should be determined and that spatially distributed collection of rainfall should be privileged to the sequential time-fractionated sampling at a single location. During these floods, the composition of exported suspended sediment could be studied along a continuum of ten nested stations draining areas between 0.1 ha and 19.8 km². Respective contributions of the different surface and subsurface sources of sediment and their propagation across the catchment, were successfully determined with the same tracers as during the May 2012 event. In future, the panel of parameters used in this thesis could be applied to characterise suspended matter behaviour during floods occurring later in the rainy season, or along a longer continuum of nested monitoring stations.L’érosion des sols est intense dans les petits bassins versants montagneux en climat tropical. Lorsqu’ils atteignent les rivières, ces sédiments posent divers problèmes à l’aval. L’identification des sources de sédiments et la compréhension de leur dynamique au sein des bassins versants est donc cruciale pour proposer des mesures de lutte efficaces contre ces problèmes. Cette thèse vise à tester et à développer des méthodes de traçage qui consistent à comparer les propriétés physico-chimiques des sols à celles des sédiments transportés dans les rivières pour en définir les sources et pour en contraindre les temps de transfert. Le site d’étude est un bassin de 22,4 km² au Laos équipé d’un emboîtement de stations de suivi et soumis à un régime de mousson. Trois campagnes de prélèvements ont permis de collecter des échantillons de sols de surface, de ravines et de berges ainsi que des eaux de pluies, de ruissellement et de rivières, les matières en suspension (MES) associées et des laisses, au cours de 3 crues de début de mousson en mai 2012 et juin 2013. Les échantillons ont été analysés afin de déterminer leurs activités en radionucléides apportés au sol par les pluies de manière ponctuelle (¹³⁷Cs) ou continue (⁷Be, ²¹ºPb), la teneur et la composition de la matière organique (C organique, N, C/N, δ¹³C et δ¹⁵N), la granulométrie des MES et la composition de l’eau (conductivité électrique, δ¹⁸O). Plusieurs expériences méthodologiques ont permis de tester et valider les hypothèses qui sous-tendent l’utilisation du marquage en ⁷Be et en excès de ²¹ºPb des sédiments (adsorption rapide sur les particules de sol, rapport ⁷Be / ²¹ºPbxs des particules transportées par le ruissellement comparable à celui de la pluie correspondante). Durant la crue de mai 2012, la contribution des sédiments récemment érodés est élevée en début d’événement (25-35%), mais elle est ensuite diluée par la remobilisation de particules déposées sur le lit du cours d’eau ou stockées dans les zones de dépôt. La caractérisation combinée du carbone organique particulaire (COP) et de l’activité en ¹³⁷Cs des particules a mis en évidence la prédominance de l’érosion des sols de surface marqués par leur signature C₃ à l’amont, et l’augmentation de la contribution de l’érosion des berges (marquage C₄) dans la partie aval du bassin. Les valeurs les plus élevées des taux d’exportation de sédiments (43,3 Mg km-2) et de C (0,83 MgC km-2), du coefficient de ruissellement (11,7%) et du pourcentage d’eau de ruissellement dans l’écoulement total (78-100%) ont été observées au niveau de la station drainant en grande partie des versants sous teck. Les teneurs en C mesurées et les flux de COP calculés ici sont beaucoup plus élevés que ceux qui ont été estimés lors de travaux réalisés il y a 10 ans dans le même bassin versant, lorsque la surface couverte de teck y était beaucoup plus faible (2,5% en 2002-2003 contre 32% en 2012). Ainsi, à l’aval, le taux d’exportation spécifique de C lors d’une seule crue en 2012 s’est révélé être 2,6 fois plus important que celui observé au cours de l’année hydrologique 2002-2003 (34 crues érosives). Lors des deux crues de juin 2013, la variabilité spatio-temporelle des retombées de ⁷Be et ²¹ºPb a pu être quantifiée. Ces résultats indiquent qu’il est indispensable de mesurer le signal d’entrée du marquage ⁷Be /²¹ºPb de chaque événement et qu’il est préférable de réaliser un échantillonnage en plusieurs points du bassin plutôt que de le fractionner en un seul point au cours du temps. Durant ces crues, la composition des sédiments a pu être étudiée le long d’un continuum de dix stations emboitées (0,1 ha à 19,8 km²). Les contributions respectives des différentes sources de sédiments, et leur propagation à travers le bassin, ont pu être déterminées à l’aide des mêmes traceurs que lors de la crue de mai 2012. À l’avenir, la combinaison de traceurs proposée dans cette thèse pourrait être appliquée à d’autres évènements ou d’autres sites

Geochemical behaviour of Radium isotopes and Radon in a coastal thermal system (Balaruc-les-Bains, South of France)

International audienceThe behaviour of the four Ra isotopes (measured by gamma spectrometry) and 222Rn (measured with an AlphaGUARD ionisation chamber) has been investigated in NaCl-rich thermal waters of Balaruc-les-Bains on the Mediterranean coast (South of France). This study allows identification of the deep thermal water signature and reveals the influence of seawater or karst water inflows on Ra isotopes and 222Rn. The deep thermal water has 226Ra and 222Rn activities of 840 and 1900 mBq/l, respectively, a (228Ra/226Ra) activity ratio of 0.59, and low (224Ra/228Ra) and (223Ra/226Ra) ratios of 0.67 and 0.025, respectively. Several arguments suggest a dominant role of radioactive decay for short-lived Ra isotopes and Rn during the relatively rapid ascent of thermal water through wide open fractures and drains. The low (223Ra/226Ra) ratio constrains the maximum ascent velocity of thermal water to 8-10 m/h. Seawater inflow into the hydrothermal system results in an enrichment in short-lived Ra isotopes and 222Rn. The high (223Ra/226Ra) ratio (0.23) suggests that 223Ra, 224Ra and 222Rn produced by alpha-recoil in the upper-Jurassic limestone are entrained by seawater percolation through the coastal basement, due to active pumping in one of the near shore production well. This process would be enhanced by a low water/rock ratio (i.e. in a low-porosity, micro-fractured limestone). Mixing of this enriched seawater with thermal water induces barite precipitation with co-precipitation of a large fraction of Ra. Short-lived Ra isotopes may thus be useful tracers of seawater flux towards the continent. Conversely, the inflow and mixing of karst groundwater result in a general dilution of all dissolved elements, only a small enrichment in 224Ra and 223Ra, but a large enrichment in 222Rn (up to 26 Bq/l). The combination of Ra isotopes and 222Rn data provides a good way to trace the dynamics of different water masses along coastal areas

Constraining sediment dynamics during flood events using fallout radionuclides (Be-7 and Pb-210(xs)): a pilot study in a tropical catchment of SE Asia

International audienceSoil erosion is particularly intense in mountainous subtropical regions where heavy storms result in the production of large quantities of suspended sediment, leading to both on-site and off-site siltation problems and conveying contaminants and biological compounds along streams. To implement efficient controls on sediment transfer, it is necessary to monitor suspended matter loads and determine whether sediments are directly exported from the catchment during a given flood or re-suspended from deposits left by previous floods. Determination of sediment residence time in catchments can be performed combining fallout radionuclides (Be-7, Pb-210 and Cs-137) measurements in both rainfall and suspended sediments, but prerequisites for their use as tracers during flood events are still required. We conducted experiments along a network of nested catchments with area increasing from 0.12 to 6.15 km$^2$ , and sampled rainfall and suspended sediment loads during a flood that occurred at the beginning of the rainy season in May 2012. The experiment was carried out in the Houay Xon catchment, part of the MSEC (Multi-Scale Environmental Changes, http://msec3.net/portal/) project in the Northern Luang Prabang province of Laos. This catchment, under shifting cultivation since the end of the 1960s for a large part of its surface, is characterized by a steep topography (slopes ranging from 3% to 350% with a mean slope of 56%) and a land use evolution prone to gully erosion. The seasonal distribution of rainfall controls the Be-7 and Pb-210(xs) activities of soils in the catchment. With a half-life of 53 days, the fallout Be-7 activity of the previous wet season should have sufficiently decayed during the 6-months dry period to be neglected compared to new supply by early rains of the following wet season. Atmospheric deposition of Be-7 and Pb-210(xs) was measured after flocculation of rainfall samples by aluminum chloride hexahydrate and after dehydration of 75 suspended sediment samples (0.5-128 g/L) collected by automatic samplers during the flood. Soils, gullies and riverbanks were also sampled across the catchment to document the potential sources of sediment supply to the river. Cumulative precipitation reached 27 mm in less than 50 minutes, leading to an overall export of ca. 274 tons of sediments from the catchment during the flood. The Be-7/Pb-210(xs) ratios of suspended particles were used to monitor "newly eroded" and "older re-suspended" sediment contributions to suspended matter transport along the network. Dynamic fingerprinting of rainwater vs. groundwater and soil-detached vs. re-suspended particles supplies at the river monitoring stations was confronted to results obtained using particles-Cs-137 activity, water-delta-O-18 and particulate organic matter-delta-C-13 measurements

Understanding sediment dynamics in rivers using fallout radionuclides: How to move forward from the lessons learnt in a tropical catchment

International audienceLand use change and the concomitant acceleration of soil erosion have led to an increase in sediment supply to rivers worldwide. This degradation results in significant on-site (e.g., decrease in soil fertility) and off-site impacts (e.g., reservoir siltation and degradation of water quality). To implement effective sediment mitigation measures, it is necessary to clearly understand catchment sediment sources and their spatial temporal dynamics. Fallout radionuclides characterized by different half-lives and origins (Be-7-53 d; Pb-210-22 y; Cs-137-30 y) provide important information required to quantify the dominant sources of sediment and also their temporal dynamics. However, the current methods have several limitations, and the hypotheses underpinning this technique require further verification. To examine these assumptions, we investigated sediment dynamics in a 10-km 2 catchment in Northern Laos during the first flood of the monsoon in June 2014. Before this event, Be-7 that labelled soil and sediment during previous storms in 2013 had completely decayed, and the material stored in the river channel was shown to be depleted in Be-7. A large set of samples (n=97) was collected to characterize the sources that may supply sediment to the river. In addition, suspended sediment (n=17) was collected in the river at several stations during this the first flood of the monsoon. A distribution modelling approach was used to quantify the relative contributions of surface and subsurface sources. Further we modelled the proportions of material eroded during this storm compared to material that previously eroded before the storm and was remobilized during this recent event. The results demonstrate that the majority of sediment transported during the first erosive storm of the year consists of older, remobilized material. Furthermore, the contribution of sediment supplied to the river by subsurface sources (i.e. channel bank erosion) increases downstream. In the future, this approach should be tested in larger catchments to improve our understanding of sediment propagation in river systems

Suspended sediment dynamics in a Southeast Asian mountainous catchment: Combining river monitoring and fallout radionuclide tracers

Part BInternational audienceSoil erosion is intense in mountainous tropical regions where heavy storms result in the supply of large quantities of sediment to rivers. The origin and dynamics of suspended sediment were analysed in a catchment located in northern Laos during the first erosive flood of the rainy season in May 2012. The catchment was equipped with 4 successive gauging stations (draining areas ranging 0.2–11.6 km$^2$). Fallout radionuclides (Beryllium-7 – $^7$Be, unsupported Pb-210 –$^{210}$Pb$_{xs}$, and Cesium-137 – $^{137}$Cs) were determined on rainfall, overland flow, stream water, suspended sediment, soil surface and subsurface samples (with $n$ = 3, 19, 75, 75, 65 and 14 respectively). Assumptions underpinning the $^7$Be-labelling method were validated by implementing experiments in the laboratory (i.e., rainwater $^7$Be sorption to soil particles) and in the field (i.e., $^7$Be:$^{210}$Pb$_{xs}$ activity ratio evolution in rainwater and related overland flow during a natural storm event). Radionuclide analyses provided a way to quantify variations in sediment dynamics and origin throughout the flood: (1) a proportion of recently eroded sediment (labelled by $^7$Be, and referred to as “fresh sediment”) ranging between $ca$. 8–35% in suspended loads; (2) higher contributions of fresh sediment at the beginning of the flood rising stage; (3) a progressive dilution of fresh sediment by particles remobilised from the riverbed/channel; (4) the dominance of particles originating from the soil surface (ca. 70–80% of total sediment load) in upper parts and a much larger contribution of subsurface material ($ca$. 64%) at the downstream station. The original contribution of 7Be-labelled particles derived from collapsed riverbanks to sediment export was also demonstrated. This pilot study supports the use of fallout $^7$Be and $^{210}$Pb$_{xs}$ in tropical catchments to constrain sediment dynamics. It also puts forward the need to better characterize the sources of sediment in order to avoid possible misinterpretations

Constraining sediment dynamics in rivers using fallout radionuclides: How to move forward from the lessons learnt in catchments of Mexico, Laos and France

International audienceLand use change (e.g. deforestation, overgrazing, cropping intensification) and the associated acceleration of soil erosion have led to an increase in sediment supply to rivers across the world during the last decades. This degradation leads to important on-site (e.g., decrease in soil fertility) and off-site (e.g., reservoir siltation and water pollution) impacts. In order to better understand the dynamics of sediment in catchments and rivers and to implement efficient mitigation measures, it is necessary to better constrain the pathways of sediment in time and space. Fallout radionuclides characterized by different half-lives and origins (Be-7, Cs-137, and unsupported Pb-210) have proved to provide very useful indications to constrain the transfer times of sediment at various scales (plot, hillslope, small catchment, large basins) and in a range of different environments and climates across the world. However, several limitations were raised regarding the assumptions underpinning the methods used to derive sediment residence time or transport distances. Furthermore, doubts regarding the validity of using those tracers in large rivers and catchments were also put forward in recently published papers. In this study, we provide additional data indicating that the hypotheses underlying the method are meaningful. We also compiled radionuclide data (Be-7, Cs-137, and unsupported Pb-210) measured in ,000 km²) and located in different environments (Central Mexico, Northern Laos, France). The results obtained in the different catchments are compared and reinterpreted, with the specific objective of dealing with the limitation that fresh Be-7 fallout may not label river bank or subsurface material. The results show that providing additional and independent information on the sources delivering sediment to the rivers helps constraining their transit times with the method based on Be-7 and unsupported Pb-210 measurements. Implications of the use of this technique in larger rivers and catchments are discussed. Their contribution to quantify the persistence time of particle-borne contaminants in those riverine systems is specifically addressed

Constraining sediment dynamics in rivers using fallout radionuclides: How to move forward from the lessons learnt in catchments of Mexico, Laos and France

International audienceLand use change (e.g. deforestation, overgrazing, cropping intensification) and the associated acceleration of soil erosion have led to an increase in sediment supply to rivers across the world during the last decades. This degradation leads to important on-site (e.g., decrease in soil fertility) and off-site (e.g., reservoir siltation and water pollution) impacts. In order to better understand the dynamics of sediment in catchments and rivers and to implement efficient mitigation measures, it is necessary to better constrain the pathways of sediment in time and space. Fallout radionuclides characterized by different half-lives and origins (Be-7, Cs-137, and unsupported Pb-210) have proved to provide very useful indications to constrain the transfer times of sediment at various scales (plot, hillslope, small catchment, large basins) and in a range of different environments and climates across the world. However, several limitations were raised regarding the assumptions underpinning the methods used to derive sediment residence time or transport distances. Furthermore, doubts regarding the validity of using those tracers in large rivers and catchments were also put forward in recently published papers. In this study, we provide additional data indicating that the hypotheses underlying the method are meaningful. We also compiled radionuclide data (Be-7, Cs-137, and unsupported Pb-210) measured in ,000 km²) and located in different environments (Central Mexico, Northern Laos, France). The results obtained in the different catchments are compared and reinterpreted, with the specific objective of dealing with the limitation that fresh Be-7 fallout may not label river bank or subsurface material. The results show that providing additional and independent information on the sources delivering sediment to the rivers helps constraining their transit times with the method based on Be-7 and unsupported Pb-210 measurements. Implications of the use of this technique in larger rivers and catchments are discussed. Their contribution to quantify the persistence time of particle-borne contaminants in those riverine systems is specifically addressed

Suspended sediment source and propagation during monsoon events across nested sub-catchments with contrasted land uses in Laos

Study region: Houay Xon catchment in northern Laos. Study focus: Because agricultural headwater catchments of SE Asia are prone to erosion and deliver a significant proportion of the total suspended sediment supply to major rivers and floodplains, the potential sources of sediments and their dynamics were studied for two successive storm flow events in June 2013. Characterization of suspended sediment loads was carried out along a continuum of 7 monitoring stations, combining analyses of fallout radionuclides, particle borne organic matter and stream water properties. New hydrological insights: Radionuclide activities showed that remobilization of soil particles deposited during the previous rainy season or supplied by riverbank erosion is the dominant process, although pulses of surface-soil derived sediments also propagate downstream. This interpretation is supported by suspended organic matter data that also fingerprints the mixing of surface soil vs. subsurface particles. The study moreover highlights the advantages and the drawbacks of combining fallout radionuclides, particle borne organic matter composition and stream water characteristics to discriminate and quantify sediment sources and dynamics in rural areas undergoing urban sprawl