Journal of Research in Environmental Science and Toxicology Vol. 9(3) pp. 51-61, October, 2020 DOI: http:/dx.doi.org/10.14303/jrest.2020.10 Available online http://www.interesjournals.org/JREST Copyright ©2020 International Research Journals Research Article Estimating Soil Erosion Using Fallout Caesium-137 and Lead- 210 in Cultivated Land of Micro Catchment in Dolosbage, Sri Lanka W A T L Weerakkody 1 , C K Dissanayake 1 , A G Chandrapala 2 , M D Kalpage 1 , T N Attanayake 1 , K M A Kendaragama 2 and Hettiarachchi A K 2 1 Sri Lanka Atomic Energy Board, Srilanka 2 Natural Resources Management Centre, Peradeniya, Srilanka Introducton Sri Lanka is a tropical island located near to the southeast tp of India. The country lies in the between longitudes 79° 39' - 81° 53' East and lattudes 5° 54' - 9° 52' North. It consists of 64,453.6 km 2 of land area and 1,156.2 km 2 of inland waters. Extensive faultng and erosion over tme have produced a wide range of topographic features, making Sri Lanka one of the most scenic places in the world (Annual Performance Report of Meteorology Meteorology, 2018). Climate features of the country are basically determined by the geographical locaton of the country in the equatorial belt and its positon in the inter tropical convergence zone. The chief determinants of the climate in Sri Lanka are rainfall and temperature. The mean temperature is 27.5°C over low lands. The oceanic infuence helps to reduce temperature in the lowlands by sea breezes. The temperature decreases at a steady rate of about 6.5°C for each 1000 meters rise and in the Montana (Annual Performance Report of Meteorology, 2018). The Central Highlands of Sri Lanka plays an important role in the economy of the country, producing a considerable amount of world class Tea, Rubber, Spices, vegetables and contributng around 7% of the natonal GDP. Soil erosion in agricultural and non-agricultural lands in central highlands caused serious onsite and ofsite impacts on sustainable land productvity, environmental stability and natonal economy of Sri Lanka. There are about 103 river basins covering 90% of the island, originated from Central Highlands. All the fve major hydropower reservoirs in Sri Lanka, which contribute to the 31.9% of power generaton in Sri Lanka (Sri Lanka Energy Balance, 2018) are also located in Central Highlands. However, the siltaton of hydropower reservoirs resultng 10% deducton of the capacity amountng 15 Million Rupees worth electricity generaton loss per year. To mitgate the adverse impacts of land degradaton especially soil erosion and decline of soil fertlity, Government of Sri Lanka enacted soil conservaton act in Abstract Dolosbage, Sri Lanka lies between Latitude - 7.08°, Longitude - 80.47° (790 m-890 m above sea level). It belongs to up country wet zone and the average annual rainfall is about 4585.0mm. There were four main land uses such as marginal tea, well grown tea, home gardens and grasslands in this catchment. Uncultivated fat grassland of the hill top which had not been subjected to erosion and deposition was selected as a reference site for the comparison of fallout radionuclide inventory. Activity concentrations of 137 Cs & 210 Pb were measured by HPGe detector with the relative efciency of 32.6%. Soil redistribution rates induced by water erosion were calculated by Mass Balance Model II for 137 Cs measurements and Mass Balance Model I for 210 Pbex (WALLING, et al., 2001). 137 Cs inventories, 210 Pbex inventories and soil redistribution rates were obtained for 24 bulk cores collected from the cultivated feld. For the reference site, the vertical distributions associated with both radionuclides were similar with exponential decrease with depth. According to the 137 Cs inventories, highest soil erosion was recorded in marginal tea lands (37.36 t ha -1 yr -1 ) followed by grassland (6.4 t ha -1 yr -1 ) and home gardens (5.31 t ha -1 yr -1 ). Well grown tea recorded the lowest soil erosion rate. According to the 210 Pbex inventories all the land uses studied in the catchment recorded the net soil deposition. Highest deposition rate was observed in grassland (16. 90 t ha -1 yr -1 ) followed by well grown tea (3.61 t ha -1 yr -1 ). Marginal tea recorded the lowest deposition rate (2.17 t ha -1 yr -1 ).