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. © 2012 Elsevier B.V.
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