The extent to which nanoparticles may impact the environment is largely dependent on their mobility in soils and in surface and groundwater systems. Size and aggregation/dispersion state are obvious parameters, whose determination (and prediction) requires a detailed knowledge of the surface chemistry of the nanophases and their reactivity with a variety of organic and inorganic nutrients and pollutants, taking into account possible competition between ligands. Hydrophobicity is another important parameter to take into consideration. The hydrophobicity can be either intrinsic (e.g. fullerene) or acquired (intentional coating during manufacturing or reaction with natural organics), and this hydrophobic quality may evolve during transport in the environment. Here we examine the mechanisms by which even minor changes in the physico-chemical conditions may results in drastic modifications of the ability of nanoparticles to be transported in environmental systems. The present work will focus on commercially available products, viz. C60, TiO2 and CeO2 based nanomaterials, as well as naturally occurring nanostructured alumino-silicates. The transformations at the surface of this nanomaterials and the structure and mobility of the resulting phases have been examined by several analytical tools (TEM, X-ray absorption and scattering, NMR...) so as to obtain an observation scale spanning over at least three orders of magnitude. The results underline the difficulty to obtain a global view of the phenomena due to competing mechanisms.