Radiocesium accumulation in mosses from highlands of Serbia and Montenegro: chemical and physiological aspects

Artificial neural network modeling in environmental radioactivity studies - A review

The development of nuclear technologies has directed environmental radioactivity research toward continuously improving existing and developing new models for different interpolation, optimization, and classification tasks. Due to their adaptability to new data without knowing the actual modeling function, artificial neural networks (ANNs) are extensively used to resolve the tasks for which the application of traditional statistical methods has not provided an adequate response. This study presents an overview of ANN-based modeling in environmental radioactivity studies, including identifying and quantifying radionuclides, predicting their migration in the environment, mapping their distribution, optimizing measurement methodologies, monitoring processes in nuclear plants, and real-time data analysis. Special attention is paid to highlighting the scope of the different case studies and discussing the techniques used in model development over time. The performances of ANNs are evaluated from the perspective of prediction accuracy, emphasizing the advantages and limitations encountered in their use. The most critical elements in model optimization were identified as network structure, selection of input parameters, the properties of input data set, and applied learning algorithm. The analysis of strategies and methods for improving the performance of ANNs has shown that developing integrated and hybrid artificial intelligent tools could provide a new path in environmental radioactivity modeling toward more reliable outcomes and higher accuracy predictions. The review highlights the potential of neural networks and challenges in their application in environmental radioactivity studies and proposes directions for future research.

Distribution of radionuclides in moss-lichen cover and needles on the same grounds of landscape-climatic zones of Siberia

The radiation status of the landscape and climatic zones of Siberia at the turn of the 20th and 21st centuries is characterized using bioindicators/biomonitors: lichens, mosses, and needles, according to the results obtained at the sites of their joint growth. The maximal activity of ¹³⁷Cs in these components is observed in the forest-tundra landscaped zone, polluted during the period of nuclear tests from the nuclear test site "Novaya Zemlya" and also due to slow migration of these elements to the soil under the arctic conditions. In the southern territories the specific activity of radiocesium in the moss-lichen cover and needles of conifers corresponds to the regional background, in the forest-tundra zone it sometimes exceeds it, but in general does not pose a threat to human health. Determined differences in the contents of radioactive elements in lichens and mosses that grow together on sites in different landscape zones of Siberia statistically not significant within one or two standard deviations, and recorded only in the range of 3ϭ at the level of significance 0.05. Specificity of radionuclide distribution in lichens, mosses and needles (differences for epigeals and epiphytic lichens; different species selected at one site, annual and perennial needles, etc.) made it possible to identify the causes of variations in their activities in different zones, along with landscape features of these zones. In the needles of conifers potassium concentration exceeds the content in lichens, at lower levels of thorium and ¹³⁷Cs. The contents of uranium and thorium in the studied components in all landscape-climatic zones correspond to the natural ones, except for the single local territories, because of the possible anthropogenic influence.

Effects of bark washing and epiphytic moss on 137Cs activity concentration in bark and stemflow in Japanese persimmon (Diospyros kaki Thunb.)

The effects of bark washing, and of epiphytic moss growing on the bark surfaces, on the ¹³⁷Cs activity concentrations in bark and stemflow samples were evaluated at two Japanese persimmon orchards located in Fukushima City and Date City, both approximately 60-65 km northwest of the Fukushima Dai-ichi nuclear power plant. The means of ¹³⁷Cs in stemflow captured by 1 g of sphagnum placed on the main trunks of washed and unwashed trees, and on sites with moss and with no moss growing on bark of the non-washed trees in Date City, were 0.37 Bq, 1.08Bq, 0.77 Bq and 0.52 Bq respectively. The ¹³⁷Cs was significantly lower in the washed bark and in the absence of moss at p = 0.00031 and p = 0.0443 by t-test respectively. The ¹³⁷Cs in directly-collected stemflow on the main trunks was significantly higher for the unwashed tree at 6.0 Bq L^-1 compared with the washed tree at 1.1 Bq L^-1 (p = 0.017 by Tukey's test). The extraction rate of dissolved ¹³⁷Cs with deionized water was 7.3% even after stirring for 40 h. These results indicate that the moss acts as reservoir of ¹³⁷Cs, and that bark-washing decontamination can reduce the additional ¹³⁷Cs supply from moss in subsequent years following the radioactive fallout.

Natural and anthropogenic radioactivity in the environment of Kopaonik mountain, Serbia

To evaluate the state of the environment in Kopaonik, a mountain in Serbia, the activity concentrations of (4) K, (226)Ra, (232)Th and (137)Cs in five different types of environmental samples are determined by gamma ray spectrometry, and radiological hazard due to terrestrial radionuclides is calculated. The mean activity concentrations of natural radionuclides in the soil are higher than the global average. However, with an exception of two sampling locations, the external radiation hazard index is below one, implying an insignificant radiation hazard. Apart from (40)K, content of the natural radionuclides is predominantly below minimum detectable activities in grass and cow milk, but not in mosses. Although (137)Cs is present in the soil, grass, mosses and herbal plants, its specific activity in cow milk is below minimum detectable activity. Amongst the investigated herbal plants, Vaccinium myrtillus L. shows accumulating properties, as a high content of (137)Cs is detected therein. Therefore, moderation is advised in consuming Vaccinium myrtillus L. tea.

Fractionation of (137)Cs and Pu in natural peatland

High Cs-137 concentrations in plants growing on peatland inspired us to investigate the quantity of its bioavailable fraction in natural peat. Our investigation aims to: a) estimate the quantity of bioavailable Cs-137 and Pu present in peat, b) verify the similarity of Cs-137 and K-40 behaviours, and c) perform a quantification of Cs-137 and Pu transfer from peat to plants. We analysed the vertical distribution of Cs-137 and Pu isotopes in the peat and their concentrations in plants growing on these places. Bioavailability of radionuclides was investigated by sequential extraction. Sequential analyses revealed that it was the upper layer which contained the majority of Cs-137 in an available form while deeper layers retained Cs-137 in immobile fractions. We can conclude that 18% of all Cs-137 in the peat is still bioavailable. Despite of the low quantity of bioavailable fraction of Cs-137 its transfer factor reached extremely high values. In the case of Pu, 64% of its total amount was associated with fulvic/humic acids which resulted in the high transfer factor from peat to plants. 27 years after the Chernobyl nuclear accident, the significant part of radionuclides deposited in peatland is still bioavailable.

Illustration of constrained composition statistical methods in the interpretation of radionuclide concentrations in the moss Pleurozium schreberi

In this work we have used moss Pleurozium schreberi as a bioindicator of radioactive materials in environment. The following radionuclides were determined in moss samples: (137)Cs, (40)K, (210)Pb, (212)Pb, (214)Pb, (214)Bi, (231)Th, (235)U and (228)Ac, but not all of them in each sample. The highest activity concentrations were found for naturally occurring (210)Pb, (40)K and the artificial isotope (137)Cs. Activities of radioisotopes were recalculated into mass concentrations and the statistical methods intended for constrained data processing were used. Analysis of our results showed at most a weak relationship between radioisotope mass concentrations in moss. It was supposed that concentrations of some elements depended on the same, currently unknown, factors.

Assessment of the impact of petroleum and petrochemical industries to the surrounding areas in Malaysia using mosses as bioindicator supported by multivariate analysis

Biomonitoring of multi-element atmospheric deposition using terrestrial moss is a well-established technique in Europe. Although the technique is widely known, there were very limited records of using this technique to study atmospheric air pollution in Malaysia. In this present study, the deposition of 11 trace metals surrounding the main petroleum refinery plant in Kerteh Terengganu (eastern part of peninsular Malaysia) has been evaluated using two local moss species, namely Hypnum plumaeforme and Taxithelium instratum as bioindicators. The study was also done by means of observing whether these metals are attributed to work related to oil exploration in this area. The moss samples have been collected at 30 sampling stations in the vicinity of the petrochemical industrial area covering up to 15 km to the south, north, and west in radius. The contents of heavy metal in moss samples were analyzed by energy dispersive x-ray fluorescence technique. Distribution of heavy metal content in all mosses is portrayed using Surfer software. Areas of the highest level of contaminations are highlighted. The results obtained using the principal components analysis revealed that the elements can be grouped into three different components that indirectly reflected three different sources namely anthropogenic factor, vegetation factor, and natural sources (soil dust or substrate) factor. Heavy metals deposited mostly in the distance after 9 km onward to the western part (the average direction of wind blow). V, Cr, Cu, and Hg are believed to have originated from local petrochemical-based industries operated around petroleum industrial area.

Radionuclide concentrations in samples of medicinal herbs and effective dose from ingestion of ¹³⁷Cs and natural radionuclides in herbal tea products from Serbian market

The objective of this study is to evaluate natural and artificial radioactivity levels in 18 different samples of medicinal herbs from Serbian market. The activity concentrations of (238)U, (232)Th, (40)K, and (137)Cs were measured using gamma spectroscopy system with a high-purity germanium detector. The activity concentrations of (238)U, (232)Th, (40)K, and (137)Cs were found to be in a range of 0.6-8.2, 1.7-15.1, 126-1243.7, and 0.3-8.8 Bq/kg, respectively. The annual whole body doses from ingestion of (137)Cs and natural radionuclides for an adult person who consumed herbal tea from these medicinal herbs were found to be 2.5-469.9 nSv for (137)Cs, 0.7-9.7 nSv for (238)U, 0.3-2.8 nSv for (232)Th, and 1026.0-10,132.0 nSv for (40)K. These doses are not hazardous to public health. The obtained results were compared with correspondent studies conducted worldwide.

Transfer of radionuclides to ants, mosses and lichens in semi-natural ecosystems

There is a scarcity of data on transfer of both natural and anthropogenic radionuclides to detritivorous invertebrates for use in the assessment of radiation exposure. Although mosses and lichens have been extensively used in biomonitoring programs, the data on transfer of radionuclides to these species are limited, particularly for natural radionuclides. To enhance the available data, activity concentrations of (137)Cs, (226)Ra and (228)Ra were measured in ants, mosses and lichens and corresponding undisturbed soil collected from semi-natural ecosystems in Serbia and Montenegro and biota/soil concentration ratios (CR) calculated. Since the majority of internal dose to biota is expected to come from (40)K, the activity concentrations of this radionuclide were also determined. The mean CR values for (137)Cs, (226)Ra and (228)Ra in ants analyzed in this study were found to be 0.02, 0.06 and 0.02, respectively. The mean CR values of radionuclides in mosses were found to be 2.84 for (137)Cs, 0.19 for (226)Ra and 0.16 for (228)Ra, while those in lichens were found to be 1.08 for (137)Cs, 0.15 for (226)Ra and 0.13 for (228)Ra. The CR values obtained in this study were compared with default CR values used in the ERICA Tool database and also with those reported in other studies.

Transfer of natural and anthropogenic radionuclides to ants, bryophytes and lichen in a semi-natural ecosystem

Few data are available to quantify the transfer of both natural and anthropogenic radionuclides to detritivorous invertebrates to facilitate estimation of the internal dose to such biota in models used to assess radiation exposure. To enhance the available data, activity concentrations of (137)Cs, (40)K, (90)Sr, (239 + 240)Pu, (241)Am, (235)U and (238)U were measured in ants (Formicidae) and corresponding undisturbed soil collected from the Zlatibor mountain in Serbia and ant/soil concentration ratios (CR) calculated. The (241)Am concentration ratios for ants were fourfold higher than those calculated for ants in a previous study whereas they are similar to the more numerous data previously reported for a range of detritivorous invertebrates in other studies. CR values for (137)Cs in ants were similar to the few other reported values and slightly lower than those for a range of detritivorous invertebrates. Those for (239 + 240)Pu were slightly higher than those for ants in two other studies but they were close to upper limit of a range of data reported for detritivorous invertebrates. All the CR values will be included in a future revision of the ERICA Tool database and will particularly improve the information available for uranium.

The distribution of (137)Cs, K, Rb and Cs in plants in a Sphagnum-dominated peatland in eastern central Sweden

We record the distribution of (137)Cs, K, Rb and Cs within individual Sphagnum plants (down to 20cm depth) as well as (137)Cs in vascular plants growing on a peatland in eastern central Sweden. In Calluna vulgaris(137)Cs was mainly located within the green parts, whereas Andromeda polifolia, Eriophorum vaginatum and Vaccinium oxycoccos showed higher (137)Cs activity in roots. Carex rostrata and Menyanthes trifoliata showed variable distribution of (137)Cs within the plants. The patterns of (137)Cs activity concentration distribution as well as K, Rb and Cs concentrations within individual Sphagnum plants were rather similar and were usually highest in the capitula and/or in the subapical segments and lowest in the lower dead segments, which suggests continuous relocation of those elements to the actively growing apical part. The (137)Cs and K showed relatively weak correlations, especially in capitula and living green segments (0-10cm) of the plant (r=0.50). The strongest correlations were revealed between (137)Cs and Rb (r=0.89), and between (137)Cs and stable Cs (r=0.84). This suggests similarities between (137)Cs and Rb in uptake and relocation within the Sphagnum, but that (137)Cs differs from K.

Quantification of transfer of (238)U, (226)Ra, (232)Th, (40)K and (137)Cs in mosses of a semi-natural ecosystem

There is a lack of appropriate data on transfer of some radionuclides on many terrestrial biota groups. To expand the available data concentration ratios of (238)U, (226)Ra, (232)Th, (40)K and (137)Cs in mosses are presented in this paper. The relationship between concentration ratios of radionuclides and physicochemical characteristics of the underlying soil was also investigated. The data on concentration ratios obtained here will provide a useful addition to the currently used database of transfer parameters, particularly for natural radionuclides.

Natural and anthropogenic radioactivity of feedstuffs, mosses and soil in the Belgrade environment, Serbia

By gamma spectroscopic measurement a content of natural radio-nuclides (40K, 238U, 226Ra, 232Th) and 137Cs was determined in samples of soil, alfalfa, maize and moss on six sites in the surroundings of Belgrade. Natural radionuclides in the soil were at the level characteristic for Serbia, whereas a relatively high level of activity of 137Cs (around 30 Bq kg-1) was determined. On the other hand, in plant samples mostly used as feed (such as alfalfa and maize) the concentration of natural radio-nuclide activity and 137Cs was relatively low, i.e. below the range of detection. The content of natural radio-nuclides in moss was within the standard range of values specific for Serbia. However, the activity level of 137Cs in moss gathered from the wider area around Belgrade, was high, the highest measured level being in the Avala-Zuce area (158-221 Bq kg-1). Our results show that this radio-nuclide is still present in the living environment of Belgrade even 20 years after the Chernobyl disaster, and that moss is a good indicator of living environment 137Cs contamination.

Analysis of mosses and topsoils for detecting sources of heavy metal pollution: multivariate and enrichment factor analysis

In order to assess the contribution of emission sources to the pollution of areas remote from industrial facilities, a combined approach of enrichment factor analysis and multivariate statistics was used for detecting the origin of heavy metal pollution in the Zlatibor ecosystem, in Serbia. Samples of moss (Pleurozium schreberi, Hylocomium splendens, Scleropodium purum, Hypnum cupressiforme and Thuidum delicatulum) and of topsoil (0-5 cm) were collected in 2005. The concentrations of seven heavy metals (Cd, Cr, Cu, Mn, Ni, Pb and Zn) were determined in moss and soil samples by atomic absorption spectrometry. The results obtained by enrichment factor analysis and two multivariate statistical methods, principal component analysis and cluster analysis, enabled discrimination of the lithologic and anthropogenic sources of heavy metals in the mosses. Enrichment factors, calculated to evaluate the contribution to the metal content in moss from anthropogenic sources, revealed pollution of the investigated area by Cd and Pb, originating from long-range transport and fossil fuel burning.

137Cs in a raised bog in central Sweden

The vertical distribution of (137)Cs activity in peat soil profiles and (137)Cs activity concentration in plants of various species was studied in samples collected at two sites on a raised bog in central Sweden. One site (open bog) was in an area with no trees and only a few sparsely growing plant species, while the other (low pine) was less than 100 m from the open bog site and had slowly growing Scots pine, a field layer dominated by some ericaceous plants and ground well-covered by plants. The plant samples were collected in 2004-2007 and were compared with samples collected in 1989 from the same open bog and low pine sites. Ground deposition of (137)Cs in 2005 was similar at both sites, 23,000 Bq m(-2). In the open bog peat profile it seems to be an upward transport of caesium since a clear peak of (137)Cs activity was found in the uppermost 1-4 cm of Sphagnum layers, whereas at the low pine site (137)Cs was mainly found in deeper (10-12 cm) layers. The migration rate was 0.57 cm yr(-1) at the open bog site and the migration centre of (137)Cs was at a depth of 10.7, while the rate at the low pine site was 0.78 cm yr(-1) and the migration centre was at 14.9 cm. Heather (Calluna vulgaris) was the plant species with the highest (137)Cs activity concentrations at both sites, 43.5 k Bq(-1) DM in 1989 decreasing to 20.4 in 2004-2007 on open bog and 22.3 k Bq kg(-1) DM in 1989 decreasing to 11.2 k Bq(-1) DM by the period 2004-2007 on the low pine site. (137)Cs transfer factors in plants varied between 0.88 and 1.35 on the open bog and between 0.48 and 0.69 m(2)kg(-1) DM at the low pine site.

Cesium-137 monitoring using mosses from W. Macedonia, N. Greece

(137)Cs activities in mosses and substrate (soil, bark) collected from W. Macedonia, Greece were measured 20 years after the Chernobyl reactor accident. Archive material from previous studies was also used for comparison and diachronic estimation of the radio-contamination status. A gradual decrease was detected which depended on various factors such as the collected species, location, growth rate and substrate. Maximum accumulation capacity of (137)Cs was observed in the epilithic mosses in comparison to the epiphytic ones. The (137)Cs content in the bark of the two broad-leaved species (oak and fagus) was higher than that of the conifer (pinus). Bark specimens of about 50 cm height were in general more contaminated than those of 200 cm. Autoradiography revealed an amount of (137)Cs distributed more or less uniformly in moss thalli. The high (137)Cs activities found in mosses 20 years after Chernobyl suggest that these primitive plants are effective, suitable and inexpensive biological detectors of the distribution and burden of radionuclide fallout pattern.

Implementation of neural networks for classification of moss and lichen samples on the basis of gamma-ray spectrometric analysis

Mosses and lichens have an important role in biomonitoring. The objective of this study is to develop a neural network model to classify these plants according to geographical origin. A three-layer feed-forward neural network was used. The activities of radionuclides ((226)Ra, (238)U, (235)U, (40)K, (232)Th, (134)Cs, (137)Cs and (7)Be) detected in plant samples by gamma-ray spectrometry were used as inputs for neural network. Five different training algorithms with different number of samples in training sets were tested and compared, in order to find the one with the minimum root mean square error. The best predictive power for the classification of plants from 12 regions was achieved using a network with 5 hidden layer nodes and 3,000 training epochs, using the online back-propagation randomized training algorithm. Implementation of this model to experimental data resulted in satisfactory classification of moss and lichen samples in terms of their geographical origin. The average classification rate obtained in this study was (90.7 +/- 4.8)%.

Classification of soil samples according to geographic origin using gamma-ray spectrometry and pattern recognition methods

Multivariate data analysis methods were used to recognize and classify soils of unknown geographic origin. A total of 103 soil samples were differentiated into classes, according to regions in Serbia and Montenegro from which they were collected. Their radionuclide (226Ra, 238U, 235U, 40K, 134Cs, 137Cs, 232Th and 7Be) activities detected by gamma-ray spectrometry were then used as the inputs in different pattern recognition methods. For the classification of soil samples using eight selected radionuclides, the prediction ability of linear discriminant analysis (LDA), k-nearest neighbours (kNN), soft independent modelling of class analogy (SIMCA) and artificial neural network (ANN) were 82.8%, 88.6%, 60.0% and 92.1%, respectively.

Extraction of caesium from mosses and incorporation into an oxalate crystalline lattice

The ratios of stable and radioactive isotopes in the environment are not constant and their effects on living organisms are not the same. Whilst radionuclides can be monitored by measuring their radioactivity levels, it is also important to determine the stable isotopes. The method described in this paper represents a simple and cheap process by which to extract caesium from moss, without significant destruction of the plant, and to incorporate the isolated caesium into a water-soluble crystal. Ammonium oxalate and phosphoric acid are water-soluble substances at 5% concentration but, when the solutions are combined, crystallisation occurs. Crystals were estimated to contain 68.30% O, 20.98% C, 4.68% H, 6.04% N and less than 0.01% P, corresponding to NH4HC2O4 x H2C2O4 x 2H2O (C4H11NO10). The crystal mass did not vary with the species of moss, although the effectiveness of caesium extraction did. Typically, 73.0 +/- 6.1% of the solubilised caesium was entrapped within the crystals.

Classification of soil samples according to their geographic origin using gamma-ray spectrometry and principal component analysis

A principal component analysis (PCA) was used for classification of soil samples from different locations in Serbia and Montenegro. Based on activities of radionuclides ((226)Ra, (238)U, (235)U, (40)K, (134)Cs, (137)Cs, (232)Th and (7)Be) detected by gamma-ray spectrometry, the classification of soils according to their geographical origin was performed. Application of PCA to our experimental data resulted in satisfactory classification rate (86.0% correctly classified samples). The obtained results indicate that gamma-ray spectrometry in conjunction with PCA is a viable tool for soil classification.