Significant quantities of radionuclides including a majority of ¹³⁷Cs have been deposited onto Fukushima landscapes following the accident of Fukushima Dai-ichi Power Plant in March 2011. Starting from late 2013, the Japanese government initiated a large-scale decontamination programme for cropland, residential areas, grassland and forest borders, which was conducted on 12% of the catchment area (8.2 km²) while forest, which is the dominant land use (88%), was not decontaminated. The surface layer of cropland and residential soils (~5 cm) concentrating radiocesium (¹³⁴Cs, ¹³⁷Cs) was removed and substituted with a fresh soil -composed of saprolite layer mined at local quarries- which represent 3% of the catchment area (1.8 km²). Thirteen years after the accident, questions remain regarding the fate of residual particle-bound ¹³⁷Cs across terrestrial environments in response to extreme precipitation (e.g. tropical storm, typhoon, extra-tropical cyclone) and associated erosion events. In particular, there is a need to identify and quantify the sources delivering sediment and associated ¹³⁷Cs to the water bodies, to reconstruct and evaluate the impact of decontamination on sediment and radiocesium transfers. To conduct this project, one sediment core was collected in undisturbed condition in June 2021 at a downstream location of the Mano Dam reservoir, which drains an early decontaminated catchment (67 km²) (2014–2016). Elemental geochemistry, organic matter, visible colorimetry, particle size, and radiocesium analyses were conducted on the sediment core, with depth increments of 1 cm. These analyses were used to provide multiple lines of evidence to define and interpret the major precipitation events recorded by the sedimentary sequence. Then, the sediment source fingerprinting technique allowed, with a multiple modelling approach (MixSIAR and BMM), to identify changes in sediment sources with variable contributions from forest, cropland, and subsoil (e.g. channel bank, fresh soil) throughout time. During abandonment (2011–2016), the contribution from cropland sharply decreased (from ~50% to 30-35%) while forest increased (from ~40% to 60-65%). Nevertheless, after the completion of decontamination, in late 2016, a significant increase of cropland contributions was observed, returning to the pre-accidental level in the most recently deposited sediment (~55%). It occurred concomitantly with that of sediment originating from the freshly-added soil (i.e. granite saprolite; from about 5% to 25%), reflecting the impact of decontamination. During abandonment, the ¹³⁷Cs activity in sediment was reduced by 40%, such as the ¹³⁷Cs flux per extreme event, which was reduced by 20%. After the completion of decontamination, although a strong decrease in ¹³⁷Cs activity in sediment was observed (up to -60%), it was not associated with such a significant decrease as ¹³⁷Cs flux per extreme event (0% to -20%). This suggests that the reduction in ¹³⁷Cs activity in the sediment following decontamination may result from a dilution of contaminated sediments originating from forest with sediment originating from decontaminated cropland fresh soil rather than the removal of contaminated soil in designated areas. To understand the impact of natural soil protection against erosion through revegetation on ¹³⁷Cs flux over a longer abandonment time, studying sediment cores from lately decontaminated catchment would be useful.

How to cite: Chalaux-Clergue, T., Chaboche, P.-A., Huon, S., Hayashi, S., Tsuji, H., Wakiyama, Y., Nakao, A., and Evrard, O.: Assessing the impact of soil decontamination on radiocesium and sediment transfers in a catchment affected by Fukushima nuclear accident, Japan, using a reservoir sediment core., EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1796, https://doi.org/10.5194/egusphere-egu24-1796, 2024.

Between 1972 and 1993, in the French West Indies, chlordecone – a toxic organochlorine insecticide – was applied to control the banana weevil. In the late 1990s, the intensification of agricultural practices (i.e. intensive ploughing, herbicide application) under banana plantations is expected to have led to accelerated soil erosion and sediment transfers (Bizeul et al., 2023) to aquatic systems and, ultimately, to marine environments (Sabatier et al., 2021). Due to the high affinity of chlordecone for organic matter and its hydrophobicity, these sediment transfers are associated with chlordecone remobilization (Mottes et al., 2021) and pesticide transfers along the land-to-sea continuum. Nevertheless, the links between soil erosion, sediment and chlordecone transfers are not well understood. The investigation of these processes is therefore essential to manage chlordecone transfers along the land-to-sea continuum.

To this end, three sediment cores were collected in an agricultural reservoir (Saint-Esprit, Martinique) and five soil cores (one-meter depth) were sampled along a transect in a banana plantation draining to the reservoir.

Regarding sediment cores, age-depth models were drawn for each core using short-lived radionuclide activities (Bruel et Sabatier, 2020). Furthermore, dry bulk density was measured to calculate mass accumulation rates. Moreover, chlordecone and organic carbon contents were measured on three cores. Overall, results show a correspondence between the increase of sediment supply to the reservoir and that of chlordecone and organic carbon fluxes. In particular, chlordecone fluxes showed an increase since 1999 (± 4 years, depending on the cores) from 200 µg.kg^-1 to 600-750 µg.kg^-1.

Regarding soil cores, radiocesium activities were measured in 5-cm increments and chlordecone contents were measured in a selection of 2 cores (uplslope and downslope of the transect). On the upper hillslope part, chlordecone contents showed a strong increase at 20 cm, from 255 µg.kg^-1 to 591 µg.kg^-1, in line with radiocesium activity increase, from 0.5 Bq.kg^-1 to 1.4 Bq.kg^-1. On the lowest hillslope part, chlordecone contents showed a strong increase at 70 cm, from 520 µg.kg^-1 to 1220 µg.kg^-1. Based on these results, we assume that chlordecone distribution follows erosion pathways and can accumulate on the foot slope of this banana plantation. Furthermore, in contrast, constant chlordecone contents observed in the upper part of the profile in each core (i.e. 20 and 70 cm) suggest an homogenization of the soil profile, probably due to ploughing operations carried out every 6-8 years for cyclical banana re-plantation.

Overall, these results confirm the transfer of chlordecone with soil particles along a cultivated hillslope and, ultimately, in the sediment deposited in the reservoir. We assume that these processes also reflect land use changes and the occurrence of erosive tropical climatic events. Further work is needed to confirm the validity of these results to other cultivated catchments across the French West Indies.

How to cite: Bizeul, R., Lajoie, O., Cerdan, O., Pak, L.-T., and Evrard, O.: Impact of soil erosion on chlordecone insecticide transfers in a tropical volcanic cultivated subcatchment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-1821, https://doi.org/10.5194/egusphere-egu24-1821, 2024.

Diatom frustules are well-preserved in marine and lacustrine sediments over hundreds or even thousands of years. In addition, although only in very small amounts, they also contain organic matter within their siliceous structure. Previous applications have shown that the ¹⁵N/¹⁴N ratio of the organic nitrogen contained in diatom frustules (diatom-bound δ¹⁵N, or δ¹⁵N_DB) can be used as a proxy for nutrient cycling in the polar oceans, and that it is not affected by diagenetic effects. However, the applicability of this paleo-proxy to lacustrine sediments has never been tested. Here, we explore the use of δ¹⁵N_DB to reconstruct the history of nitrogen dynamics in Lake Baldegg (Switzerland) over the past 300 years. This lake was heavily eutrophied due to anthropogenic activities during the 20^th century, before the implementation of lake restoration measures (i.e., artificial aeration of the lake bottom since 1982). Using a multi-proxy approach (e.g., reflectance-inferred chlorophyll a and organic carbon accumulation rates, XRF sulfur counts, bulk isotopic composition, C:N ratio), we identified two distinct eutrophication phases (1880-1950 and 1950-1980) that were characterised by an increase in organic matter accumulation and primary productivity, the occurrence of bottom water anoxia, and a change in the origin of the bulk organic matter. The implementation of re-oligotrophication measures has led to the disappearance of anoxic conditions at the bottom of the lake after 1995, and a decrease in phosphorus concentrations in the lake (the latter observed in the monitoring data), which seems to have mitigated primary productivity and organic matter accumulation. δ¹⁵N_DB increased during the first phase of eutrophication, which could be due to extended denitrification in the water column in an expanding anoxic water column zone, and/or limiting N concentrations for phytoplankton growth, leading to increased nitrate utilization. During the second phase, δ¹⁵N_DB decreased, probably because fixed N in surface waters was no longer limiting for phytoplankton. After the implementation of re-oligotrophication measures, δ¹⁵N_DB increased again, possibly the isotopic imprint of external N inputs with a high δ¹⁵N signature, such as organic fertilizers (e.g. animal manure, compost). Additionally, the δ¹⁵N of hand-picked Daphnia ephippia are lower than, and show no consistent offset to, δ¹⁵N_DB, suggesting that the N isotope signal of δ¹⁵N_DB is not transferred to the upper trophic level in that lake. Finally, we measured the offset between δ¹⁵N_DB and δ¹⁵N_BULK providing insight into the effects of early diagenesis on the N isotopic composition of bulk sediments. In Lake Baldegg, the offset reversed after the lake was artificially oxygenated, indicating a role of sediment oxygenation in the diagenetic alteration on δ¹⁵N_BULK.

How to cite: Millet, J., Dubois, N., Lehmann, M. F., and Studer, A. S.: Tracing the eutrophication history of Lake Baldegg using diatom-bound nitrogen isotopes, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17267, https://doi.org/10.5194/egusphere-egu24-17267, 2024.

The Crawford Lake sedimentary sequence in Milton, Ontario, Canada, has been chosen as the Global Boundary Stratotype Section and Point (GSSP) for the proposed Anthropocene Series/Epoch, with its inception occurring at 1952 CE in the mid-20th century. This sequence consists of seasonally deposited layers of organic matter capped by summer-deposited calcite, forming in alkaline surface waters when pH and temperature rise above 7.76 and ~15°C, respectively. These sediments preserve a range of proxies that mirror environmental shifts spanning from local, to regional, global scale, indicative of the Anthropocene's onset. Notably, a significant uptick in industrial fossil fuel combustion in the early 1950s is recorded by increased spheroidal carbonaceous particles and a shift in the sediment's nitrogen isotope composition. During the 1960s, the ratios of 239Pu:240Pu and 14C:12C peak, aligning with heightened radioactive fallout from atmospheric nuclear weapons testing, counterbalancing the old carbon effect in Crawford Lake's dolomitic basin. Post-World War II industrial growth in the Great Lakes region, part of the so-called Great Acceleration, led to acid rain that diminished calcite deposition and impacted primary productivity in the lake. This change is reflected in thinner calcite layers concurrent with the proposed GSSP. These varve thickness variations correlate with climate patterns and lake productivity trends, influenced by cycles like the Quasi-biennial Oscillation, El Nino-Southern Oscillation, the 11-year Schwabe sunspot cycle, and the Pacific Decadal Oscillation. The absence of pigments from anaerobic purple sulfur bacteria suggested an oxygen-rich monimolimnion but with elevated bottom-water salinities that was subsequently confirmed by water property data collected through the modern lake water column in all seasons.  Such an aerobic depositional environment is highly atypical for a meromictic lake and it was the elevated alkalinity and higher salinity conditions that resulted in preservation of varves. The oxygenated bottom waters serendipitously prevented the mobilization of 239Pu in the lake sediments, a key primary stratigraphic marker for the Anthropocene.

How to cite: Patterson, R. T., McCarthy, F. M., Head, M. J., Walsh, C. R., Riddick, N. L., Cumming, B. F., Hamilton, P. B., Pisaric, M. F., Gushulak, A. C., Leavitt, P. R., Lafond, K. M., Llew-Williams, B., Heyde, A., Pilkington, P. M., Moraal, J., Nasser, N. A., Garvie, M., Roberts, S., Rose, N. L., and Roe, H. M.: The varved sediment succession of Crawford Lake, Ontario, Canada: GSSP for the proposed Anthropocene Epoch , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-13348, https://doi.org/10.5194/egusphere-egu24-13348, 2024.

Past mining activities in the Orbiel Valley pose a significant risk of As contamination to its ecosystems and inhabitants. Approximately 12 million tons of tailings from last century's As and Au mining operations remain on site. Rehabilitation works have been implemented to store mining wastes and treat leaching waters. However recent studies have revealed that contamination is still present in water and sediments (Khaska et al., 2015; Delplace et al., 2022). The complexity of the area and previous findings have shown the importance of a more in-depth study of As sources and fate in the watershed, including 1/ characterizing As contamination levels in the Orbiel River and its tributaries during different hydrological periods, 2/ identifying the main sources of As and 3/ distinguishing the natural geochemical baseline from anthropogenic inputs.

Water samples (<0.22 µm) were collected in the Orbiel River and its tributaries from 2018 to 2022, representing a total of 170 samples, to analyze major element and metal(loid) concentrations, alkalinity, dissolved organic carbon, Sr isotope ratio, and As redox speciation in the dissolved fraction. Rock samples representative of the different geological formations were collected to compare the natural and anthropogenic evolution of the Sr isotope along the Orbiel Valley.

Upstream the mining district, in Orbiel River, the dissolved As concentration was about 2 µg/L and increased downstream near the main waste storage area to 7 – 71 µg/L (min-max, depending on the period) with a high proportion of As(III) (> 52 %). The anthropogenic origin of this contamination was confirmed by the ⁸⁷Sr/⁸⁶Sr ratio, which is less radiogenic than in the upstream pristine area, in relation with lime treatment implemented in the mine waste area. However, some valley limestones exhibit a Ca-arsenate-like isotopic ratio, highlighting the need to use complementary tracers to distinguish between anthropogenic and lithological sources. Finally, the mining-impacted tributaries are identified as significant contributors of As to the Orbiel River.

The present study will serve as a reference to interpret the origin, transport, and fate of metal(loid)s during future extreme flood events characteristic of this Mediterranean river.

Delplace, G., Viers, J., Schreck, E., Oliva, P., Behra, P., 2022. Pedo-geochemical background and sediment contamination of metal(loid)s in the old mining-district of Salsigne (Orbiel valley, France). Chemosphere 287 (2).

Khaska, M., Le Gal La Salle, C., Verdoux, P., Boutin, R., 2015. Tracking natural and anthropogenic origins of dissolved arsenic during surface and groundwater interaction in a post-closure mining context: isotopic constraints. J. Contaminant Hydrol. 177–178, 122–135.

How to cite: Heydon, M., Resongles, E., Casiot-Marouani, C., Schreck, E., Behra, P., Freydier, R., Marie, M., Causserand, C., Delpoux, S., Roddaz, M., Pages, A., and Viers, J.: Spatio-temporal evolution and source tracking of arsenic in surface waters of an old mining district (Orbiel Valley, France), EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-11245, https://doi.org/10.5194/egusphere-egu24-11245, 2024.

Historic metal mining and smelting have greatly enhanced the levels and fluxes of heavy metals in and through the fluvial system of the Geul river, a nearly 60 km long transboundary meandering stream in the northeast of Belgium and southeast of the Netherlands. In this study, we examined the long-term (> 1 year) effects of the extreme June 2021 flood on the transfer of sediments and heavy metals through the Geul river system. For this, we quantified the volumetric sediment budget of the channel belt of Dutch part of the Geul river using 0.5 m resolution Lidar-derived DEMs (Algemeen Hoogtebestand Nederland - AHN) for the 2018-2022 period and compared that to the similarly derived sediment budget for the 2012-2018 period. Furthermore, samples of fine sediment from the river bed and the top of the point bars were collected at more or less regular downstream intervals in 2022 and 2023, respectively. These sediment samples were analysed for total zinc and lead concentrations.

During the 2012-2018 period, the sediment of the channel belt was generally negative with an average net erosion rate of about 130 m³ km^-1 y^-1. This implies that during this period, river cut-bank erosion was not fully compensated by pointbar accretion and that the surface level of the newly formed point bars of the meandering Geul river was in general lower than the former floodplain surface. During the 2018-2022 period, the sediment budget was close to zero in the first 22 km of the Dutch reach downstream from the Belgian-Dutch border. However, in the downstream portion of the channel belt, the net deposition rate increased strongly with an average of about 380 m³ km^-1 y^-1. This positive sediment budget indicates strong aggradation of the point bars, which can most likely be attributed to backwater effects during the 2021 flood event, which upstream from a culvert underneath a canal close to the confluence of the Geul river into the Meuse river.

The zinc and lead concentrations in the fine fractions of the bed sediments shows a gradually decreasing trend in downstream direction which can be attributed to dilution from less contaminated sediment inputs from soil erosion on the upstream hillslopes and bank erosion. This pattern cannot be directly linked to the June 2021 flood event. In the reach where the sediment budget was close to zero during the 2018-2022 period, the zinc and lead concentrations in the point bar sediments are comparable to those in the fine bed sediments and show similar decreasing downstream trend. However, in the downstream reach, where net aggradation occurred during the 2018-2022 period, the metal concentrations in the point bar sediments deviate from the generally decreasing trend and increase again by a factor of about four. This downstream pattern in metal concentrations denotes that during the 2021 flood event, sediments originating from the contaminated upstream reaches of the Geul river skipped a substantial reach the Geul channel belt and were mainly deposited in the downstream portion of the channel belt.

How to cite: van der Perk, M. and Walcott, D.: Downstream transfer of metal-contaminated sediments in the Geul river as a result of the extreme June 2021 flood event, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-12652, https://doi.org/10.5194/egusphere-egu24-12652, 2024.

River deltas are reliant on upstream fluvial sediment delivery for their survival. The ultimate sediment delivery to deltas and the changing bed dynamics of river channels are strongly dependent on climate and anthropogenic changes within the entire river basin that control the increase (due to e.g. increased erosion, climate change) or decrease (due to e.g. sand mining, dam construction) of sediment supply. In the case of the Rhine-Meuse basin, suspended sediment delivery to the delta apex at Lobith has decreased since the 1950s. Therefore, we investigated changes in suspended sediment concentrations (SSC) and suspended sediment loads (SSL) over time along the main Rhine branch and its major tributaries (the Aare, Main, Mosel and Neckar) to determine the cause of the decline. We hypothesis and mathematically demonstrate that the spatial pattern in the temporal change can explain and determine specific mechanisms that are causing the decline.

Using collated SSC data of varying frequency from 1997-2014, we explored the suspended sediment transport within and along branches using the rating curve method & discharge-suspended sediment relations for a total of 26 measurements stations in the basin. These were compared with bed level data from Ylla-Arbós et al. (2021), to examine the interaction of SSC with bed dynamics.

A clear spatial trend emerged: the decrease in SSC strongly increases in an upstream direction. In the Alpine Rhine SSC has increased. There is negligible change in the upper basin/impounded section of the Rhine. However, SSC decreases emerge after the confluences with the Main and Mosel branches and this decrease becomes stronger moving towards the delta.

We find that contrary to many other river basins which are showing declining fluvial sediment delivery to deltas due to upstream dams or sediment management activities, in the Rhine-Meuse basin  the cause is actually the changing retention of channels and differing erosion rates from the river bed. Since the 19^th century there have been activities to straighten and narrow the Rhine river to embank and fix the river course for navigation. This created high amounts of incision in the river bed in the early 20^th century, but as proven by Ylla-Arbós et al. (2021) and others, this incision is now decreasing. These changes in suspended sediment supply from the river bed can be correlated to the changing supply at the delta apex. Since the 1980s efforts have been made to stabilize bed erosion and this ‘fixing’ of the river beds has led ultimately to a declining suspended sediment supply to the delta apex. This suggests that response to human interventions is not only relevant at a centurial timescale but is likely to be a defining feature of sediment supply for the coming century.  

References

Ylla Arbós, C., Blom, A., Viparelli, E., Reneerkens, M., Frings, R. M., & Schielen, R. M. J. (2021). River response to anthropogenic modification: Channel steepening and gravel front fading in an incising river. Geophysical Research Letters, 48(4), e2020GL091338.

How to cite: Cox, J., Edler, T., van der Perk, M., and Middelkoop, H.: Unravelling the mechanisms behind the spatial and temporal trends of suspended sediment in the Rhine basin , EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-17656, https://doi.org/10.5194/egusphere-egu24-17656, 2024.

Export of potential toxic elements and other particle bound pollutants from catchments is highly dependent on the flow regime. The main driver is the higher mobilisation and transport capacity for suspended particulate matter (SPM) during high flow events.  But are there further dynamics in the concentrations which are not purely driven by the SPM transport?

To answer this question, we investigated the dynamics of the concentrations of potential toxic elements, several other elements and total suspended solids during high flow events by automated sampling and subsequent analysis of dissolved and total concentrations by ICP-MS after microwave assisted acid digestion. At 3 river monitoring sites 3 high flow events were sampled with 3-6 samples per event and site, covering different parts of the flow and turbidity peaks, which were recorded by online-measurements. To complement the river monitoring with data about potential sediment sources, landuse-stratified soil sampling in the catchment and river bed sediment sampling were conducted.

Our case study was the Wulka river in eastern Austria with a catchment area of 384 km² and two if its tributaries, one with a very high share of treated waste water and the other with no permanent input of waste water. With a mean annual precipitation of 695 mm and a mean elevation of 256 m a.s.l. the river can be classified as a low land river. The landuse is dominated by agriculture including significant share of viniculture.

A first explorative principal component analysis showed, that several elements are strongly related with each other and the suspended sediment concentration. As this was expected, we used the SPM concentration to normalize the elemental concentrations and therefore taking out the variability caused by the suspended solids dynamics for further analysis. The remaining variability will be investigated regarding temporal and spatial patterns and correlation with the sediment and soil concentrations which can give indications about the emission pathways and sources.

To characterize the sampled high flow events, a hysteresis index was calculated from the discharge and turbidity signal which revealed different types of hysteresis, some clockwise hysteresis, several complex hysteresis patterns with different directions of the hysteresis during different times of the event and one small event with anticlockwise hysteresis. Different types of hysteresis can give indications about the distance of the sediment source to the observation location, further contributing to the exploration of SPM sources.

How to cite: Kittlaus, S., Milačič, R., Marković, K., Weber, N., Zessner, M., and Zoboli, O.: Mobilisation and transport dynamics of potential toxic elements during high flow events in a small river catchment, EGU General Assembly 2024, Vienna, Austria, 14–19 Apr 2024, EGU24-9548, https://doi.org/10.5194/egusphere-egu24-9548, 2024.