Tritium (H) and Carbon-14 (C) 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 C levels close to the atmospheric reference values (231 Bq·kg of C in 2015) or even lower in most of cases, and show sporadic and weak labelling near nuclear facilities. The low C 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 ofC. 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.
Tritium background levels in various environmental compartments are deeply needed in particular to assess radiological impact, especially in river systems where most of releases from nuclear facilities are performed. The present study aims to identify the main environmental factors that influence tritium background levels in rivers at the regional scale. 41 samples were collected from 2014 to 2016 along 17 small rivers in the south of France. All were located out of the influence of direct releases from nuclear facilities. Tritiated water (HTO) concentrations measured in water samples ranged from 0.12±0.11 to 0.86±0.15BqL and HTO concentrations in rains were modelled between 2015 and 2016 over the study period referring to time series acquired from 1963 to 2014 at Thonon-les-Bains monitoring station. The results of tritium concentrations in rivers studied present a significant variability and are more than twice lower than forecasted values in rain. Multiple linear regressions allowed identifying that HTO concentration in rains, watershed area and altitude were the main tested parameters that are linked to the variability of HTO concentrations in the studied rivers. Finally, HTO fluxes delivered to the Mediterranean Sea by French coastal rivers out of influence of nuclear releases were estimated. The results highlight that those account for around 1% of HTO exported while 99% are transferred by the nuclearized Rhone River.
Tritium of artificial origin was initially introduced to the environment from the global atmospheric fallout after nuclear weapons tests. Its level was increased in rainwaters by a factor 1000 during peak emissions in 1963 within the whole northern hemisphere. Here we demonstrate that tritium from global atmospheric fallout stored in sedimentary reservoir for decades as organically bound forms in recalcitrant organic matter while tritium released by nuclear industries in rivers escape from such storages. Additionally, we highlight that organically bound tritium concentrations in riverine sediments culminate several years after peaking emission in the atmosphere due to the transit time of organic matter from soils to river systems. These results were acquired by measuring both free and bound forms of tritium in a 70 year old sedimentary archive cored in the Loire river basin (France). Such tritium storages, assumed to be formed at the global scale, as well as the decadal time lag of tritium contamination levels between atmosphere and river systems have never been demonstrated until now. Our results bring new lights on tritium persistence and dynamics within the environment and demonstrate that sedimentary reservoir constitute both tritium sinks and potential delayed sources of mobile and bioavailable tritium for freshwaters and living organisms decades after atmospheric contamination.
Rivers solid discharge represents a substantial environmental issue, especially for the coastal marine environment. Unlike continental climate rivers, Mediterranean rivers show large discharge variability linked to rainfall, runoff and groundwater discharge, and can be temporary dry. Solid yields are difficult to predict due to variable source of floodwater. This paper assesses the suspended solid discharge of the Las River (SE France), a small stream in karst and urban environments, and its proportion due to flood events. Floods characteristics are analyzed to explore the variability of the solid yield and the influence of urban runoff and karst springs discharge, based on in situ data. The 35 floods events monitored during a 17 months survey largely contributed to the annual yield, with a proportion of 47% of the total water, and 69% of the total solid yielded in 11% of the time. The total rainfall and the total water discharge, related to the water levels in karst springs, drive the total solid yield, mostly composed of mud (90%). Urban runoff induces higher suspended solid concentration than karst flood, but generated a lower total solid yield. Karst springs, by expanding the catchment area of the stream, largely contribute to the total volume of water discharged to the sea and enhance the sand proportion due to their influence on the maximal water discharge. These results reveal the strong influence of the karst spring hydrodynamic functioning on the sediment yield to Mediterranean coastal environments. This study also highlights the efficiency of electric conductivity data to provide valuable insights about floodwater sources and sediment transport processes. Keywords Solid discharge; flood; runoff; urban stream; karst spring; Bay of Toulon Highlights • 69% of the total solid yield of the Las River occur during flood events • Solid discharge is composed at 90% of mud • Electrical conductivity efficiently distinguishes floodwater sources • Karst flood events generate bigger solid yield and enhance sand proportions
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