Long - term atmospheric depositional fluxes of radionuclides at Málaga (Coastal Mediterranean Station)

LONG-TERM ATMOSPHERIC DEPOSITIONAL FLUXES OF RADIONUCLIDES AT MÁLAGA (COASTAL MEDITERRANEAN STATION) 1C. DUEÑAS, 1M.C. FERNÁNDEZ,1E. GORDO, 2E. LIGER, 1S. CAÑETE , 3M. PÉREZ and 1M.CABELLO 1Department of Applied Physics I, Faculty of Sciences. 2Department of Applied Physics II, Faculty of Computer Engineering. 3Deparment of Radiology and Health Physics, Ophthalmology and OIT, Faculty of Medicine. University of Málaga (SPAIN) INTRODUCTION.7Be and 210Pb are radionuclides which have been measured routinely in many places in order to study the description of environmental processes such as aerosol transit and residence times in the troposphere, aerosol deposition velocities and aerosol trapping by ground vegetation. 7Be is one of the radionuclide produced by spallation reactions of cosmic rays with light atmospheric nuclei and rapidly associates primarily with submicron-sized aerosol particles. 210Pb is a natural radionuclide of the 238U series which is widely used as a tracer and its depositional pattern and give information on continental aerosols in lower troposphere. Therefore, these two radionuclides with their different sources are useful to understand the mechanisms of aerosol removal from the atmosphere. 40Kis also a natural radionuclide and has been associated with the arrival of coarse re-suspended material from the Africa continent. The bulk atmospheric deposition of 7Be ,210Pb and 40K has been measured at Málaga, southeastern Spain from January 2005 to December 2014,on monthly basis. MATERIAL AND METHODS The sampling site is one of the environmental radioactivity monitoring network stations operate by the Spanish Nuclear Security Council (CSN). The sampling point is located above the ground, on the roof of the Faculty of Sciences (24 m.a.s.l.), University of Málaga (4°28′80“W; 36°43´40´´ N). Monthly precipitation and dry fallout samples were routinely collectedon a monthly basis using a steel tray 1m2in area as a collecting system and polyethylene vessels of 25L capacity for rainwater samples reservoirs. Measurements by gamma spectrometry were performed to determine the 7Be , 210Pb and 40K activities of the samples using an intrinsic REGe detector. RESULTS AND DISCUSSION The results from specific activities (in Bq/L) of 7Be ,210Pb and 40K were analyzed to derive the statistical estimates characterizing the distributions. Table 1 provides: Number of cases (N), arithmetic mean (AM), geometric mean (GM), standard deviation (SD), maximum and minimum values, the coefficient of variation (CV) and Skewness(GI). Table 1. Summary statistics of the activity concentrations of the radionuclides Taking into account the local orography and meteorological features in Málaga, with strong rain events followed by long dry periods, different parameters have been considered to study their influence on the concentrations of these radionuclides: number of dust intrusions (days) from Africa, rainfall amount (mm), rainfall duration, atmospheric presion number of dry days and number of wet days. Best significant correlations found are shown in Fig. 2. The 7Be, 210Pb and 40K depositions are significantly correlated with the rainfall amount, rainfall duration, number of dry days and humidity air. Such relations have been commonly observed and explained by the fact that rainfall constitutes the major depositional pathway of these radionuclides. N AM GM SD MAX MIN CV (%) GI 7Be 120 2.5 2.1 1.6 8.30 0.29 64 4.9 210Pb 120 0.39 0.32 0.23 1.32 0.05 60 6,4 40K62 0.20 0.17 0.15 0.84 0.02 72 6,5 Fig. 1.-Seasonal variation of specific activities of 7Be , 210Pb and 40K Fig. 2.- Scatter plots of radionuclides concentrations and some factors . The next three figures show the monthly variations in the deposition fluxes (in Bq/m2)of 7Be, 210Pb and 40K (Fig. 3). The lowest values were observed from June to September and the highest values from October to April. Correlation study was also carried out (Table 3). 7Be 210Pb 40K Rainfall amount 0.86 0.87 0.66 Rainfall duration 0.85 0.89 0.60 Number of dry days -0.68 -0.69 -0.52 Number of intrusions -0.15 -0,13 -0,03 Relative humidity air 0.60 0.65 0.43 Temperature air -0.41 -0.45 -0.21 Table 3.- Linear correlation coefficient between the depositional fluxes and different precipitation parameters. Fig.3. Depositional fluxes of the three radionuclides during the ten years under study. ACKNOWLEDGMENT The authors wish to thank to Spanish Nuclear Safety Council (CSN) for financial support and to the Spanish Meteorological Agency (AEMET), for the provision of data used in this study. The specific activity of 7Be exhibit their maximum values in spring (Fig. 1). The higher values of 7Be can be due to the rapid exchange of aerosols between stratosphere and troposphere by contraction of tropopause in the spring season. The specific activities of 210Pb and 40K show minimum values in winter and autumn and maximum values in summer and spring.