ABSTRACT: The Double Paradox is a phenomenon describing unchanged or increasing concentrations of minerals in surface waters following rainfall. One proposed solution to this phenomenon is piston flow. Flow and trace experiments were performed in a laboratory sand-box-model which is also known as the intermediate scale model. The flow velocity in the saturated zone rose to 17 % and the capillary fringe upper edge decreased by 1 – 2 cm immediately after infiltration of simulated rainfall. Simultaneously, water pressure rose in the central part of the model. According to laboratory data, piston flow could explain the First Paradox quite satisfactorily. The mechanism of the Second Paradox is more complicated, but piston flow could also be involved here. Flow and geo-chemical long-term field data from the Košetice field observatory were studied and investigated for signs of the Double Paradox in the conditions of a small watershed in the Czech Republic. The double paradox appears to flourish in these conditions.