From floodplain to aquatic sediments: Radiogeochronological fingerprints in a sediment core from the mining impacted Sancho Reservoir (SW Spain)

The records of 239+240Pu and 137Cs of global fallout in Lake East Dongting Sediments and Responses to watershed environmental changes

Plutonium-239 + 240 and 137Cs in the environment can usually be used to track the impact of nuclear activities on the environment, and have become important tools in environmental geochemical studies. In this study, nine sediment cores (E1-E9) in Lake East Dongting were collected and measured for the activity concentration of 239+240Pu, 137Cs and the atomic ratio of 240Pu/239Pu, and then their vertical distribution characteristics were analyzed. The results show that: the activity concentrations of 137Cs and 239+240Pu in Lake East Dongting ranged from 5.26 ± 0.43 to 28.6 ± 2.23 Bq kg-1 and 0.29 ± 0.02 to 1.37 ± 0.09 Bq kg-1, with an average of 7.48 ± 0.68 Bq kg-1 and 0.39 ± 0.03 Bq kg-1, respectively. The atomic ratios of 240Pu/239Pu are 0.168 ± 0.012-0.211 ± 0.015, which are basically consistent with the global atmospheric deposition. The vertical profiles of 137Cs and 239+240Pu in sediment cores show obvious single-peak distribution in E1-E6 and bimodal distribution in E7-E9. The results of sedimentation rates calculated by 137Cs and 239+240Pu method ranged from 0.59 cm y-1 to 1.99 cm y-1 with a mean of 1.18 cm y-1 and 0.61 cm y-1 to 2.18 cm y-1 with a mean of 1.26 cm y-1. The inventories of 137Cs and 239+240Pu in nine sediment cores are 5.87-10.8 kBq m-2 and 307-545 Bq m-2, which are about 8-14 and 9-15 times the inventory in the global average atmospheric deposition at the same latitude respectively. Comparing the results of the sedimentation rates and the inventories from different sampling points indicates that extreme climatic events and human activities have a significant impact on sediment environment of Lake East Dongting.

Geochemical behavior of heavy metals and radionuclides in a pit lake affected by acid mine drainage (AMD) in the Muskau Arch (Poland)

Pit lakes in the 'anthropogenic lake district' in the Muskau Arch (western Poland; central Europe) are strongly affected by acid mine drainage (AMD). The studied acidic pit lake, ŁK-61 (pH <3), is also exposed to floods due to its location in the flood hazard area, which may significantly influence the geochemical behavior of elements. The elemental compositions of water and lake sediment samples were measured with ICP-OES and ICP-MS. The sediment profile was also examined for 137Cs and 210Po activity concentrations using gamma and alpha spectrometry, respectively. Grain size distribution, mineralogical composition, diatoms, and organic matter content in the collected core were also determined. The key factors responsible for the distribution of selected heavy metals (e.g., Cu, Ni, Pb, Zn) and radioisotopes (137Cs and 210Po) in the bottom sediments of Lake ŁK-61 are their coprecipitation/precipitation with Fe and Al secondary minerals and their sorption onto authigenic and allogenic phases. These processes are likely driven by the lake tributary, which is an important source of dissolved elements. The data also showed that the physiochemical parameters of Lake ŁK-61 water changed during an episodic depositional event, i.e., the flood of the Nysa Łużycka River in the summer of 2010. The flood caused an increase in the water pH, as interpreted from the subfossil diatom studies. The down-core profiles of the studied heavy metal and radionuclide (HMRs) contents were probably affected by this depositional event, which prevented a detailed age determination of the collected lake sediments with 137Cs and 210Pb dating methods. Geochemical modeling indicates that the flood-related shift in the physicochemical parameters of the lake water could have caused the scavenging of dissolved elements by the precipitation of fresh secondary minerals. Moreover, particles contaminated with HMRs have also possibly been delivered by the river, along with the nutrients (e.g., phosphorus and nitrogen).

Kinetic reactive transport explains distinct subsurface deposition patterns of pollutants in sediments. The case of the Sellafield-derived 236U, 137Cs and 239,240Pu in the Esk Estuary, UK

The kinetics of the uptake of pollutants by solids in sediments interacts with transitional eddy diffusivity in the pore fluid, leading to different depth-distribution patterns. This work aims to gain insights into the still poorly understood behaviour in the marine environment of the anthropogenic ²³⁶U, a recently postulated tracer of water masses. It is hypothesized that the transition from mobile U(VI) to highly particle-reactive U(IV) in the anoxic zone of the sediment produces a subsurface deposition pattern. A novel numerical model for kinetic reactive transport in sediments, which merges diagenetic processes for transport and box models for the uptake, is used for concept demonstration. It is applied to synthetic environments with high eddy diffusivity to obtain the singular depth-distribution patterns of pulsed inputs of tracers that mimic the anthropogenic ^239,240Pu, ¹³⁷Cs, and ²³⁶U. While the first is retained in the upper cm, the second shows an exponential penetration pattern over few cm, and ²³⁶U is deposited with a Gaussian-like pattern centred below the transition to the anoxic zone. These patterns are then merged into a diagenetic model to compute the depth distribution at decadal or centennial scales of dissolved and particle-bound inputs of these radiotracers. It is successfully applied to a real case using literature data for a sediment core from the Esk Estuary, UK, affected by radioactive releases from the Sellafield nuclear reprocessing plant. This work provides insight into until now poorly understood field data and provides a novel view of broad implications in the study of the behaviour of pollutants in surficial aquatic sediments.

On the use of 210Pb-based records of sedimentation rates and activity concentrations for tracking past environmental changes

Lead-210 from natural atmospheric fallout is widely used in multidisciplinary studies to date recent sediments. Some of the ²¹⁰Pb-based dating models can produce historical records of sediment accumulation rates (SAR) and initial activity concentrations (A₀). The former have been profusely used to track past changes in the sedimentary conditions. Both physical magnitudes are differently affected by model errors (those arising for the partial or null accomplishment of some model assumptions). This work is aimed at assessing the effects on SAR and A₀ of model errors in the CRS, CS, PLUM and TERESA dating models, due to random variability in ²¹⁰Pb fluxes, which is a usual sedimentary condition. Synthetic cores are used as virtual laboratories for this goal. Independently of the model choice, SARs are largely affected by model errors, resulting in some large and spurious deviations from the true values. This questions their general use for tracking past environmental changes. A₀ are less sensitive to model errors and their trends of change with time may reflect real changes in sedimentary conditions, as it is shown with some real cores from varved sediments.

Mollusk death assemblages in 210Pb-dated marine sediment cores reveal recent biotic changes in the Gulf of Guanahacabibes, NW Cuba

We investigated stratigraphic changes in mollusk death assemblages and geochemistry in sediment cores from four seagrass beds and one unvegetated site in the Gulf of Guanahacabibes (GG), NW Cuba. There was a transition from mangrove to seagrass beds, associated with sea level rise ∼6000 years ago. Sediment accumulation rates during the last century showed a general rise, but increased sharply after ∼1980, likely because of human activities. The GG displayed overall high mollusk γ-diversity, and our estimate of 189 species is biased toward the low end. High β-diversity was driven by inter-site differences in grain size, vegetation cover, and nutrient input. Spatial heterogeneity within the basin influenced downcore abundance and diversity metrics, highlighting the influence of local drivers. Herbivorous gastropods dominated in seagrass beds and suspension feeder bivalves were dominant on sandy bottom. In the top parts of cores, species richness declined at two sites that were subject to high, human-mediated bulk sedimentation rates and eutrophication. Conservation measures are needed to preserve this hotspot of marine diversity.

Element Content in Volcano Ash, Soil and River Sediments of the Watershed in the Volcanic Area of South Iceland and Assessment of Their Mobility Potential

In this work, we present the results of element content in river sediments and soil of the Rangárvellir river in southern Iceland as well as in the volcano ash from the Eyjafjallajökull eruption in 2010. Severe natural catastrophes have devastating impact on the environment. We selected the Rangárvellir area located in the vicinity of Mt Hekla, Iceland’s most active volcano, for our study. This study site is ideal to investigate wind and water erosion and sediment transport processes of two main glaciers rivers: the Eystri-Rangá and the Ytri-Rangá and the Hroarslækur River. Sediments of these rivers consist of volcano materials, ash and lava. In order to assess the sediment dynamics, we collected sediment, soil and volcanic ash. The fractioning of the elements was carried out using sequential extractions. The distribution of Si indicates that the sediments originate from the same geochemical basis. Li and partly B exist in the first phase of volcanic ash and river sediments but not in the mobile phases of soils. In the mobile phase of volcanic ash, P was found but it did not exist in the first phase of soil and sediments. These results suggest the different chemical fingerprints in the water sediments and surrounding soil.

Occurrence and depositional history of organochlorine pesticides in the sediments of the Zayandehrud River in the arid region of Central Iran

In this study, surface sediments along the Zayandehrud River (14 samples), and two dated core sediments (46 samples) from small artificial urban lakes at the middle section of the Zayandehrud River in the Gavkhooni basin in the central arid regions of Iran were analyzed for residual levels of 20 organochlorine pesticide (OCP) compounds. Total OCP concentrations ranged from 0.1 to 50.1 ng g-1 dry weight and from 1.9 to 51.5 ng g-1 dry weight in surface and core sediments, respectively. Dichlorodiphenyldichloroethylene (DDE) and dichlorodiphenyldichloroethane (DDD) were found to be the predominant OCPs in these sediments. The calculated metabolic and isomeric ratios confirmed the aged nature of residual dichlorodiphenyltrichloroethane (DDT) in sediments. Moreover, the isomeric ratios indicated the aged nature of technical HCH (hexachlorocyclohexane), while the contribution of γ- HCH (lindane) as a main source has increased, especially in the last two decades. Past usage, as well as current usage of endosulfan technical mixture in the Gavkhooni basin, has been found in the last four decades. Analyses of sedimentary cores, as natural archives, have shown the successful ban on the use of organochlorine pesticides (especially DDT) in the Gavkhooni basin, and to some extent, in the central plateau of Iran. In general, it can be concluded that natural factors (i.e., floods and wet years) lead to soil leachate and play an essential role in remobilization and transfer of residual OCPs from soil to inland aquatic ecosystems in the Gavkhooni basin, which is an arid region.

Environmental radioactivity and trace metals in surficial sediments from estuarine systems in Ghana (Equatorial Africa), impacted by artisanal gold-mining

This paper reports concentrations of γ-emitter radionuclides (⁴⁰K, ¹³⁷Cs, ²¹⁰Pb, ²²⁶Ra, ²²⁸Ra, ²²⁸Th and ²³⁴Th) and some metals (Al, Cr, Fe, Co, Ni, Cu, Zn, As, Sr, Cd, Sb, Cs, Pb, Th and U) in surficial sediments from the Ankobra, Pra and Volta estuaries, in Ghana. Artisanal gold-mining in the Ankobra and Pra basins promoted moderate enrichments of As, Sb, Cu, Cs and Cr in their estuarine sediments, with respect to the reference background of the Volta Estuary. Radionuclide concentrations were in the range found in the Earth's crust. Present data do not support any conclusion on their potential enrichments due to gold-mining activities. Radionuclide isotopic ratios revealed a transfer of ²²⁸Ra from sediments to the water column. Pearson correlation coefficient matrices showed different patterns, which were reasonably understood after novel approaches: i) inter-estuaries comparison of slopes in the linear regressions of element-concentrations vs Al, Fe and Cs; ii) study of Al-normalized concentrations of elements; iii) excess ²¹⁰Pb informing on local sedimentary conditions. The metal enrichments observed in the Ankobra and Pra estuaries are associated with the Fe-rich compounds in sulphide ores (such as FeAsS) transported along the river course and deposited in the estuary.

Experimental study on the use of granulometric speciation for the radiometric dating of recent sediments

Theoretical and experimental studies have shown that activity concentrations of fallout radionuclides (such as ¹³⁷Cs and excess ²¹⁰Pb) decrease with particles size in aqueous suspensions. This paper is aimed at reviewing the theoretical fundamentals for granulometric speciation of radionuclides, and at exploring its practical use in the analytical context of gamma spectrometry for the radiometric dating of recent sediments, with view to: i) improving the detection of ¹³⁷Cs (since its use as independent chronostratigraphic mark is challenging in the southern hemisphere because its low fallout rate), ii) supporting refined CIC models and normalization techniques in ²¹⁰Pb-based radiogeochronologies. The work uses surface sediments sampled from the Tinto Estuary (SW Spain), affected by mining and phosphate-fertilizer industries, and from the Ankobra Estuary (Ghana), affected by intensive artisanal gold-mining. Granulometric classes have been separated by a sieving column with decreasing mesh sizes and the obtained cumulative percentage of mass mathematically described by a Rosin-Rammler particle-size distribution. The target radionuclides for gamma spectrometry were ²¹⁰Pb, ²²⁶Ra and¹³⁷Cs, complemented with ⁴⁰K, ²³⁴Th and ²²⁸Ra. Results revealed that, far from ideal experiments, under actual environmental conditions the increase in activity concentrations with decreasing particle sizes is too moderate, and in general they are affected by larger counting uncertainties due to the small available amount of mass. Indeed, there was no correlation between grain-size and ¹³⁷Cs concentrations (p = 0.25), and similarly for excess ²¹⁰Pb (p = 0.53). No effect of the organic matter content was observed in ¹³⁷Cs (p = 0.58) and excess ²¹⁰Pb (p = 0.85) concentrations. Present results pose some concerns to the general use of granulometric speciation in the context of gamma spectrometry for supporting the radiometric dating of recent sediments. A detailed discussion on the use of normalization methods is also presented.

A 2000-year sediment record reveals rapidly changing sedimentation and land use since the 1960s in the Upper Mara-Serengeti Ecosystem

The Mara River basin is a trans-boundary basin of international importance. It forms the headwaters of the Nile River and serves as the primary dry season water source for an estimated 1.1 million rural people and the largest remaining overland migration of 1.4 million wildebeest in the Serengeti-Mara Ecosystem. Changes throughout the basin are impacting the quantity and quality of the Mara River, yet the historical context of environmental conditions in the basin is not well known. We collected sediment cores throughout the wetland at the mouth of the Mara River, and we used isotopic dating methods and a suite of analyses to examine historical patterns of sediment quantity and source, mercury contamination, and carbon and nutrient loading. Our results show that ecological conditions in the Mara River basin were fairly stable over paleoecological time scales (2000-1000 years before present), but there has been a period of rapid change in the basin over the last 250 years, particularly since the 1960s. A shift in the source and quantity of sediments in the river began in the late 1700s and became much more pronounced in the 1950s and 1960s, coincident with increasing mercury concentrations. The quantity of sediment from the Upper Mara increased, particularly since 1960, but the proportion of total sediment from this region decreased as the Talek and Middle Mara portions of the basin began producing more sediment. The decadal oscillation in sediment accumulation was congruent with known periods of extreme precipitation events. Carbon and nitrogen loading also increased since the 1960s, and the shift in the isotopic ratio of nitrogen provides evidence for increased anthropogenic loading. Altogether, these data likely reflect patterns of change also experienced in other basins throughout East Africa.

Evidence for a recent increase in delivery of atmospheric 210Pb to Oualidia lagoon, coastal Morocco

Two sediment cores were collected from the Oualidia lagoon, on the Atlantic coast of Morocco, and analyzed for ²¹⁰Pb and ¹³⁷Cs activity by gamma spectrometry. The ²¹⁰Pb profiles were characterized by high activity at specific depths in each core, which were attributed to substantial increases in atmospheric ²¹⁰Pb input to the sediment. A modified CRS model was applied to develop age-depth relations (chronologies) for the cores and calculate sediment accumulation rates, taking into account changing unsupported ²¹⁰Pb delivery and specifying the year when the increase began. Calculated ²¹⁰Pb inventories (activity/area) and fluxes (activity/area/time) depend strongly on sedimentation rates and were much higher than mean values in similar coastal systems worldwide. We attempted to use ¹³⁷Cs as a time marker to support the modified CRS chronologies for both cores. The ¹³⁷Cs profiles, however, were affected by post-depositional cesium migration in the sediment which made it difficult to identify the 1963 atmospheric bomb-testing peak, especially in the core with low sedimentation rate. We conclude that the high activities of ²¹⁰Pb detected at specific depths in the Oualidia lagoon sediment cores are a consequence of decay of radioactive ²²²Rn, which displayed periodic high concentrations in the overlying atmosphere.

Historical trends and assessment of radionuclides and heavy metals in sediments near an abandoned mine, Lavrio, Greece

Two sediment cores (coastal and offshore) and surface sediments were collected near an abandoned mine area in the marine environment of Oxygono Bay at Lavreotiki peninsula to investigate temporal and spatial variations among radionuclides and trace metals/major elements. Lavreotiki was and still is well known for the mining and metallurgical activities, which lasted from ancient times to nowadays (early 1980s). Gamma-ray and X-ray fluorescence measurements were held to determine the radionuclide and trace metal/major element concentrations, respectively. The sedimentation rate at Oxygono Bay coastal core was determined using the ²¹⁰Pb and ¹³⁷Cs tracers, while the enrichment factors were estimated to assess the anthropogenic influence due to metals in a spatial (surface sediments) and a temporal (coastal core sediments) basis. The mass flux at the coastal core was utilized to provide a baseline information at Lavreotiki peninsula. The trace metal/major element profiles were indeed associated with the mining activity in the area, revealing the mining history. The ERICA Tool was incorporated to estimate the dose rates due to natural and ¹³⁷Cs radioactivity in the marine organisms and the values were found below the screening levels.