Tracing the origin of suspended sediment in a large Mediterranean river by combining continuous river monitoring and measurement of artificial and natural radionuclides

Carbon-14 cycling in a nuclearized river: A first study in the downstream part of the Rhône River (France)

Carbon-14 (14C) has a natural origin but is also anthropogenically released from civil nuclear facilities. Due to its long decay period (half-life: 5700 ± 30 years), it is a persistent radionuclide in the environment. In rivers, the complex speciation of carbon makes the fate of industrial 14C difficult to track. This study reports a first overview of artificial 14C cycling in a nuclearized river. A one-year sampling campaign was conducted on the French nuclearized Rhône River and two of its non-nuclearized tributaries (Durance and Ardèche rivers). Isotopic (δ13C, Δ14C) and carbon concentrations analyses were performed on the particulate organic carbon (POC), dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). Chlorophyll-a (Chl-a) and tritium analyses were performed to assess the dynamic of aquatic organic matter and the nuclear industry contribution, respectively. Comparisons of Δ14C data obtained from the Rhône River with those from the tributaries highlight significant industrial radiocarbon labelling in all carbon forms, with medians of 142, 130 and 42 ‰ for POC, DOC and DIC, that are 2-3 times higher than those of the tributaries. The high values of Chl-a/POC ratios with Δ14C-enriched POC suggest a biological uptake of artificial Δ14C in DIC by aquatic photosynthesis. The relationship of Δ14C-DIC with tritium activity indicates a response to recent releases and enables the contribution of nuclear power plants to be estimated at a median of 26 %. Sampling at the Rhône's mouth would reinforce our understanding of the fate of riverine 14C when entering the marine environment.

Contrasting platinum trajectories in three major French rivers using dated sediment cores (1910-2021): From geochemical baseline to emerging source signals

Platinum (Pt) is a Technology Critical Element (TCE) which, since the 1990s, has been mainly used in the industry in catalytic converters for automobile emission control. Previous studies have shown Pt contamination of road-side sediments and surface sediments in urban rivers and lakes but few of them have addressed temporal variations. The present work presents historical Pt concentration trends in 137Cs-dated sediment cores from floodplains or secondary channels at the outlets of three major French watersheds (Loire, Rhone, and Seine Rivers) covering the past ∼110 years, i.e., from the 1910s to 2021. Platinum baseline levels in the sediment were estimated for the Loire River (0.76 ± 0.22 μg kg-1 for the period ∼1910-∼1955) and the Rhone River (1.64 ± 0.41 μg kg-1), and historical Pt variations seem to reflect variations in hydrodynamics and grain size composition. Since the early 2000s, Pt concentrations in the Loire and the Rhone River sediments tend to increase (>2.5 μg kg-1) and were attributed to the use of car catalytic converters, an emerging technology since the 1990s using >50 % of European Pt demand. High and variable historical Pt concentrations (up to 14.6 μg kg-1) in the Seine River sediments may reflect legacy Pt sources due to former anthropogenic activities in this watershed, such as the use of Pt-based catalysts for petroleum refinery since the end of the 1940s, coal handling and precious metals refining, probably concealing the likely presence of an emerging traffic-related Pt signal. This first comparison of historical Pt concentration trends in sediments from contrasting watersheds allows to distinguish signals originating from different natural and anthropogenic sources (background level, historical sources, road traffic).

Assessment of the depositional characteristics of the Yellow River estuary from 1960s by 239+240Pu and 137Cs

The spatial and vertical distribution of 239+240Pu and 137Cs in the sediments of the Yellow River Delta was studied to evaluate the deposition dynamics in the Yellow River estuary from 1960s. The activity of 239+240Pu and 137Cs in sediment core ranged from 0.001 to 0.212 Bq/kg and 0.52-2.53 Bq/kg, respectively. A maximum accumulation peak and two secondary accumulation peaks appeared in the sediment core YR2. The average deposition rate of 8.3 cm/y for the Yellow River estuary from 1964 to 1976 was obtained. The proportion of Pu from the Yellow River net input and direct deposition to the total inventory of Pu in the estuary was assessed, with a total inventory of Pu in the abandoned estuary of 7.4 × 1010 Bq and a net input of 2.2 × 1010 Bq from the Yellow River. Pu deposited in the estuary only accounts for 18 % of the total Pu transported by the Yellow River, and most of the Pu is injected into the Bohai Sea with the Yellow River.

Short review of plutonium applications for the sediment transport studies

The transport of sediments in surface waters is a natural process involving the relocation of the sediments themselves and the nutrients, pollutants and radionuclides associated with them. Plutonium isotopes have proven to be a useful tool for studying this process over several decades. In this article, we review the characteristics of the behaviour of plutonium in the water column and the main directions of its use to study the transport of sediments at different scales. The characteristic isotopic fingerprints of the sources and their known input functions, as well as the good reactivity of the particles, favour the widespread use of plutonium for the study of the chronology of sediments and deposits and for studies of the fate and migration pathways of sediments at different scales. While other radionuclides are losing relevance due to the short half-life the ²³⁹Pu and ²⁴⁰Pu will remain useable for a long time.

Sediment source fingerprinting as an aid to large-scale landscape conservation and restoration: A review for the Mississippi River Basin

Reliable quantitative information on sediment sources to rivers is critical to mitigate contamination and target conservation and restoration actions. However, for large-scale river basins, determination of the relative importance of sediment sources is complicated by spatiotemporal variability in erosional processes and sediment sources, heterogeneity in sediment transport and deposition, and a paucity of sediment monitoring data. Sediment source fingerprinting is an increasingly adopted field-based technique that identifies the nature and relative source contribution of sediment transported in waterways. Notably, sediment source fingerprinting provides information that is independent of other field, modeling, or remotely sensed techniques. However, the diversity in sampling, analytical, and interpretive methods for sediment fingerprinting has been recognized as a problem in terms of developing standardized procedures for its application at the scale of large river basins. Accordingly, this review focuses on sediment source fingerprinting studies conducted within the Mississippi River Basin (MRB), summarizes unique information provided by sediment source fingerprinting that is distinct from traditional monitoring techniques, evaluates consistency and reliability of methodological approaches among MRB studies, and provides prospects for the use of sediment source fingerprinting as an aid to large-scale landscape conservation and restoration under current management frameworks. Most MRB studies reported credible fingerprinting results and found near-channel sources to be the dominant sediment sources in most cases, and yet a lack of standardization in procedural steps makes results difficult to compare. Findings from MRB studies demonstrated that sediment source fingerprinting is a highly valuable and reliable sediment source assessment approach to assist land and water resource management under current management frameworks, but efforts are needed to make this technique applicable in large-scale landscape conservation and restoration efforts. We summarize research needs and discuss sediment fingerprinting use for basin-scale management efforts with the aim of encouraging that this technique is robust and reliable as it moves forward.

Hydrological classification by clustering approach of time-integrated samples at the outlet of the Rhône River: Application to Δ14C-POC

Within the framework of the Rhône Sediment Observatory, monthly time-integrated samples have been collected by Particle Traps in the last decade to monitor particulate contaminants in the Rhône River and its main tributaries. In this watershed with a contrasted hydrology, a clustering approach is used to classify the samples according to the main hydrological events. This approach has been applied to riverine particulate organic radiocarbon signatures (Δ¹⁴C-POC) that are strongly affected by the origin of the material and the occurrence of nuclear power plant releases. Suspended Particulate Matter (SPM) samples were collected near the outlet of the Rhône River and analysed for ¹⁴C along with particulate organic carbon (POC), chlorophyll a and tritium contents to confirm Δ¹⁴C-POC origins. Cluster Analysis, coupled to Principal Component Analysis, was performed based on monthly average water discharges of the Upper Rhône River and the five main tributaries. The classification obtained by fuzzy C-mean logic of the Rhône River hydrology into 5 clusters is similar to that already observed in the literature with Mediterranean/Cevenol flood, oceanic pluvial flood, nival flood, low-water level and baseflow clusters. The contributions of each cluster among the Δ¹⁴C-POC values demonstrate the complexity of hydrological classification of time-integrated samples. First, the samples with a unique and significantly dominant cluster are easily explained with negative Δ¹⁴C-POC values observed in the flood clusters due to input of ¹⁴C-depleted material from soil or rock weathering, and positive values observed in the low-water level and baseflow clusters due to anthropogenic input by nuclear industry. Second, samples that present a homogeneous mixture between several clusters demonstrate the occurrence of different hydrological events during the sampling periods. This tool appears as a solution to estimate the contribution of each hydrological event in time-integrated samples.

Relevance of using the non-reactive geochemical signature in sediment core to estimate historical tributary contributions

Fluvial suspended particulate matter (SPM) fluxes transport large amounts of contaminants that can affect water quality and river ecosystems. To better manage these inputs in river systems, it is essential to identify SPM and sediment sources. Many studies have applied a fingerprinting method based on using metals integrated into a numerical mixing model to estimate source contributions in a watershed. Most fingerprinting studies use contemporary SPM to trace historical inputs, whereas their metal concentrations were modified over time due to anthropogenic inputs. Moreover, total concentrations of these properties are subject to change due to diagenetic processes occurring in stored sediments. The aim of this study was to assess the relevance of using the non-reactive fraction of metals (i.e. metals and metalloids) in fingerprinting studies to estimate the historical contributions of SPM tributary inputs in a sediment core. To assess metal concentrations in the 'conservative' (i.e. non-reactive) fraction, SPM (samples of sources) and sediment core layers (targeted sediments) were subjected to total mineralization and soft extraction, and the non-reactive fraction was obtained by calculating the difference between the two extractions. This approach was applied on a sediment core from the Upper Rhône River (France), using geochemical signature in contemporary SPM of three major tributaries. We showed that the non-reactive fraction retains a higher number of metals in the range test for the deepest layers, which are characterized by significant anthropogenic inputs. Through apportionment modelling using Monte Carlo simulation, we demonstrated that the tributary contributions computed using the non-reactive fraction are more consistent with historical flood and water flow data and have lower uncertainties than with the total fraction. Working with the non-reactive fraction made it possible to decipher historical inputs of SPM using contemporary SPM samples. This approach enables robust identification of sub-catchment areas liable to provide large quantities of SPM. The non-reactive fraction can be used in a variety of environmental conditions and at various spatial and temporal scales to provide a robust quantification of sediment sources.

Field measurements on alluvial watercourses in light of numerical modeling: case studies on the Danube River

Adequate monitoring and data acquisition of proper hydraulic, sediment, and constituent parameters in alluvial watercourses have become crucial aspects of human interaction with the environment. Conducting well-organized, comprehensive, and meaningful field measurements on natural watercourses are of great importance when assessing its hydraulic, morphological, and ecological state. However, this paper presents a methodology for field measurements on alluvial watercourses in light of numerical modeling. The proposed methodology focuses on collecting field data sets to calibrate numerical models for flow, sediment, and heavy metal transport. The proposed approach targets the simultaneous measurement of hydraulic, sediment transport, and heavy metal transport parameters that are key for calibrating constants and exchange mechanisms in contemporary numerical models. Using the principles laid out in this paper, two sets of measurements were carried out on the Danube River, one on a reach near Mohács in Hungary and the other on a reach near Belgrade in Serbia. The first case study discusses the measurement and results of comprehensive hydraulic and sediment parameters. The second case study considers hydraulic and sediment measurements complemented with trace metal measurements for zinc, lead, and mercury. These measurements were used for calibrating numerical models for flow, sediment, and heavy metal transport, as a proof of concept. It has been demonstrated that the gathered data sets contain key parameters that are strongly linked through physical laws and are needed for calibration purposes, as well as parameters that can allow the newly calibrated coefficients to be confirmed through other measured phenomena. Therefore, the proposed methodology provides minimal data sets with detailed measurements for calibrating numerical models for flow, sediment, and heavy metal transport. Guidelines for future measurements that can suffice the increasing need for numerical modeling and monitoring of natural watercourses are also offered.

Suspended sediment flux at the Rhone River mouth (France) based on ADCP measurements during flood events

Suspended sediment distribution and fluxes were estimated within the dominant channel at the mouth of the Rhone River for two annual flood events. The estimates were based on ADCP acoustic backscatter intensity and using calibration and post-processing methods to account for the grain-size distribution (GSDs). The fluxes were very similar to those obtained from suspended sediment measurements based on surface sampling at an automated station located 35 km upstream. Suspended sediment concentrations (SSC) and GSDs showed little variation along the channel cross-section, except for a graduate suspension that appeared at the maximum of discharge, corresponding to velocities lower than 1 m s^-1 near the bottom. However, without post processing to account for the GSD, an under-estimation of 10% was observed during the two flood periods. The two flood events (12 November 2012 and 29 November 2012), separated by only 2 weeks, had clear differences in suspended sediment fluxes (SSF) and SSC during the peak of the river discharge, with twice more flux during the first, respectively, 925,226 and 430,879 tons of SSF.

Combining flux monitoring and data reconstruction to establish annual budgets of suspended particulate matter, mercury and PCB in the Rhône River from Lake Geneva to the Mediterranean Sea

Long term and high resolution data on water discharge, suspended particulate matter (SPM) and contaminant concentrations in rivers are required for a better understanding of particulate transfers from the continental areas to the seas. The aim of this study was to provide a novel estimation of annual fluxes of SPM and related pollutants in the Rhône River from Lake Geneva to the Mediterranean Sea by combining high frequency or time-integrative monitoring and novel relations between SPM concentration (Cs) and water discharge (Q). At six stations of the Rhône Sediment Observatory (OSR), SPM fluxes were calculated over the 2000-2016 period by combining observational data and Cs-Q relations. Monthly average concentrations of mercury (Hg) and PCB 180 were obtained by analysis of SPM samples collected in time integrative particle traps between 2011 and 2016. These pollutants were selected because of the well documented contamination of the Rhône watershed by these substances. Inter-annual fluxes at the Rhône River outlet averaged 6.6 Mt. yr^-1 for SPM, 572 kg yr^-1 for Hg and 14 kg yr^-1 for PCB 180. The Isère and Durance tributaries were found to be the main contributors of SPM fluxes. Annual SPM budgets were not balanced, suggesting deposition, remobilization of bottom sediments and/or contributions from non-monitored tributaries. The SPM sampled at the outlet was more contaminated than the combined SPM inputs from the monitored tributaries, suggesting that intermediate sources of contamination were not captured in the budget.

A brief history of origins and contents of Organically Bound Tritium (OBT) and 14C in the sediments of the Rhône watershed

Tritium (³H) and Carbon-14 (¹⁴C) are radionuclides of natural (cosmogenic) origin that have also been introduced into the environment by humans since the middle of the last century. They are therefore not compounds that have only recently been released into the environment and they do not pose a recognized health threat due to their low radiotoxicity. However, they hold an important place among current concerns because they are being discharged into the environment by the nuclear industry in large quantities compared to other radionuclides. Those both radionuclides partly integrate organic matter during metabolic processes (i.e., photosynthesis) leading to organically bound forms that can be found in sediments. Organically bound tritium (OBT) analyses carried out on the sediments of the Rhône and its tributaries indicate a significant and historical tritium labelling of sedimentary particles all along the Rhône river, as well as in several northern tributaries, in particular the Ognon and the Tille rivers (tributaries of the Saone), the Doubs River and the Loue River (a tributary of the Doubs) and the Arve river. The recorded levels (10 to over 20,000 Bq/L) are very likely to be related to the presence of synthetic tritiated particles (technogenic tritium), which were used in the past in watchmaking workshops. Although overall contamination levels decrease from north to south in the Rhône watershed and fade over time, particularly due to the radioactive decay of tritium, this contamination source of technogenic tritium in the Rhône watersheds is currently still not negligible. Carbon-14 analyses show that the Rhône sediments generally display ¹⁴C levels close to the atmospheric reference values (231 Bq·kg^-1 of C in 2015) or even lower in most of cases, and show sporadic and weak labelling near nuclear facilities. The low ¹⁴C levels in the Rhône sediments are most likely related to the solid contributions from tributaries draining areas that are rich in fossil organic matter, and therefore devoid of ¹⁴C. In the Rhône watershed, the presence in solid particles of tritium in a form organically bound to synthetic compounds and of petrogenic (fossil) organic carbon, can potentially alter the apparent assimilation rates to the food chain of these two radionuclides.

U isotopes distribution in the Lower Rhone River and its implication on radionuclides disequilibrium within the decay series

The large rivers are main pathways for the delivery of suspended sediments into coastal environments, affecting the biogeochemical fluxes and the ecosystem functioning. The radionuclides from ²³⁸U and ²³²Th-series can be used to understand the dynamic processes affecting both catchment soil erosion and sediment delivery to oceans. Based on annual water discharge the Rhone River represents the largest river of the Mediterranean Sea. The Rhone valley also represents the largest concentration in nuclear power plants in Europe. A radioactive disequilibrium between particulate ²²⁶Ra_(p) and ²³⁸U_(p) was observed in the suspended sediment discharged by the Lower Rhone River (Eyrolle et al. 2012), and a fraction of particulate ²³⁴Th was shown to derive from dissolved ²³⁸U_(d) (Zebracki et al. 2013). This extensive study has investigated the dissolved U isotopes distribution in the Lower Rhone River and its implication on particulate radionuclides disequilibrium within the decay series. The suspended sediment and filtered river waters were collected at low and high water discharges. During the 4-months of the study, two flood events generated by the Rhone southern tributaries were monitored. In river waters, the total U_(d) concentration and U isotopes distribution were obtained through Q-ICP-MS measurements. The Lower Rhone River has displayed non-conservative U-behavior, and the variations in U_(d) concentration between southern tributaries were related to the differences in bedrock lithology. The artificially occurring ²³⁶U was detected in the Rhone River at low water discharges, and was attributed to the liquid releases from nuclear industries located along the river. The (²³⁵U/²³⁸U)_(d) activity ratio (=AR) in river waters was representative of the ²³⁵U natural abundance on Earth. The (²²⁶Ra/²³⁸U)_(p) AR in suspended sediment has indicated a radioactive disequilibrium (average 1.3 ± 0.1). The excess of ²³⁴Th in suspended sediment =(²³⁴Th_xs(p)) was apparent solely at low water discharges. The activity of ²³⁴Th_xs(p) was calculated through gamma measurements and ranged from unquantifiable to 56 ± 14 Bq kg^-1. The possibility of using ²³⁴Th as a tracer for the suspended sediment dynamics in large Mediterranean river was then discussed.

Environmental evolution records reflected by radionuclides in the sediment of coastal wetlands: A case study in the Yellow River Estuary wetland

Vertical profiles of environmental radionuclides (²¹⁰Pb, ¹³⁷Cs, ²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K) in a sediment core (Y1) of the Yellow River Estuary wetland were investigated to assess whether environmental evolutions in the coastal wetland could be recorded by the distributions of radionuclides. Based on ²¹⁰Pb and ¹³⁷Cs dating, the average sedimentation rate of core Y1 was estimated to be 1.0 cm y^-1. Vertical distributions of natural radionuclides (²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K) changed dramatically, reflecting great changes in sediment input. Concentrations of ²³⁸U, ²³²Th, ²²⁶Ra and ⁴⁰K all had significant positive relationships with organic matter and clay content, but their distributions were determined by different factors. Factor analysis showed that ²³⁸U was determined by the river sediment input while ²²⁶Ra was mainly affected by the seawater erosion. Environmental changes such as river channel migrations and sediment discharge variations could always cause changes in the concentrations of radionuclides. High concentrations of ²³⁸U and ²²⁶Ra were consistent with high accretion rate. Frequent seawater intrusion decreased the concentration of ²²⁶Ra significantly. The value of ²³⁸U/²²⁶Ra tended to be higher when the sedimentation rate was low and tide intrusion was frequent. In summary, environmental evolutions in the estuary coastal wetland could be recorded by the vertical profiles of natural radionuclides.