**1. Introduction**

Soil is one of the most essential elements in life. Its functions are crucial to the ecosystem because it is considered a storehouse of carbon and a food supplier. In addition, healthy soils are a prerequisite for ensuring the ecological ecosystem functions worldwide [1,2]. Moreover, the soil has a primordial role in limiting the intrusion of pollutants in groundwater by acting as a filter [3]. In several irrigated areas, worrying signs of deterioration in water and soil quality have been reported. Agricultural practices directly impact the soil's physical, chemical, and biological properties [4,5]. The alteration of soil properties has resulted in many environmental problems, such as soil degradation, salinization, waterlogging [6], deforestation and erosion [7], and groundwater contamination. The terms of agriculture conservation must be respected by ensuring the recycling of nutrients, avoiding environmental losses, and reducing the emission of greenhouse gases, whether at the regional or national scale [8]. Nitrogen is an essential macronutrient for healthy plant growth and high-yield production. Nevertheless, the massive use of nitrogenous fertilizers has led to some environmental problems, such as nitrate leaching [9,10]. After N application, crops assimilate their nitrogen needs by absorbing nitrate and ammonium accessible in the soil. The surplus of nitrate exceeds the plants' demand and soil denitrification capacity [11] and leaches out of the root zone as one of the most common forms of groundwater contamination [12,13]. The leaching of nitrate from the soil is a major problem threatening surface and groundwater quality and therefore human health [14,15]. The nitrate form (NO<sup>3</sup> −) of nitrogen is highly soluble, easily mobile within the soil, and poorly adsorbed by the soil particles. Recent literature shows increasing global concern about the impact of nitrate leaching with regard to the environment, especially in agricultural ecosystems [10]. The nitrate background is determined not to exceed 10 mg/L, and values exceeding this concentration indicate anthropogenic pollution [16]. The factors influencing the leaching of nitrate from the soil are numerous. Still, the most important remains the nature of the soil (the content of clay, silt, and organic matter), the irrigation and precipitation rates, the dose of fertilizers, and the temperature.

The soil texture is the most important determining factor influencing the vertical movement of contaminants through the soil. In coarse-textured sandy soils, the voids between soil particles are large in volume, allowing water to flow quickly through the unsaturated zone and reach groundwater. Huang and Hartemink [17] reported that sandy soils often have high hydraulic conductivity, gas permeability, and specific heat, but low field capacity, permanent wilting point, organic carbon, and cation exchangeable capacity. Therefore, filtration or natural water treatment takes a minimal amount of time. On the contrary, in fine-textured soils such as clays, the movement of water and contaminants through the soil is prolonged, which gives the clay minerals the time to adsorb pollutants and allows bacteria and other microorganisms to degrade contaminants before reaching the groundwater. Furthermore, the groundwater level can vary considerably from season to season, depending mainly on the infiltration rates. Consequently, the percentage of clay could be a deterministic factor affecting groundwater, especially in agricultural areas.

The soil characteristics could be determined using several characterization techniques. Simultaneous use of thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in combination or association with XRD and other chemical analyses could be used for the quantitative determination of a particular mineral or the estimation of the total mineralogical composition [18]. Indeed, the knowledge of soil characteristics using different techniques (SEM, XRF, XRD, BET, TGA/DSC) allows the determination of the soil texture and its influence on the mobility, adsorption, and leaching rates of pollutants. The study of soil characteristics in agricultural areas could help decision-makers and scientists in understanding the processes that might reduce groundwater pollution. Parallelly, proposing low-cost solutions for soil remediation and optimization might be beneficial to the environment, especially in sandy soils. Many approaches and strategies are already in place to address soil pollution issues. Soil remediation techniques and their applicability (e.g., in situ or ex situ) differ according to the type of contamination, the method of treatment (physical, chemical, or bioremediation), and the cost-effectiveness of treatment [19]. Nanomaterial is a novel technology that is quickly evolving and expanding its domains of application in all areas of research [20,21]. Yaqoob et al. [22] noted that nanoparticles have become the most appealing and widely employed materials for a wide range of applications including agriculture and wastewater treatment. Nowadays, nanotechnology has the potential to offer solutions for agricultural challenges such as boosting nutrient utilization efficiency, mitigating heavy metal toxicity, and efficiently improving soil fertility [23,24]. Alessandrino et al. [25] investigated the ability of graphene to reduce the concentration of nitrate in sandy soils and concluded that, unlike other soil improvers, graphene can stimulate the denitrification process in soil. The use of biochar in reducing soil contamination has been extensively studied during the last years [26–29]. Due to their higher cation exchange capacity, complexation, precipitation, physisorption, and electrostatic interaction, alkaline substances such as cement, lime, fly ash, steel slag, and blast furnace slag are excellent stabilizers for soil contaminants [30,31]. Das et al. [32] highlighted the importance of reusing steel slag (steel processing by-products) to increase crop productivity and soil fertility, reduce greenhouse gas emissions, and stabilize heavy metals in contaminated soils. Liyun et al. [33] reported that steel slag is efficient for nitrate removal and might be used to decrease nitrate leaching from the soil. The fast growth of biomass power plants has resulted in massive amounts of ashes and slags [34]. The application of biomass ash and slag to agricultural soils is now largely recognized as the most efficient way for recycling these residues [35].

The groundwater resources in the Loukkos region are well known for their low quality resulting mainly from intensive farming activities. The sandy nature of the soil, the intensive use of fertilizers, and the shallow aquifer make the R'mel groundwater sensitive to physicochemical contamination. This vulnerability becomes more problematic as long as the area provides water intended for human consumption. According to Tanji et al. [36], in the same study area, 25 groundnut farmers used extensively six nitrogenous fertilizers with an average of 350 Kg/ha. Such excessive nitrogenous applications are unacceptable since the majority of these effluents would immediately infiltrate groundwater. Moreover, previous studies have reported the contamination of public drinking wells by higher concentrations of nitrate and pesticide residues in this region [37–39]. Contrariwise, no studies aim to explain the influence of soil properties on groundwater contamination by nitrate in this perimeter.

In this study, agricultural sandy soils were analyzed for physicochemical parameters and characterized in order to investigate the influence of soil properties and intensive farming on nitrate leaching and to determine the nitrate-retention capacity in sandy soils through column experiments. In addition, the authors proposed the potential utilization of biomass slag (BS) formed during the combustion of olive pomace as a soil additive and improver. To explore the physical and chemical properties of this residual material, the olive pomace biomass slag (OPBS) was evaluated for physicochemical parameters and heavy metal toxicity and characterized using different techniques (ICP/OES, XRD, XRF, BET, and SEM).
