M. Rizzotto

Short-term seasonal variability in 7Be wet deposition in a semiarid ecosystem of central Argentina.

The (7)Be wet deposition has been intensively investigated in a semiarid region at San Luis Province, Argentina. From November 2006 to May 2008, the (7)Be content in rainwater was determined in 58 individual rain events, randomly comprising more than 50% of all individual precipitations at the sampling period. (7)Be activity concentration in rainwater ranged from 0.7+/-0.3 Bq l(-1) to 3.2+/-0.7 Bq l(-1), with a mean value of 1.7 Bq l(-1) (sd=0.53 Bq l(-1)). No relationship was found between (7)Be content in rainwater and (a) rainfall amount, (b) precipitation intensity and (c) elapsed time between events. (7)Be ground deposition was found to be well correlated with rainfall amount (R=0.92). For the precipitation events considered, the (7)Be depositional fluxes ranged from 1.1 to 120 Bq m(-2), with a mean value of 32.7 Bq m(-2) (sd=29.9 Bq m(-2)). The annual depositional flux was estimated at 1140+/-120 Bq m(-2)y(-1). Assuming the same monthly deposition pattern and that the (7)Be content in soil decreases only through radioactive decay, the seasonal variation of (7)Be areal activity density in soil was estimated. Results of this investigation may contribute to a valuable characterization of (7)Be input in the explored semiarid ecosystem and its potential use as tracer of environmental processes.

Occupational exposure to radon and natural gamma radiation in the La Carolina, a former gold mine in San Luis Province, Argentina

Radon and gamma radiation level measurements were carried out inside the La Carolina mine, one of the oldest gold mining camps of southern South America, which is open for touristic visits nowadays. CR-39 track-etch detectors and thermoluminescent dosimeters of natural CaF(2) and LiF TLD-100 were exposed at 14 points along the mine tunnels in order to estimate the mean (222)Rn concentration and the ambient dose equivalent during the summer season (November 2008 to February 2009). The values for the (222)Rn concentration at each monitoring site ranged from 1.8+/-0.1 kBqm(-3) to 6.0+/-0.5 kBqm(-3), with a mean value of 4.8 kBqm(-3), indicating that these measurements exceed in about three times the upper action level recommended by ICRP for workplaces. The correlations between radon and gamma radiation levels inside the mine were also investigated. Effective doses due to (222)Rn and gamma rays inside the mine were determined, resulting in negligible values to tourists. Considering the effective dose to the mine tourist guides, values exceeding 20mSv of internal contribution to the effective doses can be reached, depending on the number of working hours inside the mine.

Variability of 137Cs and 40K soil-to-fruit transfer factor in tropical lemon trees during the fruit development period.

In this investigation we evaluate the soil uptake of (137)Cs and (40)K by tropical plants and their consequent translocation to fruits, by calculating the soil-to-fruit transfer factors defined as F(v) = [concentration of radionuclide in fruit (Bq kg(-1) dry mass)/concentration of radionuclide in soil (Bq kg(-1) dry mass in upper 20 cm)]. In order to obtain F(v) values, the accumulation of these radionuclides in fruits of lemon trees (Citrus limon B.) during the fruit growth was measured. A mathematical model was calibrated from the experimental data allowing simulating the incorporation process of these radionuclides by fruits. Although the fruit incorporates a lot more potassium than cesium, both radionuclides present similar absorption patterns during the entire growth period. F(v) ranged from 0.54 to 1.02 for (40)K and from 0.02 to 0.06 for (137)Cs. Maximum F(v) values are reached at the initial time of fruit growth and decrease as the fruit develops, being lowest at the maturation period. As a result of applying the model a decreasing exponential function is derived for F(v) as time increases. The agreement between the theoretical approach and the experimental values is satisfactory.

Assessment of 7Be content in precipitation in a South American semi-arid environment

There are two naturally occurring radiogenic isotopes of beryllium, ⁷Be and ¹⁰Be. These are produced when cosmic radiation interacts with oxygen and nitrogen in the atmosphere. After production, these radionuclides are input to ecosystems through wet and dry deposition. In recent years ⁷Be and ¹⁰Be have proved to be powerful tools for studying dynamic processes that occur on the surface of the earth. We measured the ⁷Be content in precipitation at a semiarid location in central Argentina. From November 2006 to March 2009, 68 precipitation events were collected. Measured ⁷Be content ranged from 0.7±0.4 Bq L⁻¹ to 3.2±0.7 Bq L⁻¹, with a mean of 1.7 Bq L⁻¹ ±0.6 Bq L⁻¹. Beryllium-7 content of rainfall did not show clear relationships with amount of rainfall (mm), mean intensity (mm h⁻¹) or duration (h⁻¹), or elapsed time between events (day). The general results indicate that for the typical range of precipitation there was no atmospheric washout and that the reload of the atmosphere is not a relevant factor, but when the amount of precipitation is very high washout may occur. On the other hand, when the ⁷Be content was measured during single rain events, a high content of this radionuclide was found to be associated with very low rainfall intensity (≈3 mm h⁻¹), this suggests that rain intensity could affect the ⁷Be content. Using all data, a good linear relationship between ⁷Be deposition and rain magnitude was obtained (r²=0.82, p<0.0001). Because of this, the slope of this linear regression equation may be applied as a tool for tracing environmental processes that affect the surface of the earth. We can do this by directly estimating erosion/sedimentation processes using ⁷Be or by estimating the input of ¹⁰Be in the environment with the aim to evaluate land degradation phenomena.

Using 222Rn as a tracer of geodynamical processes in underground environments

Radon levels in two old mines in San Luis, Argentina, were measured and analyzed, with the aim to assess the potential use of this radioactive noble gas as a tracer of geological processes in underground environments. La Carolina gold mine and Los Cóndores tungsten mine are today used as tourism mines. CR-39 nuclear track detectors were used for this purpose. Measurements were performed during both winter and summer seasons. The findings show that in these environments, significant radon concentrations are subject to large seasonal fluctuations, due to the strong dependence on natural ventilation with the outside temperature variations. For both mines, higher concentration values of (222)Rn were observed in summer than in winter; with an extreme ratio of 2.5 times between summer and winter seasons for Los Cóndores mine. The pattern of radon transport inside La Carolina mine revealed, contrary to what was believed, that this mine behaves as a system with two entrances located at different levels. However, this feature can only be observed in the winter season, when there is a marked difference between the inside and outside temperatures of the mine. In the case of Los Cóndores mine, the radon concentration pattern distribution is principally established by air current due to chimney-effect in summer and winter seasons. In both cases, the analyses of radon pattern distribution appear as a good method to trace air currents, and then localize unknown ducts, fissures or secondary tunnels in subterranean environments.

Temporal evolution of 137Cs+, K+ and Na+ in fruits of South American tropical species

Concentrations of (137)Cs, K and Na in fruits of lemon (Citrus limon B.) and of K and Na in fruits of coconut (Cocos nucifera L.) trees were measured by both gamma spectrometry and neutron activation analysis, with the aim to understand the behaviour of monovalent inorganic cations in tropical plants as well as the plant ability to store these elements. Similar amounts of K(+) were incorporated by lemon and coconut trees during the growth and ripening processes of its fruits. The K concentration decreased exponentially during the growth of lemons and coconuts, ranging from 13 to 25 g kg(-1) dry weight. The incorporation of Na(+) differed considerably between the plant species studied. The Na concentration increased linearly during the lemon growth period (0.04 to 0.70 g kg(-1) d.w.) and decreased exponentially during the coconut growth period (1.4 to 0.5 g kg(-1) d.w.). Even though radiocaesium is not an essential element to plants, our results have shown that (137)Cs incorporation to vegetable tissues is positively correlated to K distribution within the studied tropical plant species, suggesting that the two elements might be assimilated in a similar way, going through the biological cycle together. A mathematical model was developed from the experimental data allowing simulating the incorporation process of monovalent inorganic cations by the fruits of such tropical species. The agreement between the theoretical approach and the experimental values is satisfactory along fruit development.

Using 222Rn as a tracer of geophysical processes in underground environments

Radon levels in two old mines in San Luis, Argentina, are reported and analyzed. These mines are today used for touristic visitation. Our goal was to assess the potential use of such radioactive noble gas as tracer of geological processes in underground environments. CR-39 nuclear track detectors were used during the winter and summer seasons. The findings show that the significant radon concentrations reported in this environment are subject to large seasonal modulations, due to the strong dependence of natural ventilation on the variations of outside temperature. The results also indicate that radon pattern distribution appear as a good method to localize unknown ducts, fissures or secondary tunnels in subterranean environments.

Atmospheric deposition of 7Be by rain events, incentral Argentina

Beryllium‐7 is a natural radionuclide that enters into the ecosystems through wet and dry depositions and has numerous environmental applications in terrestrial and aquatic ecosystems. Atmospheric wet deposition of 7Be was measured in central Argentina. Rain traps were installed (1 m above ground) and individual rain events have been collected. Rain samples were filtered and analyzed by gamma spectrometry. The gamma counting was undertaken using a 40%‐efficient p‐type coaxial intrinsic high‐purity natural germanium crystal built by Princeton Gamma‐Tech. The cryostat was made from electroformed high‐purity copper using ultralow‐background technology. The detector was surrounded by 50 cm of lead bricks to provide shielding against radioactive background. The detector gamma efficiency was determined using a water solution with known amounts of chemical compounds containing long‐lived naturally occurring radioisotopes, 176Lu, 138La and 40K. Due to the geometry of the sample and its position close to the detec...

40K, 115Cs and 226Ra Soil and Plant Content in Seminatural Grasslands of Central Argentina

Activity concentrations of 40K, 226Ra and 137Cs have been analyzed in soil and plant samples, collected in permanent grassland in central Argentina. Two near areas (A1 and A2) under field conditions with soil undisturbed at least in the last four decades were selected. For each of the three studied radionuclides we do not find differences in the inventories between both areas. The inventories range from 143 kBq m−2 to 197 kBq m−2 for 40K and from 13 kBq m−2 to 18 kBq m−2 for 226Ra. The vertical distributions of 40K and 226Ra are uniform through de soil profile. For 137Cs the inventories range from 0.33 kBq m−2 to 0.73 kBq m−2. In spite of 137Cs inventories are similar in both areas the distribution through vertical profile is different. 137Cs activity concentration has a maximum for layers 5–10 cm depth in A1 and 10–15 cm depth in A2. For deeper layers both areas show similar activity concentrations. The diffusion coefficient (Ds) and convection velocity (vs) are estimated with a convection‐diffusion mode...