The representation of sediment source group tracer distributions in Monte Carlo uncertainty routines for fingerprinting: An analysis of accuracy and precision using data for four contrasting catchments

Previous studies comparing sediment fingerprinting un‐mixing models report large differences in their accuracy. The representation of tracer concentrations in source groups is perhaps the largest difference between published studies. However, the importance of decisions concerning the representation of tracer distributions has not been explored explicitly. Accordingly, potential sediment sources in four contrasting catchments were intensively sampled. Virtual sample mixtures were formed using between 10 and 100% of the retrieved samples to simulate sediment mobilization and delivery from subsections of each catchment. Source apportionment used models with a transformed multivariate normal distribution, normal distribution, 25th–75th percentile distribution and a distribution replicating the retrieved source samples. The accuracy and precision of model results were quantified and the reasons for differences were investigated. The 25th–75th percentile distribution produced the lowest mean inaccuracy (8.8%) and imprecision (8.5%), with the Sample Based distribution being next best (11.5%; 9.3%). The transformed multivariate (16.9%; 17.3%) and untransformed normal distributions (16.3%; 20.8%) performed poorly. When only a small proportion of the source samples formed the virtual mixtures, accuracy decreased with the 25th–75th percentile and Sample Based distributions so that when <20% of source samples were used, the actual mixture composition infrequently fell outside of the range of uncertainty shown in un‐mixing model outputs. Poor performance was due to combined random Monte Carlo numbers generated for all tracers not being viable for the retrieved source samples. Trialling the use of a 25th–75th percentile distribution alongside alternatives may result in significant improvements in both accuracy and precision of fingerprinting estimates, evaluated using virtual mixtures. Caution should be exercised when using a normal type distribution, without exploration of alternatives, as un‐mixing model performance may be unacceptably poor. The representation of source group tracer concentrations is perhaps the largest difference between sediment fingerprinting un‐mixing models. Despite this, the effects of different distributions on model accuracy have not been explored explicitly. ‘This study compared a transformed multivariate normal, a normal and a 25th–75th percentile distribution as well as a distribution replicating the retrieved source samples. The 25th–75th percentile distribution produced the lowest mean inaccuracy (8.8%), with the Sample Based being next best (11.5%). The transformed multivariate (16.9%) and untransformed normal distributions (16.3%) performed poorly

Sensitivity of source sediment fingerprinting to tracer selection methods

In a context of accelerated soil erosion and sediment supply to water bodies, sediment fingerprinting techniques have received an increasing interest in the last 2 decades. The selection of tracers is a particularly critical step for the subsequent accurate prediction of sediment source contributions. To select tracers, the most conventional approach is the three-step method, although, more recently, the consensus method has also been proposed as an alternative. The outputs of these two approaches were compared in terms of identification of conservative properties, tracer selection, modelled contributions and performance on a single dataset. As for the three-step method, several range test criteria were compared, along with the impact of the discriminant function analysis (DFA). The dataset was composed of tracer properties analysed in soil (three potential sources; n = 56) and sediment core samples (n = 32). Soil and sediment samples were sieved to 63 µm and analysed for organic matter, elemental geochemistry and diffuse visible spectrometry. Virtual mixtures (n = 138) with known source proportions were generated to assess model accuracy of each tracer selection method. The Bayesian un-mixing model MixSIAR was then used to predict source contributions on both virtual mixtures and actual sediments. The different methods tested in the current research can be distributed into three groups according to their sensitivity to the conservative behaviour of properties, which was found to be associated with different predicted source contribution tendencies along the sediment core. The methods selecting the largest number of tracers were associated with a dominant and constant contribution of forests to sediment. In contrast, the methods selecting the lowest number of tracers were associated with a dominant and constant contribution of cropland to sediment. Furthermore, the intermediate selection of tracers led to more balanced contributions of both cropland and forest to sediments. The prediction of the virtual mixtures allowed us to compute several evaluation metrics, which are generally used to support the evaluation of model accuracy for each tracer selection method. However, strong differences or the absence of correspondence were observed between the range of predicted contributions obtained for virtual mixtures and those values obtained for actual sediments. These divergences highlight the fact that evaluation metrics obtained for virtual mixtures may not be directly transferable to models run for actual samples and must be interpreted with caution to avoid over-interpretation or misinterpretation. These divergences may likely be attributed to the occurrence of a not (fully) conservative behaviour of potential tracer properties during erosion, transport and deposition processes, which could not be fully reproduced when generating the virtual mixtures with currently available methods. Future research should develop novel metrics to quantify the conservative behaviour of tracer properties during erosion and transport processes. Furthermore, new methods should be designed to generate virtual mixtures closer to reality and to better evaluate model accuracy. These improvements would contribute to the development of more reliable sediment fingerprinting techniques, which are needed to better support the implementation of effective soil and water conservation measures at the catchment scale.</p

A 300-year record of sedimentation in a small tilled catena in Hungary based on δ13C, δ15N, and C/N distribution

Purpose Soil erosion is one of the most serious hazards that endanger sustainable food production. Moreover, it has marked effects on soil organic carbon (SOC) with direct links to global warming. At the same time, soil organic matter (SOM) changes in composition and space could influence these processes. The aim of this study was to predict soil erosion and sedimentation volume and dynamics on a typical hilly cropland area of Hungary due to forest clearance in the early eighteenth century. Materials and methods Horizontal soil samples were taken along two parallel intensively cultivated complex convex-concave slopes from the eroded upper parts at mid-slope positions and from sedimentation in toe-slopes. Samples were measured for SOC, total nitrogen (TN) content, and SOMcompounds (δ13C, δ15N, and photometric indexes). They were compared to the horizons of an in situ non-eroded profile under continuous forest. On the depositional profile cores, soil depth prior to sedimentation was calculated by the determination of sediment thickness. Results and discussion Peaks of SOC in the sedimentation profiles indicated thicker initial profiles, while peaks in C/N ratio and δ13C distribution showed the original surface to be ~ 20 cm lower. Peaks of SOC were presumed to be the results of deposition of SOC-enriched soil from the upper slope transported by selective erosion of finer particles (silts and clays). Therefore, changes in δ13C values due to tillage and delivery would fingerprint the original surface much better under the sedimentation scenario than SOC content. Distribution of δ13C also suggests that the main sedimentation phase occurred immediately after forest clearance and before the start of intense cultivation with maize. Conclusions This highlights the role of relief in sheet erosion intensity compared to intensive cultivation. Patterns of δ13C indicate the original soil surface, even in profiles deposited as sediment centuries ago. The δ13C and C/N decrease in buried in situ profiles had the same tendency as recent forest soil, indicating constant SOM quality distribution after burial. Accordingly, microbiological activity, root uptake, and metabolism have not been effective enough to modify initial soil properties

Sediment source fingerprinting: benchmarking recent outputs, remaining challenges and emerging themes

Abstract: Purpose: This review of sediment source fingerprinting assesses the current state-of-the-art, remaining challenges and emerging themes. It combines inputs from international scientists either with track records in the approach or with expertise relevant to progressing the science. Methods: Web of Science and Google Scholar were used to review published papers spanning the period 2013–2019, inclusive, to confirm publication trends in quantities of papers by study area country and the types of tracers used. The most recent (2018–2019, inclusive) papers were also benchmarked using a methodological decision-tree published in 2017. Scope: Areas requiring further research and international consensus on methodological detail are reviewed, and these comprise spatial variability in tracers and corresponding sampling implications for end-members, temporal variability in tracers and sampling implications for end-members and target sediment, tracer conservation and knowledge-based pre-selection, the physico-chemical basis for source discrimination and dissemination of fingerprinting results to stakeholders. Emerging themes are also discussed: novel tracers, concentration-dependence for biomarkers, combining sediment fingerprinting and age-dating, applications to sediment-bound pollutants, incorporation of supportive spatial information to augment discrimination and modelling, aeolian sediment source fingerprinting, integration with process-based models and development of open-access software tools for data processing. Conclusions: The popularity of sediment source fingerprinting continues on an upward trend globally, but with this growth comes issues surrounding lack of standardisation and procedural diversity. Nonetheless, the last 2 years have also evidenced growing uptake of critical requirements for robust applications and this review is intended to signpost investigators, both old and new, towards these benchmarks and remaining research challenges for, and emerging options for different applications of, the fingerprinting approach

How to evaluate sediment fingerprinting source apportionments

International audiencePurpose Evaluating sediment fingerprinting source apportionments with artificial mixtures is crucial for supporting decisionmaking and advancing modeling approaches. However, artificial mixtures are rarely incorporated into fingerprinting research and guidelines for model testing are currently lacking. Here, we demonstrate how to test source apportionments using laboratory and virtual mixtures by comparing the results from Bayesian and bootstrapped modeling approaches. Materials and methods Laboratory and virtual mixtures (n = 79) with known source proportions were created with soil samples from two catchments in Fukushima Prefecture, Japan. Soil samples were sieved at 63 µm and analyzed for colorimetric and geochemical parameters. The MixSIAR Bayesian framework and a bootstrapped mixing model (BMM) were used to estimate source contributions to the artificial mixtures. In addition, we proposed and demonstrated the use of multiple evaluation metrics to report on model uncertainty, residual errors, performance, and contingency criteria. Results and discussion Overall, there were negligible differences between source apportionments for the laboratory and virtual mixtures, for both models. The comparison between MixSIAR and BMM illustrated a trade-off between accuracy and precision in the model results. The more certain MixSIAR solutions encompassed a lesser proportion of known source values, whereas the BMM apportionments were markedly less precise. Although model performance declined for mixtures with a single source contributing greater than 0.75 of the material, both models represented the general trends in the mixtures and identified their major sources. Conclusions Virtual mixtures are as robust as laboratory mixtures for assessing fingerprinting mixing models if analytical errors are negligible. We therefore recommend to always include virtual mixtures as part of the model testing process. Additionally, we highlight the value of using evaluation metrics that consider the accuracy and precision of model results, and the importance of reporting uncertainty when modeling source apportionments

Natural and anthropogenic impacts on the geochemical composition and metal speciation of fine sediment in a glacier-fed Canadian river basin

Purpose Surveillance monitoring programs can provide fast, relatively low-cost, synoptic information on key water quality drivers and help inform land management decisions. Here, we evaluate longitudinal changes in the geochemical composition and metal speciation of deposited sediment over a 967 km reach of a large glacier-fed river and its key tributaries in central Alberta, Canada. In particular, the work provides a basis to understand how a major urban conurbation influences the geochemical composition and trace element properties of deposited sediment. Methods The concentrations and spatial distribution of major elements and sediment-associated metals (As, Cd, Cr, Cu, Pb, Hg, Ni, Zn) were determined and compared to consensus-based threshold effect concentrations (TEC). Metal partitioning to geochemical properties of sediment was evaluated by sequential extraction. Results Levels of Cr exceeded the TEC in 28% and 20% of the samples in the North Saskatchewan River and tributary samples, respectively. One Ni sample (3%) also exceeds the TEC on the main stem of the North Saskatchewan River. No consistent downstream increase in major element or trace metal concentrations was observed. The majority of Cr was bound to the largely non-bioavailable silicate phase thus highlighting the importance of parent geology as a source of metals to receiving streams. Conclusion The results of this surveillance monitoring provide preliminary data on the distribution and speciation of sediment-associated metals in the North Saskatchewan River and thereby address an evidence gap common to many large river basins in Canada

Combining multiple fallout radionuclides (Cs-137, Be-7, Pb-210(xs)) to investigate temporal sediment source dynamics in tropical, ephemeral riverine systems

Land use change and the associated acceleration of soil erosion have increased sediment supply to rivers worldwide. To effectively manage sediment loads, it is important to understand sediment dynamics. Fallout radionuclides (i.e. Be-7, excess Pb-210, Cs-137) separately provide spatial and temporal sediment source information. However, their combined application has been limited by an assumption that subsurface sources do not receive Be-7 fallout. Here, we examine this assumption and investigate the utility of combining these radionuclides to simultaneously model spatial and temporal sediment source dynamics. Soil and subsurface material were sampled to characterize potential sediment sources in a 12-km(2) catchment in northern Laos. This included material that was directly sampled from surface and subsurface sources (n = 65) and also in situ sediment source samples that were collected in ephemeral flow on hillslopes (n = 19). Suspended sediment (n = 16) was sampled at two monitoring stations. A distribution modelling approach quantified the relative contributions of surface and subsurface sources, as well as the proportion of material labelled with Be-7. The results from these two end-member models were compared to a four end-member model examining contributions from recently eroded surface, recently eroded subsurface, re-suspended surface and re-suspended subsurface sources. Approximately 80 % of sediment was modelled to be supplied by sources labelled with Be-7. Subsurface sources were modelled to contribute similar to 75 % of sediment. Accordingly, there was similar to 55 % more sediment modelled from subsurface sources than expected if subsurface sources were sheltered from Be-7 fallout. Differences between subsoil and Be-7-labelled source contributions modelled by the two and four end-member models were, with one exception (0.3 %), within the range of model uncertainty, further supporting the existence of re-suspended subsurface sources. At an upstream sampling location (S4), surface sources contributed the majority of sediment (55 %) whereas subsurface sources dominated the supply of sediment downstream (S10-74 %). Importantly, re-suspended subsurface sources, labelled with Be-7, were a significant sediment source at the catchment outlet (S10-60 %) during the investigated event. This study demonstrates the utility of combining multiple radionuclides when investigating spatial and temporal sediment source dynamics in tropical, ephemeral catchments. In the future, this approach should be tested with larger source datasets during the entire wet season and in larger catchments. This research furthers our understanding of sediment propagation in tropical catchments, contributing to the implementation of efficient soil conservation measures to reduce the deleterious effects of suspended sediment loads

Tracing total and dissolved material in a western Canadian basin using quality control samples to guide the selection of fingerprinting parameters for modelling

International audienceThe source dynamics of total and dissolved material in riverine systems are being affected by anthropogenic activities resulting in the degradation of waterways worldwide. Identifying the main sources of total and dissolved material is thus central to the management of increasingly scarce water resources. Here, we utilize data generated from water quality monitoring programs to investigate the sources of total and dissolved material in a large, semi-arid basin in western Canada. Our research focuses on the confluence of two major tributaries in the South Saskatchewan River Basin (SSRB) in the Province of Alberta: the Bow River (25,611 km$^2$) and the Oldman River (28,270 km$^2$). A tributary tracing technique coupled with a Deconvolutional-MixSIAR (D-MIXSIAR) modelling approach is used to estimate the potential source contributions of total and dissolved material from major tributary sites to target node sites on the main stem of the Bow River and Oldman River in addition to target nodes downstream of their confluence. In total, 812 samples were taken from 29 sites across the SSRB. A novel approach to selecting fingerprints for modelling is presented based on the analyses of additional quality control samples (146 duplicate and 172 blank samples). Overall, the Rocky Mountain headwater catchments were found to dominate the supply of material modelled using total recoverable (68%) and dissolved (76%) metals. There were seasonal fluctuations in source dynamics evident where the Bow River dominated the supply of total (69%) and dissolved (57%) material during the ice-covered season (November-March), and the Oldman River dominated the supply of total (73%) and dissolved (59%) material during the open water season (April-October). On the one hand, these seasonal dynamics are potentially the result of the extensive regulation of flow, particularly along the Bow River. On the other hand, the intensification of agriculture in the prairie/plain catchments may also facilitate the excess supply of total relative to dissolved material. For example, the Little Bow River, with ~70% agricultural land cover, contributed ~14 times more total material than anticipated based on discharge and 1.6 times more than anticipated based on unit area during the open water season. Overall, this research has improved our understanding of the source dynamics of total and dissolved material in the SSRB, providing the foundation for focussed studies targeting the main sources of total and dissolved material in this large, semi-arid basin in western Canada. In addition, our research highlights the potential of using existent data generated from water quality monitoring programs along with quality control best practices to help improve our understanding of the source dynamics of total and dissolved material in waterways around the world

Rainfall erosivity in catchments contaminated with fallout from the Fukushima Daiichi nuclear power plant accident

The Fukushima Daiichi nuclear power plant (FDNPP) accident in March 2011 resulted in the fallout of significant quantities of radiocesium over the Fukushima region. After reaching the soil surface, radiocesium is quickly bound to fine soil particles. Thereafter, rainfall and snowmelt run-off events transfer particle-bound radiocesium downstream. Characterizing the precipitation regime of the fallout-impacted region is thus important for understanding post-deposition radiocesium dynamics. Accordingly, 10 min (1995&ndash;2015) and daily precipitation data (1977&ndash;2015) from 42 meteorological stations within a 100&thinsp;km radius of the FDNPP were analyzed. Monthly rainfall erosivity maps were developed to depict the spatial heterogeneity of rainfall erosivity for catchments entirely contained within this radius. The mean average precipitation in the region surrounding the FDNPP is 1420&thinsp;mm&thinsp;yr^&minus;1 (SD 235) with a mean rainfall erosivity of 3696 MJ mm ha^&minus;1 h^&minus;1 yr^&minus;1 (SD 1327). Tropical cyclones contribute 22&thinsp;% of the precipitation (422 mm yr^&minus;1) and 40&thinsp;% of the rainfall erosivity (1462 MJ mm ha⁻¹ h⁻¹ yr⁻¹ (SD 637)). The majority of precipitation (60&thinsp;%) and rainfall erosivity (82&thinsp;%) occurs between June and October. At a regional scale, rainfall erosivity increases from the north to the south during July and August, the most erosive months. For the remainder of the year, this gradient occurs mostly from northwest to southeast. Relief features strongly influence the spatial distribution of rainfall erosivity at a smaller scale, with the coastal plains and coastal mountain range having greater rainfall erosivity than the inland Abukuma River valley. Understanding these patterns, particularly their spatial and temporal (both inter- and intraannual) variation, is important for contextualizing soil and particle-bound radiocesium transfers in the Fukushima region. Moreover, understanding the impact of tropical cyclones will be important for managing sediment and sediment-bound contaminant transfers in regions impacted by these events

Depth distribution of radiocesium in Fukushima paddy fields and implications for ongoing decontamination works

International audienceLarge quantities of radiocesium were deposited across a 3000 km2 area northwest of the Fukushima Dai-ichi nuclear power plant after the March 2011 accident. Although many studies have investigated the fate of radiocesium in soil in the months following the accident, the potential migration of this radioactive contaminant in rice paddy fields requires further examination after the typhoons that occurred in this region. Such investigations will help minimize potential human exposure in rice paddy fields or transfer of radioactive contaminants from soils to rice. Radionuclide activity concentrations and organic content were analysed in 10 soil cores sampled from paddy fields in November 2013, 20 km north of the Fukushima power plant. Our results demonstrate limited depth migration of radiocesium with the majority concentrated in the uppermost layers of soils (137Cs inventories was still found within the 137Cs to maintenance (grass cutting - 97% of 137Cs in the upper 5 cm) and farming operations (tilling - 83% of 137Cs in the upper 5 cm). As this area is exposed to erosive events, ongoing decontamination works may increase soil erodibility. We therefore recommend the rapid removal of the uppermost - contaminated - layer of the soil after removing the vegetation to avoid erosion of contaminated material during the subsequent rainfall events. Remediation efforts should be concentrated on soils characterised by radiocesium activities > 10 000 Bq kg-1 to prevent the contamination of rice. Further analysis is required to clarify the redistribution of radiocesium eroded on river channels