4. Conclusions
The application of four different olive mill wastes to two types of soil produced leachates having minimum potential impact on groundwater quality. In fact, when OMW was directly applied, the acidity, electrical conductivity and polyphenols concentration of the leachate were always negligible with respect to the corresponding values of the incoming water. On the contrary, the amount of nitrate potentially reaching groundwater was dependent on the specific soil characteristics, so that the higher the hydraulic conductivity of the investigated soil, the lower the outcoming nitrate concentration. Moreover, pre-treatment of OMW by catalytical digestion was found not useful to obtain the lowest values of the above parameters. With regards to the olive pomace compost, its application resulted completely safe in terms of potential leaching of contaminants.