Nuclear weapons fallout 137Cs in temperate and tropical pine forest soils, 50 years post-deposition

Following nuclear releases to the environment, 137Cs (half-life 30 years) is a long-term contaminant of many ecosystems, including forests. We recently sampled soils under pine forests in temperate and tropical climates to test the hypothesis that migration of 137Cs, 50 years after nuclear weapons fallout, is coupled with organic matter (OM) accumulation in these soils. Depth profiles of 137Cs, naturally-occurring 210Pb and weapons-derived 241Am were measured. After 50 years, migration of 137Cs into the temperate and tropical soils is limited to half-depths of 7–8 cm and 2–3 cm, respectively. At both locations, most 137Cs is associated with OM that accumulated from the early to mid-1960s. Illite, which immobilises radiocaesium, was undetectable by X-ray diffraction in the layer of peak 137Cs accumulation in the temperate forest soil, but apparent in the zone of peak concentration in the tropical soil. Data indicate that long-term (50 year) fate of 137Cs in organic-rich, temperate forest soil is coupled with OM accumulation; fixation of 137Cs by illite is more important in the tropical forest soil where OM is rapidly decomposed. Models of long-term radiocaesium migration in forest soils should explicitly account for the role of OM, especially when considering forests under contrasting climatic regimes

Three dimensional quantification of soil hydraulic properties using X-ray Computed Tomography and image based modelling

We demonstrate the application of a high-resolution X-ray Computed Tomography (CT) method to quantify water distribution in soil pores under successive reductive drying. We focus on the wet end of the water release characteristic (WRC) (0 to -75 kPa) to investigate changes in soil water distribution in contrasting soil textures (sand and clay) and structures (sieved and field structured), to determine the impact of soil structure on hydraulic behaviour. The 3D structure of each soil was obtained from the CT images (at a 10 µm resolution). Stokes equations for flow were solved computationally for each measured structure to estimate hydraulic conductivity. The simulated values obtained compared extremely well with the measured saturated hydraulic conductivity values. By considering different sample sizes we were able to identify that the smallest possible representative sample size which is required to determine a globally valid hydraulic conductivity

Iodine soil dynamics and methods of measurement: a review

Iodine is an essential micronutrient for human health: insufficient intake can have multiple effects on development and growth, affecting approximately 1.9 billion people worldwide. Previous reviews have focussed on iodine analysis in environmental and biological samples, however, no such review exists for the determination of iodine fractionation and speciation in soils. This article reviews the geodynamics of both stable 127I and the long-lived isotope 129I (t1/2 ¼ 15.7 million years), alongside the analytical methods for determining iodine concentrations in soils, including consideration of sample preparation. The ability to measure total iodine concentration in soils has developed significantly from rudimentary spectrophotometric analysis methods to inductively coupled plasma mass spectrometry (ICP-MS). Analysis with ICP-MS has been reported as the best method for determining iodine concentrations in a range of environmental samples and soils due to developments in extraction procedures and sensitivity, with extremely good detection limits typically <mg L_1. The ability of ICP-MS to measure iodine and its capabilities to couple on-line separation tools has the significance to develop the understanding of iodine geodynamics. In addition, nuclear-related analysis and recent synchrotron light source analysis are discussed

Kinetics of uranium(VI) lability and solubility in aerobic soils

Uranium may pose a hazard to ecosystems and human health due to its chemotoxic and radiotoxic properties. The long half-life of many U isotopes and their ability to migrate raise concerns over disposal of radioactive wastes. This work examines the long-term U bioavailability in aerobic soils following direct deposition or transport to the surface and addresses two questions: (i) to what extent do soil properties control the kinetics of U speciation changes in soils and (ii) over what experimental timescales must U reaction kinetics be measured to reliably predict long-term of impact in the terrestrial environment? Soil microcosms spiked with soluble uranyl were incubated for 1.7 years. Changes in UVI fractionation were periodically monitored by soil extractions and isotopic dilution techniques, shedding light on the binding strength of uranyl onto the solid phase. Uranyl sorption was rapid and strongly buffered by soil Fe oxides, but UVI remained reversibly held and geochemically reactive. The pool of uranyl species able to replenish the soil solution through several equilibrium reactions is substantially larger than might be anticipated from typical chemical extractions and remarkably similar across different soils despite contrasting soil properties. Modelled kinetic parameters indicate that labile UVI declines very slowly, suggesting that the processes and transformations transferring uranyl to an intractable sink progress at a slow rate regardless of soil characteristics. This is of relevance in the context of radioecological assessments, given that soil solution is the key reservoir for plant uptake

Iodine binding to humic acid

The rate of reactions between humic acid (HA) and iodide (I-) and iodate (IO3-) have been investigated in suspensions spiked with 129I at concentrations of 22, 44 and 88 µg L-1 and stored at 10oC. Changes in the speciation of 129I-, 129IO3- and mixed (129I-+129IO3-) spikes were monitored over 77 days using liquid chromatography inductively coupled plasma mass spectrometry (LC-ICP-MS). In suspensions spiked with 129I- 25% of the added I- was transformed into organic iodine (Org-129I) within 77 days and there was no evidence of 129IO3- formation. By contrast, rapid loss of 129IO3- and increase in both 129I- and Org-129I was observed in 129IO3--spiked suspensions. However, the rate of Org-129I production was greater in mixed systems compared to 129IO3--spiked suspensions with the same total 129I concentration, possibly indicating IO3-—I- redox coupling. Size exclusion chromatography (SEC) demonstrated that Org-129I was present in both high and low molecular weight fractions of the HA although a slight preference to bond with the lower molecular weight fractions was observed indicating that, after 77 days, the spiked isotope had not fully mixed with the native 127I pool. Iodine transformations were modelled using first order rate equations and fitted rate coefficients determined. However, extrapolation of the model to 250 days indicated that a pseudo-steady state would be attained after ~ 200 days but that the proportion of 129I incorporated into HA was less than that of 127I indicating the presence of a recalcitrant pool of 127I that was unavailable for isotopic mixing

Overview of selected soil pore water extraction methods for the determination of potentially toxic elements in contaminated soils : operational and technical aspects

Chemical elements that are either present naturally in the soil or introduced by pollution are more usefully estimated in terms of ‘availability’ of the element, because this property can be related to mobility and uptake by plants. A good estimation of ‘availability’ can be achieved by measuring the concentration of the element in soil pore water. Recent achievements in analytical techniques allowed to expand the range of interest to trace elements, which play a crucial role both in contaminated and uncontaminated soils and include those defined as potentially toxic elements (PTE) in environmental studies. A complete chemical analysis of soil pore water represents a powerful diagnostic tool for the interpretation of many soil chemical phenomena relating to soil fertility, mineralogy and environmental fate. This chapter describes some of the current methodologies used to extract soil pore water. In particular, four laboratory-based methods, (1) high-speed centrifugation-filtration, (2) low (negative-)-pressure Rhizon™ samplers, (3) high-pressure soil squeezing and (4) equilibration of dilute soil suspensions, are described and discussed in detail. A number of operational factors are presented: pressure applicable (i.e. pore size involved), moisture prerequisites of the soil, pore water yielding, efficiency, duration of extraction, materials and possible contaminations for PTE studies. Some consideration is then taken to assess advantages and disadvantages of the methods, including costs and materials availability

The transfer of arsenic to sheep tissues

There is the potential for arsenic to enter the human food chain via ingestion by grazing animals. Data on the transfer of arsenic to ruminants have been too sparse to allow the development of dynamic models to predict changes in the arsenic contents of different tissues following ingestion. A study is described during which a group of 6-month-old lambs were given a single oral administration of 73AsCl3. Subsequently, concentrations of 73As in the tissues of groups of lambs slaughtered at intervals over a period of 181 days were determined. A true absorption coefficient of 0·46±0·055 (mean±S.E.) was determined which is considerably lower than expected from previous studies of non-ruminant animals which demonstrate complete absorption for inorganic arsenic. The resultant data were used to develop a compartment model to describe arsenic behaviour in sheep tissues. The derived model accounted for 80 % (n = 100) of the observed variation in the data. The model predicts that arsenic concentrations in tissues rapidly (< 40 days) reach equilibrium with the dietary intake level. Equilibrium transfer coefficient values (the ratio of the arsenic concentration in a tissue to the daily dietary intake of arsenic) for the important food-chain tissues were calculated as: (2·5±0·67)×10−3 days/kg for muscle, (9·1±1·96)×10−3 days/kg for liver and (1·1±0·14)×10−2 days/kg for kidney

Does soil adhesion matter when predicting radiocaesium transfer to animals?

A sward will often have significant amounts of soil adhered to the vegetation surfaces which will be ingested by grazing animals. If the soil is contaminated by radioactive fallout then it can serve as a dietary source of radionuclides, in addition to any root uptake by the plants. This study is an attempt to quantitatively assess the importance of soil adhesion as a source of radiocaesium to sheep using the RUINS model which simulates radiocaesium transfer in grazing systems. The method of simulating the contamination of vegetation surfaces used by the RUINS model is described, and the importance of the availability of radiocaesium associated with adhered soil relative to plant incorporated radiocaesium discussed. Two sets of simulations are presented: one in which the soil is treated as a medium providing a uniform availability of radiocaesium, and the second in which account is taken of the partitioning of radiocaesium in the soil between ‘fixed’ and ‘labile’ phases. The results demonstrate that, because of the reduced absorption in the gut of radiocaesium associated with soil, animals grazing pastures with significant amounts of radiocaesium associated with adhered soil will not be as contaminated as radiocaesium activity concentrations measured in bulk vegetation samples would suggest. Therefore, the extent of soil adhesion needs to be considered if predictions of radiocaesium contamination of animal products are to be made on the basis of measured activities of sampled vegetation. However, soil adhesion is unlikely to be a significant dietary source of available radiocaesium, unless the soil concerned exhibits an unusually high bioavailability of radiocaesium. Moreover the simulation results indicate that differences in availability between soil types observed experimentally are consistent with the partitioning between fixed and labile soil compartments made by the RUINS model