In most arid and semi-arid areas around the world where water demand exceeds the available freshwater, poor quality water like treated wastewater is becoming the major sources for irrigating farmlands. The treated wastewater (TWW) is being in use for irrigating agricultural lands in many arid and semi-arid regions for supplementing the scarcity of freshwater. Even though the quality of TWW depends mainly on the quality of the sewage water sources, and the level of treatment applied (Assouline and Narkis, 2011; Levy et al., 1999; Pedrero and Alarcón, 2009), it has a poor quality relative to freshwater. Specifically, TWW has higher electrical conductivity (EC), dissolved organic matter (DOM), total suspended solids (TSS), chemical oxygen demand (COD), biological oxygen demand (BOD), sodium adsorption ratio (SAR) and more (Levy et al., 2014a) (Chen et al., 2011; Feigin et al., 1991; Gerstl and Graber, 2011; Iannelli and Giraldi, 2011; Levy, 2011; Marcar et al., 2011; Paranychianakis et al., 2011) relative to freshwater. Irrigating agricultural land on a continual base with poor quality water including treated wastewater adversely affects soil physicochemical characteristics particularly in clay soil. The change in soil physicochemical property is one of the important factors limiting orchards' performance and sustainable use of the available farmland to support the growing needs of the world population. Irrigation with poor quality water like treated wastewater due to the scarcity of fresh water is among the major constraints distorting soil physicochemical properties on the daily bases. Irrigation with TWW has shown positive impact in agriculture by improving soil fertility (Bedbabis et al., 2015; Levy, 2011; Morgan et al., 2008; Nadav et al., 2013; Pedrero and Alarcón, 2009; Singh et al., 2012; Speir et al., 1999; Yadav et al., 2002), and therefore is becoming a reliable source of water for enhancing irrigation system especially in arid and semi-arid areas. Contrary to its benefits, long-term irrigation with TWW causes different levels of harm to soil physicochemical properties (Levy, 2011; Pedrero and Alarcon, 2009; Singh et al., 2012; Yadav et al., 2002).
For instance, increased soil electrical conductivity (EC) (Ayoub et al., 2016; Jahany and Rezapour, 2020; Lado et al., 2012; Rosabal et al., 2007; Schacht and Marschner, 2015), higher sodium adsorption ratio (SAR) (Ayoub et al., 2016; Jahany and Rezapour, 2020; Lado et al., 2012; Levy et al., 2014a), higher Na concentration (Ayoub et al., 2016; Bedbabis et al., 2015), higher Cl concentration (Bedbabis et al., 2015), degradation of soil structure and stability (Levy, 2011), lower hydraulic conductivity (HC) (Bardhan et al., 2016; Schacht and Marschner, 2015), and relatively high degree of soil water repellency (Nadav et al., 2013; Schacht and Marschner, 2015; Wallach et al., 2005) are some of the negative impacts of long-term irrigation with TWW on soil physicochemical properties. Levy et al. (2014) suggested that long-term irrigation with TWW having SAR in the range of 3-5 will lead to more than 8% accumulation of ESP in subsurface soil. The authors reported 4.5% and 8% increase in ESP in the topsoil layer (0-30 cm) and the deepest soil layer (90-120 cm) of clay soil irrigated with TWW compared with freshwater, respectively but no significant difference in sandy soil. This shows that the negative impact of TWW on soil quality depends on the inherent properties of the soil that influences its drainage efficiency. Moreover, the low rainfall, higher evapotranspiration, and low leaching in arid and semi-arid regions might have contributed to higher salt accumulations in clay soils. Therefore, the aim of this study is to evaluate the effect of water quality remediation, irrigation scheduling and soil remediating alternatives improve the negatively affected chemical properties of an avocado orchard long term irrigation with treated wastewater. To test our objectives four mitigating measures were evaluated alongside with TWW (control) at field level.
The studied remediation alternatives were irrigating with freshwater (FW), diluting TWW with an equal proportion of FW (MIX), high volume and long irrigation intervals (LFI), and installation of tuff trenches (TUF). Obviously, the low levels of salts and other harmful materials in freshwater makes it the first choice for irrigation. Freshwater irrigation might improve physicochemical properties of soil already negatively affected by long term irrigation with TWW (Assouline and Narkis, 2011; Graber and Gerstl, 2011; Lado et al., 2012; Levy et al., 2014a; Nadav et al., 2013; Schacht and Marschner, 2015) compared with poor quality water (e.g., TWW). However, there is a scarcity of FW in Arid and semi-arid areas, thus its use as a sole irrigation water source may not be feasible and/or economical. So that another feasible alternative that would either reverse or prevents further damages to soil are required. The second alternative remediation studied, MIX might be a reliable alternative for sustainable agriculture. This amendment might reduce the salinity, and ions concentration in irrigation water thereby reducing the SAR content of the irrigation water, which may reduce the negative effects of TWW on the soil. Reducing the SAR of the TWW before irrigation is important for sustainability and prevent soil degradation (Assouline et al., 2016).
Pedrero and Alarcón (2009) reported improved quality of reclaimed wastewater by mixing it with well water of equal amount. The third remediation evaluated, LFI might enhance leaching of salinity that would improve the aeration of the soil negatively affected with long term irrigation with treated wastewater. Higher rate of irrigation with the effluent of medium salinity might leach salts to the lower horizons and might reduce salinity buildup (Stewart et al., 1990). Tuff trenches installation is the fourth remediation alternative evaluated. Tuff has become a popular growth media in Israel due to its high porosity (Papadopoulos et al., 2008; Silber and Raviv, 1996), high saturated hydraulic conductivity (Silber and Raviv, 1996) and high surface area (Papadopoulos et al., 2008). The high porosity and high-saturated hydraulic conductivity of tuff are excellent for leaching of solutes from the soil profile and reversing the limited hydraulic conductivity of the soil irrigated with TWW for long-term. In a numerical analysis of solute transport, Russo et al. (2008) found a fast vertical flow of water and solute in tuff pond strips as compared with local soil and sand. It also creates better aeration and water transport in the soil that may favour root growth. Moreover, the improved infiltration of water in the top 30 cm of soil covered with tuff might allow the water to reach the soil profile where it cannot be easily lost due to evaporation.