Long-term impact of irrigation with olive mill wastewater on aggregate properties in the top soil

Olive mill wastewater (OMW) is the main waste product generated by the olive oil extraction process. The uncontrolled disposal of OMW is becoming a serious environmental problem. The objective of this study was to investigate the long-term effects of OMW irrigation on soil aggregate stability, on solute diffusion into aggregates and on aggregate structure formation. The soil aggregates were sampled from three sites: non-irrigated with OMW (T0) and regularly irrigated with untreated OMW for 5 (T5) and 15 (T15) years. The results showed that the regular application of OMW for 5 and 15 years increased the soil aggregate stability, as a result of a rising organic matter content of OMW sites. OMW application furthermore reduced the effective diffusion coefficient into aggregates, because the organic matter of OMW forms a coating on the aggregates and blocks the pore mouths. OMW is characterized by its adhesive behaviour that affects the aggregation and structure of the topsoil by binding micro-aggregates together to form macro-aggregates and larger pore spaces between micro-aggregates. Consequently, the use of OMW for irrigation over long time periods alters the surface layer of the soil and makes it fragmented, which may increase the risk for preferential solute transport.