Impact of olive mill wastewater (OMW) on the soil hydraulic and solute transport properties

The Mediterranean area concentrates the world’s largest production area of olive oil. The olive oil industry represents, in this basin, one of the leading sectors of the agri-food economy. Olive mill water (OMW) is the principal waste effluent produced by the olive oil industry. Due to its high pollution load, this aqueous by-product cannot be directly disposed of in domestic wastewater treatment plants (especially those with a biological treatment unit). Untreated OMW is currently used for agronomic purposes in several countries, mainly because it is rich in valuable plant nutrients. However, OMW is characterized by toxic phenols, high organic matter, high salinity, suspended solids and several other components that may have possible negative effects on chemical and physical soil properties, as well as soil biological activities. In the present research, we focused on the effects of OMW application on transport and hydraulic soil properties. Three distinct soils from a pedological point of view were selected and a series of laboratory steady-state miscible flow tests were conducted under saturated conditions, on both OMW-treated and -untreated soil columns. Tests were conducted on disturbed and undisturbed soil columns. The approach proposed by Kachanoski, based on soil impedance (Z) measurements via the time domain reflectometry (TDR) technique, was used to monitor the leaching experiments. The breakthrough curves (BTCs) exhibited different shapes that allowed the repercussions of OMW applications on soil transport behaviour to be distinguished. Several additional tests conducted on OMW-treated and -untreated soil cores to determine water retention curves (SWRCs) and saturated hydraulic conductivity Ks allowed us to infer the probable mechanisms involved in soil hydrological behaviour changes under OMW treatments. The results show that when OMW leaches into the soil immediately after its disposal there is little effect on the evaluated hydraulic and hydrodispersive properties. By contrast, we demonstrated that a short incubation period (i.e. a short contact time between OMW and soil) of 10 days is enough to exert a great influence on all the values determined (e.g. soil pore velocity v and Ks reduced by up to one order of magnitude). These effects were especially evident in undisturbed soil samples. Graphic Abstract: [Figure not available: see fulltext.]