This study presents the results of systematic experiments on tritium (3H) concentrations in ground level air against those in rainwater near a pressurized heavy water reactor in a tropical region. The samples were collected over the rainy season of year 2011 from eight locations in the environment around Narora Atomic Power Station. The specific activity ratio of 3H between rainwater and air moisture at ground level was calculated for each data set. The average specific activity ratio was found to be ranged from 0.12 to 1.1. A correlation ( to 0.76, ) was observed between the total rain hours in a day and the rainwater 3H activity. Higher rain duration with slower rain rate yielded higher 3H concentrations as more time was available for the scavenging/wash-out process to take effect together with lower dilution. Annual tritium (HTO) wet deposition has been measured and calculated for the year 2011 within 0.8？km distance from 145？m high stack of Narora Atomic Power Station (NAPS) at nine locations in different directions. The range of deposition velocity, (m·s？1), at nine locations for the years 2011 is found to be from 4.43E？？？04 to 6.42E？？？03. The average value for wet deposition velocity for NAPS site is estimated as 3.17E？？？03？m·s？1. 1. Introduction Tritium (3H) is one of the major long-lived radioisotopes in the gaseous effluent released from any Pressurized Heavy Water Reactor (PHWR). Tritium produced in a PHWR is released into the atmosphere in the form of tritiated water vapour (HTO) [1]. Because of its physicochemical similarity with water, HTO is incorporated in all environmental matrices such as soil, air, and biota. In order to carry out an impact assessment due to 3H, it is necessary to understand the kinetics of transfer of 3H through the various environmental matrices. The process of 3H removal from the atmosphere is by either wet or dry deposition [2]. When raindrops pass vertically through an HTO plume, rainwater scavenges 3H from the air due to a wash-out process. Scavenging by rain is one of the phenomena which transfers substances present in the atmosphere to the ground. It consists of two steps, known as in-cloud scavenging (rain out) and below cloud scavenging (wash out) [3]. Scavenging ratios represent the vertical scavenging efficiencies of transfer from the atmosphere to rainwater. Velarde and Perlado [4] reported that wet deposition is critical for the incorporation of tritiated water vapour into the natural biological chain. It has been reported that the wash-out coefficient of ？3H depends upon the distance from the source,
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