Radioactivity of soil in Croatia I: naturally occurring decay chains.

The assessment of environmental radioactivity much relies on radionuclide content in soil. This stems from the significant contribution of soil to both external and internal exposure to ionising radiation via direct emission of gamma radiation and soil-to-plant radionuclide transfer, respectively. This motivated us to carry out a systematic research on the radioactivity of soil in Croatia to obtain relevant data that can be used as a basis for understanding the related effects of geomorphological, biogeographical, and climatological properties of the environment. We collected samples of the surface layer of uncultivated soil (0-10 cm) at 138 sites from all over the country and measured them for radionuclide activity concentrations by means of high-resolution gamma-ray spectrometry. This resulted in radioactivity maps containing data on activity concentrations of representative radionuclides in the environment. In this paper, which is the first in our two-part presentation, we focus on the naturally occurring 232Th and 238U decay chains and their correlations with the diversity of Croatian regions. For both of the chains, activity concentrations were the highest in the Dinaric region, the lowest in the Pannonian region, and intermediate in the Adriatic region. Relatively high concentrations of 226Ra in the soil of the Dinaric region implied a possibility of an enhanced emanation of its progeny 222Rn into the air. Activity concentrations of 210Pb were additionally elevated in areas with dense vegetation, most probably due to an atmospheric deposition of airborne 210Pb onto the surface of plants and their eventual decomposition on the ground.

Application of a new in situ calibration technique for gamma spectrometry and comparison of in situ and laboratory measurements.

In situ gamma ray spectrometry was developed to quickly measure large areas of land following nuclear accidents. However, a proper calibration of detectors for in situ measurements is a long and complicated process. One tool designed to make this calibration quick is the InSiCal software. We compared 5,000 s in situ measurements with two different HPGe detectors calibrated using the InSiCal software and laboratory measurements of samples collected at the same locations. Our findings suggest that in situ gamma spectrometry using InSiCal software can provide reasonably accurate data, but some improvements are needed.

Radioactivity of soil in Croatia II: 137Cs, 40K, and absorbed dose rate.

We took samples of uncultivated soil from the surface layer (0-10 cm) at 138 sites from all over Croatia and measured their radionuclide activity concentrations with high-resolution gamma-ray spectrometry. This second part of our report brings the results on 40K and 137Cs to complement those on the 232Th and 238U decay chains addressed in the first part. Together they give the most complete picture of radioactivity of Croatian soil so far. Activity concentrations of 40K were the highest in the Pannonian region, and there was an opposite trend for 137Cs. We found that the concentrations of 137Cs tended to increase with altitude, annual precipitation, and vegetation density. The concentration ratio of 137Cs and K in soil, which indicates the potential for 137Cs entering food chains via uptake by plants, was the lowest in agriculturally important areas in the east of the Pannonian region. In addition, we used the obtained results on activity concentrations to calculate the related absorbed dose rate as a measure of external exposure to ionising radiation from soil. The sum of the absorbed dose rates for naturally occurring radionuclides and 137Cs showed that external exposure was generally the highest in the Dinaric region and Istrian Peninsula.

Baseline radioecological data for the soil and selected bioindicator organisms in the temperate forest of Plitvice Lakes National Park, Croatia.

The aim of this study was to provide baseline radioecological data for the temperate forest ecosystem in Plitvice Lakes National Park. Emphasis was placed on the determination of naturally occurring radionuclides since there is an acknowledged lack of data for these radionuclides in non-accident conditions in wildlife, even for bioindicator organisms. Activity concentrations of 238U, 226Ra, 210Pb, 232Th, 40K, 134Cs, and 137Cs were measured by gamma spectrometry in soil and bioindicators: earthworms, conifer needles, mosses, and lichens. From the measured activity concentrations, concentration ratios were calculated to quantify the transfer of these radionuclides from soil to bioindicators. Our results show that soil activity concentrations are biased toward results from other studies conducted within the Dinaric mountain region. However, in moss and lichen samples, we measured higher activity concentrations of 226Ra and lower activity concentrations of 40K and 137Cs in comparison to similar studies. Also, we estimated lower concentration ratios for all radionuclides from soil to these organisms, except for 210Pb, in comparison to generic values. The transfer of 238U was generally low for all of the bioindicator organisms. For conifer needles, a correlation was found between activity concentrations of 226Ra and 137Cs in soil and related concentration ratios. Correlation was also found between the activity concentration of 40K in soil and transfer of 40K and 137Cs to mosses and lichens. A comparison with literature data highlighted the lack of 226Ra related concentration ratios for conifer trees and especially for earthworms. Therefore, the results of this study could supplement the sparse data currently available on radionuclide background data in similar ecosystems and related soil-to-wildlife transfer of radionuclides. Dose rate assessments, performed by the ERICA Tool, estimated that 96% of the overall exposure of wildlife in the Park area is due to the background dose rates, while 0.06 µGy h-1 on average can be attributed as an incremental dose rate from 134Cs and 137Cs.

The role of physics in radioecology and radiotoxicology.

This article gives an overview of physical concepts important for radioecology and radiotoxicology to help bridge a gap between non-physicists in these scientific disciplines and the intricate language of physics. Relying on description and only as much mathematics as necessary, we discuss concepts ranging from fundamental natural forces to applications of physical modelling in phenomenological studies. We first explain why some atomic nuclei are unstable and therefore transmute. Then we address interactions of ionising radiation with matter, which is the foundation of both radioecology and radiotoxicology. We continue with relevant naturally occurring and anthropogenic radionuclides and their properties, abundance in the environment, and toxicity for the humans and biota. Every radioecological or radiotoxicological assessment should take into account combined effects of the biological and physical half-lives of a radionuclide. We also outline the basic principles of physical modelling commonly used to study health effects of exposure to ionising radiation, as it is applicable to every source of radiation but what changes are statistical weighting factors, which depend on the type of radiation and exposed tissue. Typical exposure doses for stochastic and deterministic health effects are discussed, as well as controversies related to the linear no-threshold hypothesis at very low doses.

Assessing the radiological load on the environment in the middle Danube river basin on the basis of a study of the Kopacki Rit Nature Park, Croatia.

A study of the environmental radioactivity in the Kopacki Rit Nature Park, Croatia, is presented. This wildlife reserve is part of the Middle Danube River Basin, and it is exposed to various pollutants due to a number of human activities in the surroundings, where there is a nuclear power plant and also urban centres and areas of intense agricultural production. Results for the activity concentrations of soil and surface water samples do not indicate any elevated radioactivity level, which is confirmed by on-site measurements of ambient dose rate equivalent. An assessment of the radiological load on the local biota, carried out using the ERICA tool, implies an overall low radiological risk even if conservative values of the risk quotient are used. Therefore, human activities do not have a significant effect on the radiological load on the Kopacki Rit area. A similar conclusion might be made with regard to numerous similar environments in the Middle Danube River Basin.

Current radiological situation in Slavonski Brod, Croatia: is there a reason for concern?

For years, the town of Slavonski Brod in Croatia has been facing serious problems with air pollution, which is usually attributed to an oil refinery across the Sava River in Bosnia and Herzegovina. While the air quality is being monitored rigorously with regard to nonradioactive matter, no attention has been paid to the possibility of a coincidental radioactive pollution. This study is the first to have addressed this issue. We measured ambient dose rate equivalents at 150 sites and found that none exceeded 120 nSv h-1, while the average was 80 nSv h-1. Gamma-ray spectrometry of the collected river water and soil samples did not reveal any unusual radioactivity either. In other words, we have found no evidence of radioactive pollution that would endanger the health of the residents of Slavonski Brod.

Plant uptake of 238U, 235U, 232Th, 226Ra, 210Pb and 40K from a coal ash and slag disposal site and control soil under field conditions: A preliminary study.

The aim of this study was to investigate the uptake of 238U, 235U, 232Th, 226Ra, 210Pb and 40K by plants that grow on a coal ash and slag disposal site known for its higher content of naturally occurring radionuclides. Plant species that were sampled are common for the Mediterranean flora and can be divided as follows: grasses & herbs, shrubs and trees. To compare the activity concentrations and the resultant concentration ratios of the disposal site with those in natural conditions, we used control data specific for the research area, obtained for plants growing on untreated natural soil. Radionuclide activity concentrations were determined by high resolution gamma-ray spectrometry. Media parameters (pH, electrical conductivity and organic matter content) were also analysed. We confirmed significantly higher activity concentrations of 238U, 235U, 226Ra and 210Pb in ash and slag compared to control soil. However, a significant increase in the radionuclide activity concentration in the disposal site's vegetation was observed only for 226Ra. On the contrary, a significantly smaller activity concentration of 40K in ash and slag had no impact on its activity concentration in plant samples. The calculated plant uptake of 238U, 235U, 226Ra and 210Pb is significantly smaller in comparison with the uptake at the control site, while it is vice versa for 40K. No significant difference was observed between the disposal site and the control site's plant uptake of 232Th. These results can be the foundation for further radioecological assessment of this disposal site but also, globally, they can contribute to a better understanding of nature and long-term management of such disposal sites.

Air sampling by pumping through a filter: effects of air flow rate, concentration, and decay of airborne substances.

This paper tackles the issue of interpreting the number of airborne particles adsorbed on a filter through which a certain volume of sampled air has been pumped. This number is equal to the product of the pumped volume and particle concentration in air, but only if the concentration is constant over time and if there is no substance decomposition on the filter during sampling. If this is not the case, one must take into account the inconstancy of the concentration and the decay law for a given substance, which is complicated even further if the flow rate through the filter is not constant. In this paper, we develop a formalism which considers all of these factors, resulting in a single, compact expression of general applicability. The use of this expression is exemplified by addressing a case of sampling airborne radioactive matter, where the decay law is already well known. This law is combined with three experimentally observed time dependence of the flow rate and two models for the time dependence of the particle concentration. We also discuss the implications of these calculations for certain other situations of interest to environmental studies.

Determination of gamma-ray self-attenuation correction in environmental samples by combining transmission measurements and Monte Carlo simulations.

We develop a simple and widely applicable method for determining the self-attenuation correction in gamma-ray spectrometry on environmental samples. The method relies on measurements of the transmission of photons over the matrices of a calibration standard and an analysed sample. Results of this experiment are used in subsequent Monte Carlo simulations in which we first determine the linear attenuation coefficients (µ) of the two matrices and then the self-attenuation correction for the analysed sample. The method is validated by reproducing, over a wide energy range, the literature data for the µ of water. We demonstrate the use of the method on a sample of sand, for which we find that the correction is considerable below ~400keV, where many naturally occurring radionuclides emit gamma rays. At the lowest inspected energy (~60keV), one measures an activity that is by a factor of ~1.8 smaller than its true value.

137Cs in soil and fallout around Zagreb (Croatia) at the time of the Fukushima accident.

This paper addresses the noticeable increase of 137Cs activity concentrations in soil and fallout in the area surrounding Zagreb (Croatia) that occurred at the time of the 2011 Fukushima accident. This topic is important for public health as 137Cs is highly toxic due to its long half-life of radioactive decay and chemical similarity to potassium. 137Cs concentrations in fallout were much greater than in soil, but remained present longer in the latter. While being detectable in our measurements, 137Cs did not spread through the food chain in amounts exceeding the maximum allowed level of radioactive food contamination. However, more thorough and consistent measurements need to be done in order to establish the precise activity trends of 137Cs in Zagreb soil and fallout.