Effects of olive mill wastewater on soil leachates composition under Tunisian climatic conditions: a lysimeter pilot study

Tunisia, as a major olive oil producer, generates substantial quantities of olive mill wastewater (OMW), frequently applied to agricultural soils due to its fertilizing potential. However, OMW contains high levels of (poly)phenolic compounds, which can persist in soils and thereby affect basic soil properties, soil wettability, and pose risks of groundwater contamination. The semi-arid to arid climate of Tunisia, characterized by pronounced seasonal variations, may strongly influence the degradation, leaching, and environmental fate of OMW-derived compounds. This study aimed to investigate the dynamics of OMW application in soil columns under controlled conditions simulating Tunisian seasonal climates. Soil lysimeters were used to monitor soil leachate quality over 18 weeks, encompassing two winter periods, a spring, and a summer season. Parameters analyzed in leachates included soluble phenolic compounds (SPC), pH, electrical conductivity (EC), water drop penetration time (WDPT), and dissolved organic carbon (DOC) quality via SUVA ₂₅₄ . Results showed that wet winter conditions promoted OMW percolation, leading to elevated SPC concentrations in leachates, while moderate spring conditions favored degradation processes, reducing SPC and soil water repellency. Hot and dry summer conditions induced polymerization and (re)accumulation of OMW-derived compounds at the soil surface, whereas the second winter period exhibited lower SPC levels than the first. The findings highlight the significant role of seasonal climatic conditions on OMW behavior in soils, underlining the need for season-specific management strategies to minimize environmental risks associated with its utilization as soil amendment.