Seasonal changes of naturally occurring radionuclides and fallout 137-Cs in the samples of leaves of deciduous trees

Abstract

Specifična aktivnost prirodnih radionuklida 238U, 235U, 226Ra, 210Pb, 232Th i 40K i proizvedenog 137Cs je merena u uzorcima lišća dve vrste listopadnog drveća, kestena (Aesculus hippocastanum) i lipe (Tilia spp) koji potiču iz tri gradska parka na užem području Beograda. Uzorci su sakupljani u vreme prolećne (n=7), letnje (n=2) i jesenje (n=8) sezone u periodu od 2002-2012. godine. Iz svake pojedinačne sezone uzorci lišća su spojeni po vrstama pri čemu je dobijen jedan integralni (sezonski) uzorak za svaki ispitivani park čime je postignuta veća efikasnost detekcije niskog nivoa gama zračenja. Za određivanje aktivnosti primenjen je metod spektrometrije gama emitera. U svim pojedinačnim uzorcima suve materije lišća detekcija radionuklida 210Pb i 40K je bila moguća i njihova specifična aktivnost se nalazila intervalu 15-52 Bq/kg i 310-650 Bq/kg, respektivno. Iako je aktivnost 226Ra bila veoma niska, u intervalu 0,5-5,6 Bq/kg, ovaj radionuklid je bio izmeren u svim uzorcima (osim u dva) i svim sezonama. Aktivnost 210Pb i 226Ra se povećavala tokom godine, pri čemu su njihove najveće vrednosti izmerene u lišću kestena u letnjoj sezoni. Suprotno, aktivnost 40K u lišću lipe i kestena je opadala tokom godine. Za razliku od prethodno navedenih, radionuklidi 238U, 235U i 232Th su retko detektovani samo u letnjem i/ili jesenjem periodu i po četiri uzorka lišća su sadržali 238U (2,7-11,7 Bq/kg) i 235U (0,13-1,0 Bq/kg), a tri 232Th (1,8-3,3 Bq/kg). Proizvedeni radionuklid 137Cs detektovan je u svim uzorcima lišća (osim u dva), a bez jasnog trenda ponašanja po sezonama i nalazio se u intervalu 0,3-1,2 Bq/kg. Cilj rada bio je ispitivanje akumulacije prirodnih radionuklida i njihovog potencijalnog variranja tokom godine u lišću listopadnog drveća.The specific activity of naturally occurring radionuclides 238U, 235U, 226Ra, 210Pb, 232Th, and 40K and fallout 137Cs was measured in the samples of leaves of two deciduous tree species, chestnut (Aesculus hippocastanum) and linden (Tilia spp.) common for the Belgrade urban area. Samples were collected in the spring (n = 7), summer (n = 2) and autumn (n = 8) during the vegetation period from 2002 to 2012. From each season, samples of leaves were combined according to two species and one single integral (seasonal) sample was obtained in order to improve detection efficiency of low gamma radiation level. Specific activities were determined using the gamma-ray spectrometry method. 210Pb and 40K radionuclides were detected in all the samples of leaves and their specific activity was in the range (Bq/kg dry weight): 15 - 52 and 310 - 650, respectively. Although the 226Ra level was very low and in the range from 0.5 - 5.6 Bq/kg, this radionuclide has been measured in all samples (except in the two) and each season. 210Pb and 226Ra activity increased during the vegetation period and their highest values were found in the leaves of chestnut trees in the summer season. In contrast, the activity of 40K in the leaves of linden and chestnut has decreased. Detection 238U, 235U and 232Th radionuclides was scarce and only in the summer and/or autumn. Four samples of leaves contained 238U (2.7 - 11.7 Bq/kg dry wt) and 235U (0,13 - 1.0 Bq/kg dry wt) and three of them 232Th (1.8 - 3.3 Bq/kg dry wt). Man-made radionuclide 137Cs (0.3-1.2 Bq/kg dry wt) was detected in all samples of leaves (except in two) and no clear trend according to seasons was found. The aim was to investigate the accumulation of naturally occurring radionuclides and fallout 137Cs in the leaves of deciduous trees and their potential variations during the vegetation period