Radioactive contamination of tropical rainforest soils in Southern Costa Rica

Radionuclide content in soils from four locations in a tropical rainforest near Golfito in Southern Costa Rica was investigated. For comparison, two nearby locations in open grassland were also studied. From each site 5 soil cores down to a depth of 15 cm were taken. The median contamination with 137Cs was 584 Bq m-2 (reference date 1 January 1996) and the coefficient of variation (CV) was 50%. This contamination can be attributed to global fallout from atmospheric nuclear weapon tests between 1945 and 1980. The mean contamination is slightly lower than the value expected for the latitude (8 degrees 42': 700 Bq m-2), which may be explained by migration of radiocaesium to subsoil below 15 cm or by uptake into the living biomass. Out of the total variability of 50%, around 20% can be attributed to the sampling and measuring process uncertainties, thus leaving a 45% contribution of spatial variability. A significant difference between forest and meadow sites could be detected: the meadow sites showed lower radiocaesium soil inventories (median: 291 Bq m-2) than the forest sites (643 Bq m-2). This may be explained by the agricultural activities carried out on meadow sites which lead to an increased redistribution of caesium in the soil profile and therefore a larger fraction of the total 137Cs lying below 15 cm. Another reason for higher contamination levels under forest can be attributed to the high interception potential of dense tree canopies for dry deposition. Extrapolating the 137Cs concentration below the sampling horizon, i.e. accounting for the cut-off of the profiles by the sampling technique, results in an estimated mean of 710 Bq m-2 for the forest sites, which is very close to the expected figure. The mainly mineral part of the forest soil profiles was analysed for the 137Cs transport parameters, apparent convection velocity (v = 0.14 +/- 0.09 cm a-1) and apparent diffusion constant (D = 0.79 +/- 0.49 cm2 a-1). The maximum concentration can be found at 5.3 +/- 2.9 cm depth, the half-value depth being 7.4 +/- 1.3 cm. The mean 40K activity concentration was 175 Bq kg-1 dry matter (CV = 69%) and 226Ra and 228Ra concentrations of 9.90 Bq kg-1 (CV = 23%) and 7.93 Bq kg-1 (CV = 20%) have been found, respectively.     

Radioactive contamination of wood and its products

This paper presents research on radioactive contamination of the three most common kinds of wood in Croatia--beech, oak and fir as well as acorn. Gamma-spectrometric measurements carried out on the samples of bark and wood of beech, oak, fir and acorn have shown radioactivity contents ranging from 1.6 +/- 0.1 to 37.3 +/- 0.5 Bq/kg from deposited 137Cs, whose concentrations in the soil of Croatia have increased after the Chernobyl accident. Measurements have also shown the radioactivity originating from 40K and 214Bi, which are part of the natural composition of the soil. The distribution of the radionuclides in wood has been discussed, as well as the impact of radioactive contamination of wood by the artificial radionuclide 137Cs upon the forest ecosystem. According to the corresponding model, it has been calculated that a 10 h daily stay in a typical family house increases the annual radiation dose of the population, due to the deposited 137Cs in the structure or furniture, by 343 microSv.     

Long-term investigations of radiocaesium activity concentrations in carp in North Croatia after the Chernobyl accident

Long-term investigations of radiocaesium activity concentrations in carp in the Republic of Croatia are presented. The radiocaesium levels in carp decreased exponentially and the effective ecological half-life of (137)Cs was estimated to be about 1 year during 1987-2002 and 5 years during 1993-2005. The observed (134)Cs:(137)Cs activity ratio in carp was found to be similar to the ratio observed in other environmental samples. The concentration factor for carp (wet weight) was estimated to be 128+/-74 Lkg(-1), which is in reasonable agreement with model prediction based on K(+) concentrations in water. Estimated annual effective dose received by adult members of the Croatian population due to consumption of carp contaminated with (134)Cs and (137)Cs are small: per capita dose from this source during 1987-2005 was estimated to be 0.5+/-0.2 microSv. Due to minor freshwater fish consumption in Croatia and low radiocaesium activity concentrations in carp, it can be concluded that carp consumption was not a critical pathway for the transfer of radiocaesium from fallout to humans after the Chernobyl accident.     

Radiocaesium fallout behaviour in volcanic soils in Iceland

The retention of 137Cs in various types of Andosols in Iceland was investigated. Soils were sampled at 29 sites with varying precipitation and environmental conditions. Samples were obtained from 0 to 5, 5 to 10, and 10 to 15 cm depths. The amount of radiocaesium present was quite variable, ranging between 300 and 4800 Bq m(-2) and correlated closely to total annual precipitation (r2=0.71). The majority of 137Cs, 82.7% on average, was retained in the uppermost 5 cm of the soil. The greatest penetration of 137Cs was observed for organic Histosols (76.3% in top 5 cm). The Icelandic Vitrisols (barren, poorly developed Andosols) are coarse grained with only 2-5% clay content and contain little organic matter (<1%). Yet these soils retained 74% of 137Cs in the top 5 cm. The results indicate that radiocaesium fallout is strongly retained by colloidal materials characteristic of Andosols, such as allophane and ferrihydrite. Most soils in Iceland are subject to severe and prolonged freezing and waterlogging; despite this, 137Cs is retained in the upper soil horizons and vertical migration is negligible in Icelandic Andosols. However, erosion and aeolian activity can markedly influence the amount and vertical distribution of radiocaesium in Icelandic soils.     

Cesium-137 contamination of oak (Quercus petrae Liebl.) from sub-mediterranean zone in South Bulgaria

This study focuses on the cesium-137 (¹³⁷Cs) contamination in grass and in different compartments of oak trees growing in ecosystems, located in the zone with sub-mediterranean climate in South Bulgaria, characterized with high summer temperatures, low precipitation and often periods of drought. In 2008, three experimental sites - PP1, PP2, PP3 - were sampled in oak ecosystems from Maleshevska Mountain at 900 m above sea level. Samples from grass species and oak tree leaves, branches with different diameter, wood disks and bark were analyzed for ¹³⁷Cs activity with γ-spectrometry. The soil-to-plant transfer factor (TF) values for ¹³⁷Cs were estimated differentiating different tree compartments. Our findings showed relatively high activity concentrations of ¹³⁷Cs in oak trees even 22 years after the Chernobyl accident. The grass under oak was less contaminated compared with the oak trees. The different organs of oak trees could be distinguished according to the ¹³⁷Cs contamination as follows: bark > branches (d < 1 cm) > leaves > branches (d > 3 cm) > wood. The relatively higher contamination of bark compared with the new-formed biomass suggested that a significant part of ¹³⁷Cs was accumulated as a result of direct adsorption at the time of the main contamination event. The TF values obtained and the presence of ¹³⁷Cs in the branches, leaves and in the wood formed after 1986 confirmed that 22 years after the contamination, the main mechanism of ¹³⁷Cs entrance in tree biomass was the root uptake.     

239+240Pu, 90Sr and 137Cs inventories in surface soils of Vietnam

Fallout 239+240Pu, 238Pu, 90Sr and 137Cs inventories in surface soils were measured for 20 locations in northern Vietnam yielding the mean values (+/- standard error) of 26.5+/-3.8 Bq m(-2) for 239+240Pu, 1048+/-143 Bq m(-2) for 137Cs and 212+/-28 Bq m(-2) for 90Sr. The concentrations of 137Cs and plutonium isotopes strongly correlate with each other resulting in a stable 239+240Pu/137Cs inventory ratio of 0.025+/-0.002. Among soil parameters, organic matter and fulvic acids strongly correlate with caesium and plutonium isotopes, especially in the 0-10 cm layer. 137Cs and 239+240Pu are distributed rather similarly over the 0-10 cm and 10-20 cm layers. At locations with high contents of sand (82-93%) along the South China Sea coast, the downward percolation by rainwater results in a higher accumulation of 239+240Pu and 137Cs in the 10-20 cm layer. The mean 137Cs/ 90Sr inventory ratio is 9.3+/-2.2, and the correlation is weak between these isotopes.     

Artificial radionuclides in sediments of the Don River Estuary and Azov Sea

The Don River Estuary-Azov Sea system is an extension of the shallow continental shelf area of the Black Sea. A large data set of artificial radionuclides in bottom sediments of the Azov Sea has been compiled in order to examine the storage and migration of radionuclides within this highly restricted inland sea and to estimate the annual dose received by individuals in the local population who regularly consume fish. In recent years (1997-1999), the radionuclide content of surface sediments has been: 137Cs < or = 0.5-100 Bq/kg d.w. (mean = 33.8 +/- 25.9 Bq/kg d.w., n = 57), 90Sr = 0.2-5.7 Bq/kg d.w. (mean = 2.1 +/- 1.4 Bq/kg d.w., n = 34) and 239,240Pu = 0.31-0.51 Bq/kg d.w. (n = 2). In general, 137Cs activities increase with distance from the mouth of the Don River and correlate negatively with sediment grain size (r2 = 0.77, n = 21). The annual 137Cs-derived dose received by an individual through the trophic chain water-fish-humans (approximately 10(-6) Sv/yr) is well below regulatory recommended limits, indicating that current levels of radioactivity in the environment pose no threat to commercial fisheries operations for the bordering nations of Russia and Ukraine.     

137Cs distribution among annual rings of different tree species contaminated after the Chernobyl accident

The distributions of 137Cs among annual rings of Pinus sylvestris and Betula pendula at four experimental sites located in the most contaminated areas in the Russian territory after the Chernobyl accident in 1986 were studied. Trees of different ages were sampled from four forest sites with different tree compositions and soil properties. The data analysis shows that 137Cs is very mobile in wood and the 1986 rings do not show the highest contamination. The difference between pine and birch in the pattern of radial 137Cs distribution can be satisfactorily explained by the difference in radial ray composition. 137Cs radial distribution in the wood can be described as the sum of two exponential functions for both species. The function parameters are height, age and species dependent. The distribution of 137Cs in birch wood reveals much more pronounced dependence on site characteristics and/or the age of trees than pines. The data obtained can be used to assess 137Cs content in wood.     

Predicting the transfer of radiocaesium from organic soils to plants using soil characteristics

A model predicting plant uptake of radiocaesium based on soil characteristics is described. Three soil parameters required to determine radiocaesium bioavailability in soils are estimated in the model: the labile caesium distribution coefficient (kd1), K+ concentration in the soil solution [mK] and the soil solution-->plant radiocaesium concentration factor (CF, Bq kg-1 plant/Bq dm-3). These were determined as functions of soil clay content, exchangeable K+ status, pH, NH4+ concentration and organic matter content. The effect of time on radiocaesium fixation was described using a previously published double exponential equation, modified for the effect of soil organic matter as a non-fixing adsorbent. The model was parameterised using radiocaesium uptake data from two pot trials conducted separately using ryegrass (Lolium perenne) on mineral soils and bent grass (Agrostis capillaris) on organic soils. This resulted in a significant fit to the observed transfer factor (TF, Bq kg-1 plant/Bq kg-1 whole soil) (P < 0.001, n = 58) and soil solution K+ concentration (mK, mol dm-3) (P < 0.001, n = 58). Without further parameterisation the model was tested against independent radiocaesium uptake data for barley (n = 71) using a database of published and unpublished information covering contamination time periods of 1.2-10 years (transfer factors ranged from 0.001 to 0.1). The model accounted for 52% (n = 71, P < 0.001) of the observed variation in log transfer factor.     

Derivation of 137Cs deposition density from measurements of 137Cs inventories in undisturbed soils

The 137Cs inventories in undisturbed soils were measured for 292 locations across the territory of Vietnam. The logarithmic inventory values were regressed against characteristics of sampling sites, such as geographical coordinates, annual rainfall and physico-chemical parameters of soil. The regression model containing latitude and annual rainfall as determinants could explain 76% of the variations in logarithmic inventory values across the territory. The model part was interpreted as the logarithmic 137Cs deposition density. At the 95% confidence level, 137Cs deposition density could be predicted by the model within +/- 7% relative uncertainty. The latitude mean 137Cs deposition density increases northward from 237 Bq m(-2) to 1097 Bq m(-2), while the corresponding values derived from the UNSCEAR (1969) global pattern are 300 Bq m(-2) and 600 Bq m(-2). High 137Cs inputs were found in high-rainfall areas in northern and central parts of the territory.     

Ecological half-life of 137Cs in plants associated with a contaminated stream

Ecological half-life (Te) is a useful measure for studying the long-term decline of contaminants, such as radionuclides, in natural systems. The current investigation determined levels of radiocesium (137Cs) in two aquatic (Polygonum punctatum, Sagittaria latifolia) and three terrestrial (Alnus serrulata, Myrica cerifera, Salix nigra) plant species from a contaminated stream and floodplain on the U.S. Department of Energy's Savannah River Site. Current 137Cs levels in plants were used in conjunction with historical data to determine Te of 137Cs in each species. Median concentrations of 137Cs were highest in S. latifolia (0.84 Bq g(-1)) and lowest in M. cerifera (0.10 Bq g(-1)). Te's ranged from 4.85 yr in M. cerifera to 8.35 yr in S. nigra, both terrestrial species. Te's for all aquatic (6.30 yr) and all terrestrial (5.87) species combined were very similar. The Te's of the two aquatic primary producers (P. punctatum and S. latifolia) in the Steel Creek ecosystem were somewhat longer than Te values previously reported for some consumers from this ecosystem.     

Activity concentration of caesium-137 in agricultural soils

In this study, we measured 137Cs activity concentrations in the soil samples taken from agricultural lands in the Buyuk Menderes Basin in Turkey in 1997 and 1998. The soil samples were collected from 42 sites in this Basin. The activity concentration of 137Cs was found to range between 2.81+/-0.17 Bq.kg(-1) and 20.75+/-0.29 Bq.kg(-1). The effect of organic matter, clay, silt and sand contents and pH of the soil on the relative adsorption of the 137Cs on the soil surface were also studied.     

Radionuclide tracing of water masses and processes in the water column and sediment in the Algerian Basin

Caesium-137 and (239,240)PU were analysed in the water column along the Algerian coast. The (137)Cs activity concentration in surface water increased from the west to the east from 1.6 to 3.3 mBq L(-1), documenting a presence of Modified Atlantic Water (MAW) in the region. Higher concentrations observed in deep waters may be due to an intrusion of Levantine Intermediate Water (LIW), which has been carrying higher levels of (137)Cs from Chernobyl accident. The (239,240)Pu sub-surface concentration peaked at about 250 m water depth as a result of biogeochemical processes in the water column. The observed (239,240)Pu/(137)Cs activity ratio at the surface (0.003) was significantly lower than that in global fallout (0.04). This decrease exceeds that expected from radioactive decay of (137)Cs, and confirms that Pu due to its adsorption on sinking particles is more effectively removed from surface layers than is (137)Cs. An increase of the (239,240)Pu/(137)Cs activity ratio with depth suggests that (239,240)Pu, similarly as (137)Cs, should be also transported by advection to maintain the observed ratios in deep waters. An intrusion of LIW may enhance therefore both the (137)Cs and (239,240)Pu concentrations in deep waters. The average (238)Pu/(239+240)Pu activity ratio in seawater was 0.03+/-0.02, confirming a global fallout origin of Pu in the Algerian Basin. Caesium-137 and (239,240)Pu inventories in the water column were estimated to be from 2.7+/-0.5 kBq m(-2) to 3.8+/-0.7 kBq m(-2), and from 13.8+/-2.6 Bq m(-2) to 41+/-7B qm(-2), respectively. The (137)Cs massic activities in surface sediment were almost constant, the average activity was 9.0+/-0.8 Bq kg(-1). Sedimentation rates obtained using the (210)Pb method were from 0.1 to 0.7 cm y(-1), and resulting penetration depths of (137)Cs in the sediment cores were from 15 to over 40 cm. The (137)Cs peaks found in the sediment cores were associated with the Chernobyl accident (1986) and global fallout (1964). The (137)Cs inventories in the sediment were increasing from the west (180 Bq m(-2)) to the east (350 Bq m(-2)).     

Bioaccumulation of 137Cs in pelagic food webs in the Norwegian and Barents Seas

Knowledge and documentation of the levels of radioactive contamination in fish stocks important to Norwegian fisheries is of major importance to Norwegian consumers and fish export industry. In the present study, the bioaccumulation of caesium-137 ((137)Cs) has been investigated in marine food webs in the Barents and Norwegian Seas. The contents of (137)Cs in the different organisms were generally low (<1 Bq kg(-1) wet weight), but a marked bioaccumulation was apparent: The concentration of (137)Cs was about 10-fold higher in the harbour porpoise Phocoena phocoena, representing the upper level of the food web, than in the amphipod Themisto sp., representing the lower level of the food web. The Concentration Factors (CF=Bq kg(-1) wet weight/Bq l(-1) seawater) increased from 10+/-3 for a mixed sample of krill and amphipods to 165+/-5 for harbour porpoises.     

Variation of the activity concentrations and fluxes of natural (210Po, 210Pb) and anthropogenic (239,240Pu, 137Cs) radionuclides in the Strait of Gibraltar (Spain)

The activity concentrations and fluxes of natural (210Pb, 210Po) and anthropogenic (239,240Pu, 137Cs) radionuclides have been determined in the different water masses crossing the Strait of Gibraltar. New data have been gathered during four multidisciplinary and multinational sampling campaigns, performed between 1997 and 1999 within the framework of the CANIGO-FLUGIST Project. Mean activity concentrations of 210Po (1.53+/-0.34 Bq m(-3), n = 30) and 210Pb (1.16+/-0.50 Bq m(-3), n = 31) in the Atlantic water entering the Mediterranean basin are about double those measured in the Mediterranean outflow, namely 0.84+/-0.34 Bq m(-3) (n = 22) for 210Po and 0.66+/-0.34 Bq m(-3) (n = 22) for 210Pb. The opposite trend is observed for 231,240Pu, with average concentrations of 9.9+/-3.0 mBq m(-3) (n = 29) in the incoming Atlantic flow and 22.0+/-3.0 mBq m(-3) (n = 22) in the outpouring Mediterranean water. In the case of 137Cs, the same concentrations were quantified in the waters moving inwards (2.52+/-0.28 Bq m(-3), n = 27) and outwards (2.14+/-0.52 Bq m(-3), n = 21) from the Mediterranean Sea. On this basis, the Mediterranean basin experiences a net annual input flux of 14 TBq of 210Pb and 19 TBq of 210Po, and a net annual loss of 0.34 TBq of 239,240Pu, while--at present--137Cs input and output fluxes appear to be balanced.     

Levels of natural radionuclides in some Nigerian cereals and tubers

The activities of radionuclides in the two major groups of foodstuff widely consumed in Nigeria have been determined. The 40K concentration range from 9.9+/-3.6 Bq kg-1 to 298+/-14 Bq kg-1 (av. 130+/-8.12 Bq kg-1). (238)U concentration range from 1.47+/-0.91 Bq kg-1 to 39.5+/-9.9 Bq kg-1 (av. 11.5+/-3.86 Bq kg-1), and 232Th range from 3.50+/-1.85 Bq kg-1 to 10.5+/-2.13 Bq kg-1 (av. 6.78+/-2.13 Bq kg-1). 137Cs was not detected in any of the foodstuffs analysed.     

Surface ground contamination and soil vertical distribution of 137Cs around two underground nuclear explosion sites in the Asian Arctic, Russia

Vertical distributions of 137Cs have been determined in vegetation-soil cores obtained from 30 different locations around two underground nuclear explosion sites--"Crystal" (event year - 1974) and "Kraton-3" (event year - 1978) in the Republic of Sakha (Yakutia), Russia. In 2001-2002, background levels of 137Cs surface contamination densities on control forest plots varied from 0.73 to 0.97 kBq m(-2) with an average of 0.84+/-0.10 kBq m(-2) and a median of 0.82 kBq m(-2). 137Cs ground contamination densities at the "Crystal" site ranged from 1.3 to 64 kBq m(-2); the activity gradually decreased with distance from the borehole. For "Kraton-3", residual surface contamination density of radiocaesium varied drastically from 1.7 to 6900 kBq m(-2); maximal 137Cs depositions were found at a "decontaminated" plot. At all forest plots, radiocaesium activity decreased throughout the whole vertical soil profile. Vertical distributions of 137Cs in soil for the majority of the plots sampled (n=18) can be described using a simple exponential function. Despite the fact that more than 20 years have passed since the main fallout events, more than 80% of the total deposited activity was found in the first 5 cm of the vegetation-soil cores from most of the forested landscapes. The low annual temperatures, clay-rich soil type with neutral pH, and presence of thick lichen-moss carpet are the factors which may hinder 137Cs transport down the soil profile.     

Uptake and elimination of radiocaesium in fish and the "size effect."

A number of hypotheses have previously been developed concerning the rates of uptake and elimination of radiocaesium (137Cs) in fish. These include the influence of potassium and other water chemical parameters on both uptake and elimination, and the effect of fish size on accumulation. In order to test these hypotheses, we have assembled a data set comprising more than 1,000 measurements of radiocaesium (137Cs) in predatory fish (perch, pike and brown trout) in nine European lakes during the years after Chernobyl. These data have been analysed using simple models for uptake and excretion of 137Cs in fish, showing that: 1. Fish-water concentration factors (CF) were inversely proportional to potassium [K+] concentration of the different lakes, in agreement with previous studies. 2. The uptake rate of 137Cs in fish was negatively correlated with lake [K+], but excretion rate was independent of [K+]. 3. Lower than expected CF values were found in one lake, Iso Valkj?rvi, Finland. This is attributed to inhibition of the K+ (and therefore 137Cs) high affinity transport system in aquatic plants and fish by low pH and/or low Ca2+. 4. The inclusion of fish weight as a parameter in our dynamic model significantly improves the ability of the model to fit the observed measurements of 137Cs. 5. The model developed from the above hypotheses was able to fit the data from nine different lakes to within approximately a factor of 3 of the observed values.     

Chernobyl radioactivity persists in reindeer

Transfer of 137Cs in the soil-plant/lichen-reindeer food chain was studied in central (?stre Namdal) and southern Norway (V?g?) during 2000-2003. Reindeer from these areas have been continuously subjected to countermeasure application since the 1986 Chernobyl accident. In both areas no decline in 137Cs concentrations was detectable in reindeer slaughtered in autumn since 1995, or in reindeer slaughtered in winter since 1998-1999. Seasonal differences in 137Cs concentrations in reindeer have been less pronounced in recent years, with 137Cs concentrations occasionally higher in autumn than in winter. Soil-to-plant 137Cs transfer was significantly higher in ?stre Namdal than in V?g?. Climatic influences on lichen growth and abundance, and on soil properties that influence the availability of 137Cs for plant uptake, are hypothesized to have a larger impact on long-term transfer of radiocaesium in the soil-plant/lichen-reindeer food chain than has been previously observed.     

A comparison of three models of 137Cs transfer in forest ecosystems.

The predictions of three models of 137Cs transfer in forest ecosystems (FOA, LOGNAT and FORESTLAND) were compared. The scenario for the model-model comparison consisted of an acute dry deposition of 137Cs over a coniferous forest. The model predictions were subsequently compared (model-data comparison) with values derived from experimental data measured in forests of the Bryansk region in Russia that were contaminated by the Chernobyl accident and that have similar characteristics to the forests described in the scenario. The predictions of radiocaesium levels in the litter-soil layer, berries, needles, wood, whole tree and moose made with the models were in relatively good agreement with each other (within a factor of 1.4-2.9). The best agreement was observed for berries and moose and the worst for wood. There was also good agreement between the model predictions for the same variables and the experimental data (within a factor of 1.2 3.2). In this case, the best agreement was observed for the litter-soil layer and the worst for wood and the whole tree. Overall, at least for the studied scenario and for the first 10 years after deposition, any of the models can be used if the final aim is to estimate average concentrations in different forest components. The agreement between the model predictions worsens with time and there were differences in the form of the time dependencies predicted by the models, especially for wood. This may lead to larger differences between the model predictions and the experimental data for times beyond the period for which data were available for comparison (10 years after the deposition).