which occurred since the beginning of the experiment. Over time, the organic matter content of OMW, which is respon- sible for the sealing of soil particles (Mahmoud et al., 2010), enhanced the effect of the preferential flow pathways in lysime- ters amended with olive mill wastes with respect to the control ones. The only exception was treatment 2 in the soil S2, which was comparable to the control. The average volume of leachate was always greater for S2 (140 L) than S1 (40 L), due to the different granulometric composition causing different hydraulic properties. For this reason, the volume of leachate collected from S1 was very little or even null during the first months of the experimentation (Fig. 3, upper left).