  Citation: Gargouri, B.; Ben Brahim, S.; Marrakchi, F.; Ben Rouina, B.; Kujawski, W.; Bouaziz, M. Impact of Wastewater Spreading on Properties of Tunisian Soil under Arid Climate. Sustainability 2022, 14, 3177. https:// doi.org/10.3390/su14063177 Academic Editors: Matia Mainardis, Arianna Catenacci and Fabiano Asunis Received: 16 January 2022 Accepted: 21 February 2022 Published: 8 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Impact of Wastewater Spreading on Properties of Tunisian Soil under Arid Climate Boutheina Gargouri 1 , Samia Ben Brahim 1 , Fatma Marrakchi 1 , Bechir Ben Rouina 2 , Wojciech Kujawski 3, * and Mohamed Bouaziz 1,4, * 1 Laboratoire d’Electrochimie et Environnement, Ecole Nationale d’Ingénieurs de Sfax, University of Sfax, BP “1173”, Sfax 3038, Tunisia; boutheina_fss@yahoo.fr (B.G.); samia.benbrahim@fss.rnu.tn (S.B.B.); fmar@et.aau.dk (F.M.) 2 Laboratoire d’Amélioration de la Productivité Oléicole et des Arbres Fruitiers, Olive Tree Institute of Sfax, BP “1087”, Sfax 3000, Tunisia; rouina.bechir@agrinet.iresa.tn 3 Faculty of Chemistry, Nicolaus Copernicus University in Torun, 7 Gagarina Street, 87-100 Toru´ n, Poland 4 Institut Supérieur de Biotechnologie de Sfax, University of Sfax, BP “1175”, Sfax 3038, Tunisia * Correspondence: kujawski@chem.umk.pl (W.K.); mohamed.bouaziz@fsg.rnu.tn (M.B.) Abstract: The environmental impacts of irrigating an olive field with olive mill wastewater (OMW) and treated wastewater (TWW) on soil properties were investigated. The effect of different irrigation treatments of OMW (50 m 3 ha −1 , 100 m 3 ha −1 , and 200 m 3 ha −1 ) and TWW at different soil depths was studied. The obtained findings revealed that TWW application augmented soil pH; EC values; and P, K and Ca contents in all soil layers. However, Mg and Na concentrations, as well as organic matter content (OM), were proven to decrease with TWW irrigation throughout the experiment. Whereas soil adjusted with OMW showed a decrease in K, Ca, Mg and Na contents with soil depth, a significant increase was observed with the increase in applied OMW dose. On the other hand, total phenols and OM content increased significantly with the rise in OMW levels in all the investigated layers compared to the control sample. Keywords: olive mill wastewater; treated wastewater; irrigation; soil chemical properties 1. Introduction Freshwater demand frequently surpasses water accessibility. As the global population expands, water resources are limited or even diminishing, which will aggravate water use issues and widen regional conflicts [1]. Urban and semi-urban centers, industries and agri- culture are in competition for freshwater use, which puts agriculture (especially irrigated agriculture) under severe pressure. Nowadays, the vegetables and fruits irrigated with wastewater are consumed by at least one-tenth of the world’s population [2]. Agricultural irrigation with wastewater is commonly used in arid areas around the world and has been gradually adopted by more regions, owing to the increase in food demand and the scarcity of freshwater [3]. Given its water scarcity, Tunisia, which is an arid region, has introduced the treatment and reuse of wastewater as an option as an irrigation resource to preserve the country’s environment and natural capital [4]. Treated wastewater (TWW) has been used in Tunisia for the irrigation of citrus fruits since 1965. It is authorized for different cultures, such as fruit trees, namely dates, palms and vines. TWW maintains a considerable amount of organic and metallic compounds (C, N, P, and K), which have a beneficial impact on the growth of some crops [5,6]. However, Tunisia suffers from a large amount of extremely polluting olive mill wastewater (OMW) produced after the olive oil extraction process. This residue causes serious environmental problems, as its efficient remediation and disposal is difficult. Different treatment methods have been developed to reduce the environmental impact of OMW, including its direct amendment in agricultural soils as an alternative irrigation method. In fact, some OMW components are possibly Sustainability 2022, 14, 3177. https://doi.org/10.3390/su14063177 https://www.mdpi.com/journal/sustainability