The impact of olive mill wastewater spreading on soils using integrated approach of proximal soil survey, spatial, and multivariate analyses

Olive oil production generates a large amount of olive mill wastewater (OMW), the most difficult to treat of agro-industrial effluents. Spreading of OMW across the soilscape has become the most frequently used practice in several Mediterranean basin countries but is hotly debated because of its potential to impair soil environs. The research hypothesis states that soil deterioration is correlated strongly with the spatiotemporal rate of OMW application; thus, the spatial pattern of the soil attributes should be established in conjunction with the rate of OMW spreading. The spatial pattern was ascertained using proximal soil sensing that measures the apparent electrical conductance (ECₐ). Eight representative locations were identified using grouping analysis of the ECₐ data. The soils were analyzed for selected physical and chemical attributes known to be affected by OMW spreading as well as ancillary parameters needed for the calibration of the ECₐ. Discriminant analysis successfully categorized 76% of the ECₐ groups, selecting CEC, sand, pH, and ESP as the most powerful discriminatory variables in the grouping analysis. The correlation coefficients between the measured ECₐ, and the calculated ECₐ, and soil moisture were very high (r > 0.77, P < 0.05), suggesting that the proximal soil survey results are well calibrated. The spreading of untreated OMW over more than 7 years did not impair the soils under study. No significant changes in soil chemo-physical properties such as pH (< 7.66), electrical conductivity in saturated paste (< 3.58 dS m⁻¹), sodium adsorption ratio (< 2.3), potassium adsorption ratio (< 0.33), exchangeable sodium percentage (< 3.85%), and unsaturated hydraulic conductivity (< 0.3 cm h⁻¹) were found in comparison with untreated soils. The results support the premise that moderate quantities of OMW (50–70 m³ ha⁻¹ year⁻¹) equally spread over the soilscape will have little impact on soil health.