The measurements of gross alpha and gross beta radioactivity in groundwater samples from Guarani aquifer in Brazil are reported in this paper together with the activity concentration of the natural dissolved radionuclides K-40, U-238, U-234, Ra-226, Rn-222, Po-210, Pb-210, Th-232, Th-228 and Ra-228. Most of the gross alpha radioactivity values were below the critical level of detection corresponding to 1 mBq/L, however, the whole data set for the gross beta radioactivity and radionuclides K-40, U-238, U-234, Ra-226, Rn-222, Po-210, Pb-210, and Ra-228 was submitted to a statistical treatment, considering class intervals arranged in geometric progression, because of the great variability of the activity. The analysis indicated lognormal distribution of the data, as usually observed in samples taken from the natural context. An inverse relationship between the gross alpha and gross beta activity has been identified and is related to an increase in the K content in the water. The mobility coefficient has been estimated for U-238, Ra-226, Th-232 and Ra-228 in Guarani aquifer and the results indicated that the radioelement solubility in the studied system varies according to the following order: radium > uranium > thorium. The implications of the data obtained in terms of standards established for defining the drinking water quality have also been discussed. (c) 2008 Elsevier Ltd. All rights reserved.Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)