[en] A pre-concentration method for ¹³⁷Cs from large water samples was developed. The method allows an in-situ pre-concentration of 100-liter water samples in less than 1 h and is based on the percolation of the samples through a column containing Prussian blue impregnated ion-exchange resin. The method was applied in Goiania, Brazil, site of a radiological accident in 1987, in order to evaluate the contamination of ground and surface waters. The average chemical yield was (70±4)% and ¹³⁷Cs concentrations ranging from 1.7 to 105 mBq liter^-1 were found. (Author)