Desorption of radioactivity from nearshore sediment

Radiocesium distribution in the sediments of the Odaka River estuary, Fukushima, Japan

Radiocesium that originated from the Fukushima Daiichi Nuclear Power Plant accident was deposited on the ground surface and has been transported via fluvial discharge, primarily in the form of particulates, to downstream areas and eventually to the ocean. During transportation, some of the radiocesium accumulated on the riverbed. In this study, we quantified the radiocesium deposition on the riverbed in the Odaka River estuary and investigated the radiocesium sedimentation process of the river bottom. Our results show that the radiocesium inventory in the seawater intrusion area is larger than those in the freshwater and marine parts of the estuary. Moreover, the particle-size distribution in the seawater intrusion area shows a high proportion of silt and clay particles compared with the distribution in other areas. The increased radiocesium inventory in this area is attributed to the sedimentation of fine particles caused by hydrodynamic factors (negligible velocity of the river flow) rather than flocculation factor by salinity variation.

A review on cesium desorption at the freshwater-seawater interface

Understanding the processes governing the behavior of radiocesium in the sea is still essential to make accurate assessments of its potential impacts on marine ecosystems. One of the most important of this process is the desorption that may occur at the river-sea interface due to changes in physico-chemical conditions, including ionic strength and solution composition. It has been the subject of many studies using field measurements or laboratory experiments, but there was no global interpretation of these works and their results. The present review summarizes relevant laboratory experiments studying desorption of Cs (stable or radioactive) from particles in sea or brackish waters. To date, 32 experimental studies have been carried out on 68 Cs-bearing samples since 1964. A wide range of desorbed fraction (0-86%) was observed, partly depending on the experimental design. For particles containing radiocesium issued from a contamination in the environment, the desorption ranges from 0 to 64% of the particulate activity, with a median at only 3%. Particles contaminated in laboratory show a range between 6 and 86% with a multimodal distribution. The desorption initiates at low salinity (3-4) and rapidly reaches a threshold around 10-15. Laboratory experiments show that two first-order reactions govern the kinetics of the process, with half-life reaction times of 1 h and a few days. These two reactions are probably linked to the adsorption of Cs onto different particles sites. Also, the dynamic of Cs desorption depends on its initial distribution on these different sites, in relation with the history of its contamination and an aging effect.

Cesium desorption behavior of weathered biotite in Fukushima considering the actual radioactive contamination level of soils

For the better understanding of radioactive contamination in Fukushima Prefecture at present and in future, Cs desorption experiments have been conducted mainly using weathered biotite (WB) collected from Fukushima Prefecture and considering the actual contamination level (∼10^-10 wt%) of radiocesium in Fukushima Prefecture. In the experiments, ¹³⁷Cs sorbed to WB by immersing in ¹³⁷Cs solution for one day was mostly desorbed by solutions of 1 M NaNO₃, 1 M LiNO₃, 10^-1 M HCl, and 10^-1 M HNO₃, although it was barely desorbed by 1 M KNO₃, 1 M CsNO₃, 1 M NH₄NO₃, and natural seawater. X-ray diffraction analysis of WB after immersing in these solutions suggested that the collapse of the hydrated interlayers in WB suppressed the desorption of Cs. On the other hand, ¹³⁷Cs was barely desorbed from WB even by the treatments with solutions of NaNO₃ and LiNO₃ if the duration for the sorption was longer than approximately two weeks, as well as radioactive WB collected from actual contaminated soils in Fukushima Prefecture. This result implies that Cs sorbed in WB became more strongly fixed with time. Probably removal of radiocesium sorbed in weathered granitic soil at Fukushima Prefecture is difficult by any electrolyte solutions, as more than seven years have passed since the accident.

Radioactive cesium dynamics derived from hydrographic observations in the Abukuma River Estuary, Japan

Large quantities of radioactive materials were released into the air and the ocean as a result of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident, caused by the 2011 Tohoku earthquake and the subsequent major tsunami off the Pacific coast. There is much concern about radioactive contamination in both the watershed of the Abukuma River, which flows through Fukushima Prefecture, and its estuary, where it discharges into the sea in Miyagi Prefecture. We investigated radioactive cesium dynamics using mixing diagrams obtained from hydrographic observations of the Abukuma River Estuary. Particulate radioactive cesium dominates the cesium load in the river, whereas the dissolved form dominates in the sea. As the salinity increased from <0.1 to 0.1-2.3, the mixing diagram showed that dissolved radioactive cesium concentrations increased, because of desorption. Desorption from suspended particles explained 36% of dissolved radioactive cesium in estuarine water. However, the dissolved and particulate radioactive cesium concentrations in the sea decreased sharply because of dilution. It is thought that more than 80% of the discharged particulate radioactive cesium was deposited off the river mouth, where the radioactive cesium concentrations in sediment were relatively high (217-2440 Bq kg(-1)). Radioactive cesium that was discharged to the sea was transported southward by currents driven by the density distribution.

Persistent chlordane concentrations in Long Island Sound sediment: implications from chlordane, 210Pb, and 137Cs profiles

Concentrations of chlordane, a banned termiticide and pesticide, were examined in recently collected surficial sediment (10 sites) and sediment cores (4 sites) in Long Island Sound (LIS). The highest chlordane concentrations were observed in western LIS, near highly urbanized areas. Chlordane concentrations did not decrease significantly in the past decade when compared to the data collected in 1996, consistent with the observation of near-constant chlordane levels in blue mussel tissues collected during the same time period. Chlordane concentrations in many of the sites exceeded levels above which harmful effects on sediment-dwelling organisms are expected to frequently occur. Chlordane concentrations in two of the four sediment cores showed a peak below the sediment surface, suggesting reduced chlordane inputs in recent years. The lack of a chlordane concentration maximum below the sediment surface in the other two cores, coupled with the lack of a well-defined 137Cs peak, indicated significant sediment mixing. Simulations of 137Cs and 210Pb profiles in sediment cores with a simple sediment-mixing model were used to constrain both the deposition rate and the bioturbation rate of the sediment. Simulations of the chlordane profiles indicated continued chlordane input to LIS long after chlordane was phased out in the U.S. Continued chlordane input and significant sediment mixing may have contributed to the persistent chlordane concentrations in surficial sediment, which poses long-term threats to benthic organisms in LIS.

Adsorption of copper and zinc on pseudomonas putida CZ1: Particle concentration effect and adsorption reversibility

The adsorption and desorption processes of Cu(II) and Zn(II) on the biomass of Pseudomonas putida CZ1 as a function of particle concentrations (C(p)) were studied. In a 0.01 M KNO3 solution, the Cu-biomass and Zn-biomass adsorption systems displayed a clear C(p) effect. The overall adsorption isotherms under three C(p) conditions could be described as a Freundlich-type C(p) effect isotherm equation: gamma = 2.553C(p)(-0.7106) C(eq)(0.8971) for Cu-biomass system, gamma = 2.412C(p)(-0.8305) C(eq)(0.6504) for Zn-biomass system. The results of experiments, designed to eliminate several typical sources of experimental artifact, agree with the prediction of the metastable-equilibrium adsorption theory. Results from laboratory equilibration studies also indicate that biomass-adsorbed Cu(II) or Zn(II) fractions may be comprised of both reversibly and strongly bound or resistant components. A computational method has been derived to allow prediction of the magnitude of the reversible and more strongly adsorbed Cu(II) or Zn(II) fractions from conventional isotherm data. This methodology provides an initial quantitative approximation of the strongly bound, resistant, biomass fractions while utilizing relatively simple experimental adsorption-desorption data.

Effect of eutrophication upon radionuclide dynamics in the Sacca di Goro lagoon (Po River Delta, Italy): a combined field, experimental and modeling study

The focus of this paper is on the relationship between eutrophication and radionuclide circulation at the whole ecosystem scale in the shallow estuarine environment of the Sacca di Goro (Po River Delta, Italy). This lagoon is frequently affected by dystrophic crises, due to decomposition of huge amounts of macroalgae (mainly Ulva rigida), and critical conditions created at the interface between sediment and water are such that Cs-137 accumulated in the sediment can be mobilized and made available in the water column. The release of cesium from sediment in this ecosystem has been evaluated through a field experiment in which chemical conditions typical of anoxic crises were artificially created in enclosures. Also a lab experiment was carried out to shed light on possible cesium release by decomposing macroalgae. The two experiments allowed drawing conclusions on crucial factors controlling cesium release in the Sacca di Goro, the first objective of this research. The second objective was understanding the fate of radiocesium once transported in the water column. To this end ecological information gathered during the experiments and a yearly sampling campaign, has been converted into whole-system seasonal networks describing ecosystem flow structure for the Sacca di Goro. Analyzed by network analysis this model has provided clues about the dynamics of Cs-137 in terms of preferential pathways, sinks, sources, and cycling activity. Sediment, together with seston and dissolved cesium, appear to be the most significant components in the circulation of Cs-137; while macroalgal biomasses play a crucial role as an indirect causal factor.

The loading history of trace metals and nutrients in Altata-Ensenada del Pabellón, lagoon complex, northwestern Mexico

This paper summarizes the geochemical investigations about the origin and loading history of some trace metals (Ag, Cu and Zn) and nutrients (N and P) in the coastal lagoon complex of Altata-Ensenada del Pabellón, Mexico, by using the radioactive chronometers 210Pb and 228Th and the stable isotopes of C and N. The examination of sediment cores collected at different locations in the lagoon system identified a slight enrichment in metals and nutrients in some points, which was mainly associated to organic matter accumulation. Stable C and N isotope ratios revealed wastewater inputs to the lagoon system and the 210Pb geochronology showed that anthropogenic impact started 50 years ago, with the beginning of the agriculture development and the associated urban growth of the surrounding area. Several atypical 210Pb and 228Th/232Th profiles demonstrated that biological and physical disturbances are common phenomena in these environments, that frequently mask the pollution records; and therefore, considering that the contaminated sediments at some locations in the lagoon system are frequently resuspended and re-oxygenated, the pollutants will continue to be easily remobilized in the food chain.

Recent sedimentary history of anthropogenic impacts on the Culiacan River Estuary, northwestern Mexico: geochemical evidence from organic matter and nutrients

210Pb geochronology and sediment profiles of carbon, phosphorus and nitrogen were used to study time dependent changes in nutrients fluxes to Culiacan River Estuary. Results indicate that the release of urban sewage and agriculture wastes transported through Culiacan River has produced historically increased carbon, phosphorus and nitrogen fluxes to the study area. C:N:P elemental ratios showed that increments in the nutrients input begins simultaneously for C, N and P in 1948 with the clearing of the catchment for agriculture; although excess of nutrients input increased most importantly around the 1970s to roughly follow the rapidly growing population of Culiacan City. C/N ratios, delta13C and delta15N suggested that nutrient enrichment is mostly influenced by sewage delivered through Culiacan River.

Determination of radionuclide exchangeability in freshwater systems

Two freshwater sediments were spiked with 57Co, 85Sr and 134Cs and left for adsorption times ranging from 1 day to over 60 days. Following adsorption, the 'exchangeable' pool of each radionuclide was measured using ammonium acetate extractions and a sequential leach procedure (Tessier et al., 1979), and the results were compared. Exchangeability was found to depend upon the sediment, radionuclide, sorption time and the identity of extracting agent. All three radionuclides showed a shift with increasing adsorption time from regular exchange sites to sites which are sterically hindered, but a fixation within the sediment was only observed for 57Co and 134Cs, with similar ammonium acetate extraction yields for both radionuclides. Misleading results were obtained during the sequential leach procedure due to redistribution and, therefore, the inclusion of a NH4+ leach after the MgCl2 extraction step was suggested. A mathematical model of element speciation was fairly successful at defining the 'exchangeable' fraction, suggesting that the chemically- and mathematically-defined fractions were similar.