Basin scale sources of siltation in a contaminated hydropower reservoir.

Siltation and the loss of hydropower reservoir capacity is a global challenge with a predicted 26 % loss of storage at the global scale by 2050. Like in many other Latin American contexts, soil erosion constitutes one of the most significant water pollution problems in Chile with serious siltation consequences downstream. Identifying the sources and drivers affecting hydropower siltation and water pollution is a critical need to inform adaptation and mitigation strategies especially in the context of changing climate regimes e.g. rainfall patterns. We investigated, at basin scale, the main sources of sediments delivered to one of the largest hydropower reservoirs in South America using a spatio-temporal geochemical fingerprinting approach. Mining activities contributed equivalent to 9 % of total recent sediment deposited in the hydropower lake with notable concentrations of sediment-associated pollutants e.g. Cu and Mo in bed sediment between the mine tributary and the reservoir sediment column. Agricultural sources represented ca. 60 % of sediment input wherein livestock production and agriculture promoted the input of phosphorus to the lake. Evaluation of the lake sediment column against the tributary network showed that the tributary associated with both dominant anthropogenic activities (mining and agriculture) contributed substantially more sediment, but sources varied through time: mining activities have reduced in proportional contribution since dam construction and proportional inputs from agriculture have increased in recent years, mainly promoted by recent conversion of steep lands from native vegetation to agriculture. Siltation of major hydropower basins presents a global challenge exemplified by the Rapel basin. The specific challenges faced here highlight the urgent need for co-design of evidence-led, context-specific solutions that address the interplay of drivers both within and without the basin and its communities, enhancing the social acceptability of sediment management strategies to support the sustainability of clean, hydropower energy production.

Optimisation of plutonium separations using TEVA cartridges and ICP-MS/MS analysis for applicability to large-scale studies in tropical soils.

The analysis of plutonium (Pu) in soil samples can inform the understanding of soil erosion processes globally. However, there are specific challenges associated for analysis in tropical soils and so an optimal analytical methodology ensuring best sensitivity is critical. This method aimed to demonstrate the feasibility of sample preparation and analysis of Pu isotopes in African soils, considering the environmental and cost implications applicable to low-resource laboratories. The separation procedure builds upon previous work using TEVA columns, further demonstrating their usefulness for the reduction of uranium (U) interference in ICP-MS analysis with enhanced selectivity for Pu. Here several steps were optimised to enhance Pu recovery, reducing method blank concentration, and improving the separation efficiency through the determination of the elution profiles of U and Pu. The elimination of the complexing agent in the eluent, increased the spike recovery by improving matrix tolerance of the plasma, and simplified the separation procedure, improving throughput by 20%. The subsequent method was validated through the analysis of Certified Reference Material IAEA-384, where high accuracy and improved precision of measurement were demonstrated (measured value 114 ± 12 versus certified value 108 ± 13 Bq kg-1). Optimisation of the column separation, along with the analysis of the samples using O2 gas in ICP-MS/MS mode to mass shift Pu isotopes away from interfering molecular U ions provided a simple, robust, and cost-effective method with low achievable method detection limits of 0.18 pg kg-1 239+240Pu, applicable to the detection of ultra-trace fallout Pu in African soils.

Aquatic phosphorus behaviour within a UK Ramsar wetland: Impacts of seasonality and hydrology on algal growth and implications for management.

Fundamental to all life, phosphorus is an essential nutrient and, contrastingly, a significant threat to surface water biodiversity globally as one of the most common causes of eutrophication in surface waters worldwide. Freshwater wetland ditches affected by these conditions undergo a conversion from primarily submerged aquatic vegetation to algae or duckweed dominance, leading to anoxic conditions. However, macrophyte biomass harvesting in eutrophic water systems is a promising means of remediation and nutrient recycling. This study seasonally assesses spatial distribution and chemical fractionation of surface water phosphorus, as well as surface biomass abundance and total phosphorus content in the ditch systems at West Sedgemoor (Somerset, UK), a designated site of special scientific interest. Elevated phosphorus concentrations in the surface water were observed across the site, with all sites exceeding e hi the Common Standards Monitoring environmental quality standard value of 0.1 mg L-1 during summer and autumn Sites lacking hydrological flow connectivity with contaminated freshwater inputs, typically had lower surface water phosphorus concentrations than the rest of the moor, with localised hotspots associated with likely cattle related agricultural activity. Summer and autumn were determined as the dominant duckweed growth seasons, in which an estimated 39 kg of phosphorus could be removed via duckweed biomass harvesting, per harvest period.

Distribution and storage of uranium, and its decay products, in floodplain sediments.

Industrialised rivers contain legacy contaminants stored in their sediments and floodplain soils which may inhibit attainment of environmental quality criteria. The River Fal catchment, SW England, is impacted by inputs from uranium mining and clay production and serves as an exemplar for understanding the consequences of medium-term process dynamics in contaminated basins. Radionuclides were determined, by gamma spectroscopy, in six cores from the river floodplain with the aim of quantifying the activities of 238U, and its decay products, and the bomb fallout radionuclides137Cs and 241Am. Activity concentrations of 238U implied inputs from mining, accentuated by flood events and historic industrial accidents, whereas 210Pb activities included a significant input of unsupported 210Pb linked to processed mine spoil. The radionuclide inventories did not decrease systematically downstream revealing evidence of attenuation of particulate radionuclides within the river floodplain sediment column. Storage of legacy contaminants in fluvial systems, at levels in excess of contemporary environmental quality guidelines, emphasises the challenges posed by changing climatic conditions. This scenario raises significant consequences for the management of uranium-contaminated, fertile riverine floodplains within Europe.

Composition of deposited sediment and its temporal variation in a disturbed tropical catchment in the Kelantan river basin, Peninsular Malaysia.

Pristine tropical river systems are coming under increasing pressure from the development of economic resources such as forestry and mining for valuable elements. The Lebir catchment, north eastern Malaysia, is now under development as a result of unregulated tree felling and mining for essential and rare metals. Two sediment cores, one in the upstream reaches and the other from the downstream reaches, were taken from flood prone area of the Lebir River, Malaysia, and analysed for their elemental composition by XRF, specifically Al, Si, Fe, Ca, K, Mg, Mn, V, Cu, Ni, Pb, Cr, Zn, As, Th and U. Activities of fallout radionuclides, 137Cs and 210Pb were also determined to from a geochronological context. The elemental concentrations in the soils were assessed in terms of their enrichment factor and Si, Ca, K, Mg, Mn, V, Cu, Ni and Zn were found not to be enriched, whereas As, Th and U had elevated enrichment factors. The Th and U were particularly enriched in the downstream core indicating inputs from a tributary that drains a catchment with known deposits of Th and possibly U. The results suggest that the growth in economic development is fostering the transport of contaminants by the major rivers which, in turn, is contaminating the riverine floodplains. This points to the need for a more integrated and holistic approach to river basin management to maintain the environmental quality of these fragile aquatic systems.

Microscale spatial variability of Beryllium-7 at a reference site in southwest China.

Quantitative assessment of soil erosion and deposition rates using fallout radionuclides (FRNs), including Beryllium-7 (7Be), requires establishment of a reliable reference inventory i.e. the inventory of a non-eroding point. Little information, however, is currently available on the microscale spatial variability of 7Be inventory within reference sites. This is important information to inform sample design and replication, and in addition, to evaluate the uncertainty of derived soil redistribution data. In this study, soil samples were taken systematically at grid points on a 5 m × 12 m experimental reference plot with a bare soil surface, at two sampling occasions (2019 and 2021) in southwest China. 7Be activities were measured to explore the potential variability of 7Be inventory at the microscale. To determine possible causes of 7Be inventory variation, physicochemical characteristics including organic matter content (OM), pH, cation exchange capacity (CEC) and grain size compositions were analyzed at each sample location. 7Be inventories for the two periods were estimated at 211.1 ± 20.0 and 456.1 ± 43.8 Bq m-2 (mean ± 2 SEM, n = 44), with coefficients of variation of 31.4 and 31.9% for the 2019 and 2021 sampling cases, respectively. No significant correlations were observed between 7Be activity and the measured soil compositional properties, suggesting observed spatial variability is primarily a result of random variation due to rainsplash and other processes, although sampling and measuring processes may contribute some uncertainties. Using the traditional method, ca. 40 independent reference samples are required to estimate the mean 7Be inventory, i.e. to represent input across the site, with an allowable error of 10% at 95% confidence, while application of a bootstrap approach suggests that ca. 28 would be adequate under similar accuracy. Overall, results of this study emphasize that the simple assumption of uniform distribution of 7Be across the reference area needs detailed examination on a case-by-case basis, if this radionuclide is to be used effectively to assess patterns and rates of soil redistribution from field to hillslope scale.

Evaluating spatio-temporal soil erosion dynamics in the Winam Gulf catchment, Kenya for enhanced decision making in the land-lake interface.

Soil erosion accelerated by poor agricultural practices, land degradation, deprived infrastructure development and other anthropogenic activities has important implications for nutrient cycling, land and lake productivity, loss of livelihoods and ecosystem services, as well as socioeconomic disruption. Enhanced knowledge of dynamic factors influencing soil erosion is critical for policymakers engaged in land use decision-making. This study presents the first spatio-temporal assessment of soil erosion risk modelling in the Winam Gulf, Kenya using the Revised Universal Soil Loss Equation (RUSLE) within a geospatial framework at a monthly resolution between January 2017 and June 2020. Dynamic rainfall erosivity and land cover management factors were derived from existing datasets to determine their effect on average monthly soil loss by water erosion. By assessing soil erosion rates with enhanced temporal resolution, it is possible to provide greater knowledge regarding months that are particularly susceptible to soil erosion and can better inform future strategies for targeted mitigation measures. Whilst the pseudo monthly average soil loss was calculated (0.80 t ha-1 month-1), the application of this value would lead to misrepresentation of monthly soil loss throughout the year. Our results indicate that the highest erosion rates occur between February and April (average 0.95 t ha-1 month-1). In contrast, between May and August, there is a significantly reduced risk (average 0.72 t ha-1 month-1) due to the low rainfall erosivity and increased vegetation cover as a result of the long rainy season. The mean annual gross soil loss by water erosion in the Winam Gulf catchment amounts to 10.71 Mt year-1, with a mean soil loss rate of 9.63 t ha-1 year-1. These findings highlight the need to consider dynamic factors within the RUSLE model and can prove vital for identifying areas of high erosion risk for future targeted investigation and conservation action.

Evaluating the effectiveness of soil conservation at the basin scale using floodplain sedimentary archives.

Evaluation of the spatial and temporal composition of floodplain sediments and soils is critical in the creation of soil management strategies for impacted riverine catchments. The objective of this study was to determine the distribution, and to identify the sources, of particulate trace elements and fallout radionuclides in the catchment of the River Avon (SW England), where sedimentary processes had been altered by reservoir construction in the 1950s. The catchment was compartmentalized into its main functional units namely, cultivated land, pasture, woodland, wet moorland, and channel bank. Surface soils were collected in each unit, along with four strategically-placed cores, all of which were analyzed for particle size, fallout radionuclides and elemental concentrations. Sediment particle sizes and sediment accumulation rates were affected by the construction of the reservoir, specifically the distributions of silt and clay. The concentrations of fertilizer constituent Cr and P were highly correlated in the mid-catchment but were unrelated downstream due to elevated concentrations of Cr from geological deposits. Copper, As, Pb and Sn had variable down-core distributions, with pulses in concentrations due to mining inputs. The contributions of the end-member sources of particulate elements in the sedimentary mixtures were evaluated, quantitatively, using a Bayesian Mixing Model and the cultivated land was identified as a significant contributor to the mixtures, independent of space and time. The results contribute to advances in soil quality and conservation measures as components of a catchment management plan for the Avon, an approach maybe applicable to other small catchments in the UK and internationally.

Spatial distribution of sediment phosphorus in a Ramsar wetland.

Eutrophication is a significant threat to surface water biodiversity worldwide, with excessive phosphorus concentrations being among the most common causes. Wetland ditches under these conditions shift from primarily submerged aquatic vegetation to algae or duckweed dominance, leading to excessive shading and anoxic conditions. Phosphorus, from both point (e.g. wastewater treatment works) and diffuse (largely agricultural runoff) sources, is currently the central reason for failure in the majority of surface water bodies in England to meet required water quality guidelines. This study assesses phosphorus storage in the ditch systems at West Sedgemoor, a designated site of special scientific interest. Elevated phosphorus concentrations in sediment was observed across the Moor up to 4220 mg Kg-1, almost 10 times that which may be expected from background levels. The highest concentrations were generally observed at the more intensively farmed sites in the north of the moor, near key inlets and the outlet. Based upon their chemical and physical properties, clear distinction was observed between sites outside and within the Royal Society of the Protection of Birds nature reserve, using principal component analysis.

Determining tributary sources of increased sedimentation in East-African Rift Lakes.

Temporal and spatial sediment dynamics in an East-African Rift Lake (Lake Manyara, Tanzania), and its river inputs, have been evaluated via a combination of sediment tracing and radioactive dating. Changes in sedimentation rates were assessed using radioactive dating of sediment cores in combination with geochemical profile analysis of allogenic and autogenic elements. Geochemical fingerprinting of riverine and lake sediment was integrated within a Bayesian mixing model framework, including spatial factors, to establish which tributary sources were the main contributors to recent lake sedimentation. The novel application of Bayesian source attribution on sediment cores and subsequent integration with sedimentation data permitted the coupling of changes in the rate of lake sedimentation with variations in sediment delivery from the tributaries. These complimentary evidence bases demonstrated that Lake Manyara has experienced an overall upward trajectory in sedimentation rates over the last 120 years with distinct maxima between 0.80 and 0.85 g cm-2 yr-1 in the 1960s and between 0.84 and 1.81 g cm-2 yr-1 in 2010. Increased sedimentation rates are largely a result of a complex interaction between increased upstream sediment delivery following changes in land cover and natural rainfall fluctuations. Modelling results identified two specific tributaries as responsible for elevated sedimentation rates, contributing 58% and 38% of the recently deposited lake sediment. However, the effects of sedimentation were shown to be spatially distinct given the domination of different tributaries in various areas of Lake Manyara. The application of source-tracing techniques constrained sedimentation problems in Lake Manyara to specific tributary sources and established a link between upstream land degradation and downstream ecosystem health. This novel application provides a solid foundation for targeted land and water management strategies to safeguard water security and environmental health in Lake Manyara and has potential application to fill knowledge gaps on sediment dynamics in other East-African Rift Lakes.

Extreme levels of fallout radionuclides and other contaminants in glacial sediment (cryoconite) and implications for downstream aquatic ecosystems.

Glaciers in most parts of the world are retreating, releasing water and sediments to downstream rivers. Studies have found elevated levels of fallout radionuclides (FRNs) and other contaminants in glacial sediments, especially cryoconite, in European glaciers and Greenland. However, there are no equivalent studies for glaciers in North America. We report concentrations of FRNs (i.e. 137Cs, 210Pbun and 241Am) and other contaminants (i.e. metal(loids), phosphorus) in cryoconite and proglacial sediments from a glacier in British Columbia, Canada, and compare values to suspended sediments from the downstream river. The mean concentrations of 137Cs, 210Pbun and 241Am in cryoconite were 2,123 ± 74, 7,535 ± 224 and 11.5 ± 3.0 Bq kg-1, respectively, which are an order of magnitude greater than those for most soils and surficial materials. FRNs were much lower in suspended sediments and decreased with distance away from the glacier. Geochemical elements were enriched in cryoconite relative to local clastic materials and upper continental crust. Concentrations of FRNs in cryoconite were correlated with organic matter, which suggests this is important in controlling the scavenging of hydrophobic contaminants in glacial meltwater. Low concentrations of FRNs and contaminants in suspended sediments suggest that glacial meltwater and the delivery of cryoconite have limited impact on downstream aquatic ecosystems.

Fingerprinting changes in source contribution for evaluating soil response during an exceptional rainfall in Spanish pre-pyrenees.

In the Mediterranean region, floods are expected to increase as a result of climate change and knowledge of soil erosion hot spots during exceptional rainfalls is required to support mitigation measures. This study quantifies the main sediment sources during an exceptional rainfall event in 2012 (235 mm) at the outlet of two catchments located in NE Spain. To this purpose, suspended sediments were collected during the flood event, complemented with entrapped sediments in mat taken one year after the event. We used fingerprinting methodology and applied the FingerPro unmixing model to estimate the contribution from main sources. The selected tracers clearly distinguished agricultural, rangeland, subsoil and channel banks as the four potential sources in both catchments. In the Vandunchil catchment, the 8 time-integrated suspended sediment samples revealed changes in source contribution during the 2-h sampling sequence. There were relatively high contributions from rangeland, agriculture and subsoil at the beginning of the sampling, representing 30, 40 and 35% of the total source contributions, respectively. Our records captured the delivery of pulses of eroded surface soil transported by runoff with direct connectivity to the stream. The sequence was followed by a sharp increase in channel bank contribution (up to 90%) in comparison to the other sources, reflecting streambank erosion and landslide occurrence, which manifested during the flood. In contrast, in the La Reina catchment, agricultural soils contributed the most (65%) and, together with subsoils (32%), were the main sources. These results reflect the effect of the higher connectivity and slope gradient of these cultivated fields of the La Reina catchment in comparison with those of the Vandunchil catchment. We discuss the possibility of using different properties, such as radionuclides, geochemistry and magnetic measurements, as tracers to distinguish between potential sources during an exceptional event in upland Mediterranean catchments. Our results support the use of fingerprinting techniques to determine variations in source contribution and sediment provenance during flood events, as extreme rainfalls are main drivers of sediment mobilization and key factors in changing landscapes. This is essential in identifying vulnerable hot spots, in which early-stage interventions are needed, and for helping policy makers with management of soil and water resources.

A deconvolutional Bayesian mixing model approach for river basin sediment source apportionment.

Increasing complexity in human-environment interactions at multiple watershed scales presents major challenges to sediment source apportionment data acquisition and analysis. Herein, we present a step-change in the application of Bayesian mixing models: Deconvolutional-MixSIAR (D-MIXSIAR) to underpin sustainable management of soil and sediment. This new mixing model approach allows users to directly account for the 'structural hierarchy' of a river basin in terms of sub-watershed distribution. It works by deconvoluting apportionment data derived for multiple nodes along the stream-river network where sources are stratified by sub-watershed. Source and mixture samples were collected from two watersheds that represented (i) a longitudinal mixed agricultural watershed in the south west of England which had a distinct upper and lower zone related to topography and (ii) a distributed mixed agricultural and forested watershed in the mid-hills of Nepal with two distinct sub-watersheds. In the former, geochemical fingerprints were based upon weathering profiles and anthropogenic soil amendments. In the latter compound-specific stable isotope markers based on soil vegetation cover were applied. Mixing model posterior distributions of proportional sediment source contributions differed when sources were pooled across the watersheds (pooled-MixSIAR) compared to those where source terms were stratified by sub-watershed and the outputs deconvoluted (D-MixSIAR). In the first example, the stratified source data and the deconvolutional approach provided greater distinction between pasture and cultivated topsoil source signatures resulting in a different posterior distribution to non-deconvolutional model (conventional approaches over-estimated the contribution of cultivated land to downstream sediment by 2 to 5 times). In the second example, the deconvolutional model elucidated a large input of sediment delivered from a small tributary resulting in differences in the reported contribution of a discrete mixed forest source. Overall D-MixSIAR model posterior distributions had lower (by ca 25-50%) uncertainty and quicker model run times. In both cases, the structured, deconvoluted output cohered more closely with field observations and local knowledge underpinning the need for closer attention to hierarchy in source and mixture terms in river basin source apportionment. Soil erosion and siltation challenge the energy-food-water-environment nexus. This new tool for source apportionment offers wider application across complex environmental systems affected by natural and human-induced change and the lessons learned are relevant to source apportionment applications in other disciplines.

Comparing catchment sediment fingerprinting procedures using an auto-evaluation approach with virtual sample mixtures.

Information on sediment sources in river catchments is required for effective sediment control strategies, to understand sediment, nutrient and pollutant transport, and for developing soil erosion management plans. Sediment fingerprinting procedures are employed to quantify sediment source contributions and have become a widely used tool. As fingerprinting procedures are naturally variable and locally dependant, there are different applications of the procedure. Here, the auto-evaluation of different fingerprinting procedures using virtual sample mixtures is proposed to support the selection of the fingerprinting procedure with the best capacity for source discrimination and apportionment. Surface samples from four land uses from a Central Spanish Pyrenean catchment were used i) as sources to generate the virtual sample mixtures and ii) to characterise the sources for the fingerprinting procedures. The auto-evaluation approach involved comparing fingerprinting procedures based on four optimum composite fingerprints selected by three statistical tests, three source characterisations (mean, median and corrected mean) and two types of objective functions for the mixing model. A total of 24 fingerprinting procedures were assessed by this new approach which were solved by Monte Carlo simulations and compared using the root mean squared error (RMSE) between known and assessed source ascriptions for the virtual sample mixtures. It was found that the source ascriptions with the highest accuracy were achieved using the corrected mean source characterisations for the composite fingerprints selected by the Kruskal Wallis H-test and principal components analysis. Based on the RMSE results, high goodness of fit (GOF) values were not always indicative of accurate source apportionment results, and care should be taken when using GOF to assess mixing model performance. The proposed approach to test different fingerprinting procedures using virtual sample mixtures provides an enhanced basis for selecting procedures that can deliver optimum source discrimination and apportionment.

Initial flux of sediment-associated radiocesium to the ocean from the largest river impacted by Fukushima Daiichi Nuclear Power Plant.

This study aimed to quantify the flux of radiocesium in the Abukuma Basin (5,172 km(2)), the largest river system affected by fallout from the Fukushima Daiichi Nuclear Power Plant (FDNPP) event. In the period from 10 August 2011 to 11 May 2012 an estimated 84 to 92% of the total radiocesium transported in the basin's fluvial system was carried in particulate form. During this monitoring period Typhoon Roke (September 2011) was observed to induce a significant and temporally punctuated redistribution of radiocesium. The storm-mobilised radiocesium was an estimated 6.18 Terabecquerels corresponding to 61.4% of the total load delivered to the coastal zone during the observation period. The total flux of radiocesium into the Pacific Ocean estimated at the outlet station (basin area 5,172 km(2)) was 5.34 TBq for (137)Cs, and 4.74 TBq for (134)Cs, corresponding to 1.13% of the total estimated radiocesium fallout over the basin catchment (890 TBq). This was equivalent to the estimated amount of direct leakage from FDNPP to the ocean during June 2011 to September 2012 of 17 TBq and the Level 3 Scale Leakage on 21 August 2013 (24 TBq).

A 700 year record of combustion-derived pollution in northern Spain: tools to identify the Holocene/Anthropocene transition in coastal environments.

There is an uneven geographical distribution of historic records of atmospheric pollutants from SW Europe and those that exist are very limited in temporal extent. Alternative data source is required to understand temporal trends in human impacts on atmospheric pollution. Polycyclic aromatic hydrocarbons (PAHs), heavy metal content and stable Pb isotopic ratios in a sediment core from a salt marsh in northern Spain were used to reconstruct the regional history of contaminant inputs over the last 700 years. Pre-1800s concentrations of Pb and PAHs represented baseline concentrations, i.e. pre-Industrial, conditions. During the initial stages of the Industrial Revolution, 1800s to 1860s, PAH concentrations increased by a factor of about two above baseline levels in the sediment column. By the 1930s, PAH levels reached ca. 10 times pre-Industrial levels and, along with Pb, reached a peak at ca. 1975 CE. Since then, sedimentary PAH and Pb concentrations decreased significantly. A combination of PAH isomer and Pb stable isotope ratios suggests that the contaminant sources are regional, likely derived partially from wood, but mainly coal used by the metallurgic industry in the Basque country since the 1800s and until the 1970s when leaded petrol saw increased use. This chronology of regional atmosphere-derived pollution expands current southwest Europe emission records and shows coastal salt marsh sediments to be useful in reconstructing the Anthropocene.

Fate of 90Sr and U(VI) in Dounreay sediments following saline inundation and erosion.

There is concern that sea level rise associated with projected climate change will lead to the inundation, flooding and erosion of soils and sediments contaminated with radionuclides at coastal nuclear sites, such as Dounreay (UK), with seawater. Here batch and column experiments were designed to simulate these scenarios and sequential extractions were used to identify the key radionuclide solid phase associations. Strontium was exchangeable and was mobilised rapidly by ion exchange with seawater Mg(2+) in both batch and column experiments. In contrast, U was more strongly bound to the sediments and mobilisation was initially limited by the influence of the sediment on the pH of the water. Release was only observed when the pH increased above 6.9, suggesting that the formation of soluble U(VI)-carbonate species was important. Under dynamic flow conditions, long term release was significant (47%), but controlled by slow desorption kinetics from a range of binding sites.

Mobilization of technetium from reduced sediments under seawater inundation and intrusion scenarios.

Predicted sea level rise would increase the vulnerability of low lying coastal legacy nuclear sites to inundation and intrusion with oxygenated seawater. This could have a significant impact on the mobility of redox-sensitive radionuclides such as Tc. Here, batch and column experiments were used to simulate and investigate the effect of these processes on the mobilization of Tc from sediments under a range of geochemically reduced conditions. Batch experiments showed that only a small proportion of Tc was rapidly (within 5 days) released from the sediments into seawater and groundwater. The subsequent Tc release was slowest and ultimately limited to the greatest extent (17%) in initially Fe-reducing sediments, when they were reoxidized in seawater. Thus, the cycling of iron and the impact of the water chemistry on iron mineralogy were important for hindering Tc release. Column experiments showed that iron minerals were less effective at retarding Tc release under flow-through conditions. Kinetically controlled and solubility limited Fe dissolution led to ongoing Tc release from the sediments; i.e. the retarding effect of iron phases was temporary, and significantly more Tc was mobilized (79-93%) compared with the batch experiments (17-45%). These results demonstrate the potential for Tc(IV) to be oxidized and mobilized from sediments at coastal nuclear sites resulting from predicted intrusion and inundation with oxic seawater.