The alluvial deposits of the Agrio River in SW Spain have been studied using terrace mapping, boreholes, trenching and vertical electrical sounding to select an adequate place for a Permeable Reactive Barrier. Geological and hydrogeological data available prior to the barrier construction suggested a simple geological model based on three terraces, the most modern being deeper than the oldest. The barrier was accordingly trenched through the youngest terrace. However, excavation of the barrier andsubsequent subsoil data demonstrated that the internal structure of the Agrio alluvial deposits does not follow such a simple model. A revised model, less favourable to the existing barrier design, revealed the oldest terrace to be deeper than the youngest, and to form a palaeochannel oblique to the surface terraces and river trends. The study methods used are standard, but proved to have insufficient resolution to detect key features of the alluvial geology. It is concluded that such characterization methods, though widely used, are not appropriate where alluvial terraces may display complex internal structures not reflected in the modern surface geomorphology. In hindsight, the study area should have been larger, so as to encompass at least the width of the alluvial plain and to extend for a hundred metres or so up and downstream from the proposed barrier location. Lithological logs of the boreholes should have been carefully described from both cuttings and downhole geophysical logs, which would have allowed more accurate delineation ofthe stratigraphy of the alluvial deposit. Subsequentgeophysical methods should have been calibrated to this stratigraphy to characterize the internal structure and basal contact of the alluvial deposit.