Short-term effects of olive oil mill wastewater application on soil water repellency

Land spreading of olive mill wastewater (OMW) allows a cheap and environmentally sound effluent disposal. However, this practice requires suitable application protocols, in order to avoid negative effects on soil hydrological properties, such as the increase of soil water repellency (SWR). The effects of OMW land spreading on SWR, mainly evaluated in the long term, have been rarely measured few days or weeks after land spreading. To this purpose, this study has evaluated the short-term effects of OMW land spreading on SWR of olive groves (with silt, loam or silty clay loam soil texture) using the Water Drop Penetration Test (WDPT) at laboratory scale. SWR significantly (p < 0.001) varied with the soil texture and depth (surface layer or 10-cm depth) as well as the treatment (land spreading of OMW or fresh water, FW) and time elapsed from land application. More specifically, SWR was significantly higher (p < 0.001) in the topsoil than the sub-surface layer. Compared to the soils irrigated with FW, a two-fold WDPT (thus a slightly higher SWR) was found after OMW application in both soil layers, regardless of the texture. However, this weak SWR disappears just after 2 weeks from land spreading, and the hydrophobicity of the soils treated with OMW and FW becomes very similar. Moreover, the topsoil and sub-surface layer showed the same SWR after 4 weeks. The high coefficients of determination (r2 > 0.86) in the linear regressions between WDPT and OM content of soils proved the clear influence of the organic compounds on SWR, which decreases with OM, as expected. Overall, OMW land spreading has not significantly changed SWR, at least under the limited hydraulic and organic loads adopted in this study, and less noticeably on loam or silty clay loam soil compared to sandy loam texture. However, this practice is more viable in spring or autumn, since in these seasons the risk of groundwater contamination is particularly reduced.