On the use of orchards to support soil aquifer treatment systems
Introduction
Water is a limiting factor for economic and social development in most arid and semi-arid regions across the globe (Boretti and Rosa, 2019, FAO, 2017). The exploitation of natural freshwater sources, chiefly groundwater depletion and overdraft, constitutes a major risk to drinking water supplies and irrigation-dependent agriculture (Bierkens and Wada, 2019). In an effort to reduce and reverse groundwater depletion, methods such as managed aquifer recharge (MAR) are applied (Dillon et al., 2019).
Soil aquifer treatment (SAT) is a MAR technology that enhances aquifer storage using treated wastewater (TWW) as its source (Sharma and Kennedy, 2017). Typically, SAT is executed through intermittent TWW spreading, flooding, and drying cycles in infiltration basins (Dillon, 2005). These cycles are an integral part of SAT operation as they allow the aeration of the vadose zone beneath the SAT basin and, together with periodic tillage of the basin's surface, allow for steady infiltration rates (Icekson-Tal et al., 2003, Nadav et al., 2012, Negev et al., 2020). While percolating through the infiltration basin, the TWW undergoes contaminant and pathogen removal in the SAT basin’s vadose zone (e.g., biodegradation, filtration, adsorption, chemical precipitation) (Amy and Drewes, 2007, Drewes et al., 2006, Elkayam et al., 2018, Fox et al., 2001).
The recovered effluent (i.e., reclaimed TWW after percolation to the aquifer via the vadose zone) is of high quality. It can be used for various applications (e.g., irrigation, aquaculture, industrial use) (Huertas et al., 2008). The SAT approach integrates groundwater recharge and sustainability, along with TWW reclamation and reuse, thus promoting eco-friendly awareness while serving as a low-cost system suitable for arid and semi-arid regions worldwide (Bouwer, 2002, Sharma et al., 2012).
A major limitation in implementing SAT is land availability (Cikurel et al., 2010, Tsangaratos et al., 2017). SAT requires vast land surface area for installing infiltration basins. This is especially pertinent in urban areas where there is direct competition over real estate since land is limited and expensive (Niswonger et al., 2017). Moreover, to establish a SAT site, specific conditions must be present, including: (1) a thick vadose zone; (2) an unconfined underlying aquifer; (3) a vadose zone with high soil permeability; (4) proximity to a wastewater treatment plant (WWTP); and (5) proximity to locations of potential demand for the recharged effluent (Dillon et al., 2006). These requirements make selecting a SAT site challenging.
Furthermore, current SAT systems often experience an increase in effluent volumes (per unit of area) originating from WWTPs, meant for recharge via percolation in infiltration basins (i.e., hydraulic loading rates), for different reasons (e.g., the influx of stormwater into the water system (De Bénédittis and Bertrand-Krajewski, 2005; Joannis et al., 2002)). A direct result of intensive hydraulic loading rates is the shortening of drying cycles (i.e., the time between TWW flooding events in the SAT basin), which reduces the quality of the treatment, primarily due to a lack of soil aeration (Ben Moshe et al., 2020, Negev et al., 2020).
This may be especially relevant during the winter when lower temperatures impede the infiltration rates. One reason for this is reduced microbial activity, which is temperature-dependent and, thus, lower during the winter months—14 °C and 28 °C, on average, during January and July, respectively (Israel Meteorological Service, 2020). As a result, organic matter decomposition is diminished, causing its accumulation in the topsoil and impeding infiltration. Another effect of lower temperatures is the increase in water viscosity, which has an inverse relation to hydraulic conductivity. This results in decreased flux rates of effluent infiltrating through the basin (Casanova et al., 2016, Hillel, 1980, Lin et al., 2003, Nadav et al., 2012). On the other hand, volumes headed for SAT infiltration may be larger due to undesired intrusion of stormwater into the sewer system.
Since land area for SAT is limited, an alternative solution may be to use farmlands as temporary SAT infiltration basins, thus alleviating stress from active SAT basins. This method is hereafter called agricultural soil aquifer treatment (Ag-SAT) (Grinshpan et al., 2021). This procedure can be executed on vast agricultural landscapes when many crop species are in a dormant period (e.g., orchards during the winter chill) or when no crops are being grown. Farmlands that meet SAT’s geo-physical requirements may serve as temporary SAT basins. To enable crop production in an Ag-SAT setting, crops must be tolerant to both waterlogged conditions and variable TWW quality.
Observations from previous studies concerning irrigation with TWW demonstrate different and diverse impacts on the crop's health and yield (Paudel et al., 2016, Vergine et al., 2017, Zolti et al., 2019). Within the scope of Ag-SAT, TWW irrigation might reduce fertilizer needs (Jaramillo and Restrepo, 2017, Van Lier and Huibers, 2004) and improve soil properties (e.g., texture, fertility) (Jiménez, 2006). On the other hand, TWW irrigation may have adverse impacts, such as root uptake of contaminants (Goldstein et al., 2014, Malchi et al., 2014) and the buildup of a clogging layer that reduces infiltration rates, thus creating prolonged waterlogged conditions (Van Lier and Huibers, 2004) and the development of hydrophobicity (Arye et al., 2011, Nadav et al., 2012). Although flooding agricultural plots for the purpose of groundwater recharge has been tested—a practice known as agricultural managed aquifer recharge (Ag-MAR) (Bachand et al., 2014; Dahlke et al., 2018; Waterhouse et al., 2021)—to the best of our knowledge, no study has explored the outcome of flooding farmlands using TWW as a means of both irrigation and aquifer recharge. The potential advantages and disadvantages of Ag-SAT, as well as matters of concern, are summarized in Grinshpan et al. (2021).
The goal of this study was to test the feasibility of the Ag-SAT concept in terms of water quality and plant response. In our study, we tested the use of a citrus orchard as a supplementary infiltration basin for TWW transferred from a nearby SAT infiltration basin. Plots were planted with citrus (orange) trees in the vicinity of a SAT basin that is part of Israel's Dan Region Reclamation Project, also known as the Shafdan. Two conventional agricultural settings were tested: trees planted on leveled soil, hereafter named “flat”, and trees planted on a raised soil bed, hereafter named “ridge”. The use of ridges has numerous potential benefits: improved crop yield, soil structure, and, most importantly, minimization of waterlogging in the root zone (i.e., improved aeration) (Bakker et al., 2005, Hamilton et al., 2000, Manik et al., 2019). The feasibility of Ag-SAT was investigated in terms of soil-water chemistry (i.e., treatment quality), soil and plant response, technical operation, and future implementations.
Section snippets
Experiment site
The Shafdan is Israel's largest wastewater treatment facility, currently serving over 2 million inhabitants and treating around 130–140 Mm3 of raw wastewater annually (Aharoni et al., 2019, Messing and Sela, 2016). The TWW (secondary effluent) is intermittently discharged into infiltration basins located close to the Shafdan plant on sand dunes above Israel's coastal aquifer (Goren et al., 2011). Herein, the effluent percolates through the vadose zone (approximately 20–40 m deep (Goren et al.,
Source and porewater chemistry
The study focuses on the topsoil (≤ 60 cm) since most of the SAT processes (organic matter and N degradation) occur at shallow depths of the vadose zone due to enhanced aeration (Abel, 2014, Aharoni et al., 2011, Amy and Drewes, 2007, Ben Moshe et al., 2020, Essandoh et al., 2013, Fox et al., 2005, Goren et al., 2014, Lin et al., 2008, Quanrud et al., 2003, Quanrud et al., 1996, Wilson et al., 1995). The observed time series of TOC, NH4+, and NO3- concentrations obtained at different depths and
Summary and conclusions
The use of an orange orchard as an alternative SAT basin was tested during 24 and 48-h winter flooding experiments. TWW was diverted from an active SAT infiltration basin and spread over two agricultural plots containing young citrus trees with different setups: flat soil and a ridge. The ORP conditions and the VWC state were continuously monitored under the plots. Water samples were collected during flooding experiments at various depths. Water quality results indicated that in both the flat
Funding
This work was supported by the US-Israel Binational Agricultural Research and Development Fund, BARD (grant no. IS-5125–18 R). This work was also supported by the Israel-U.S. Collaborative Water-Energy Research Center (CoWERC) via the Binational Industrial Research and Development Foundation (BIRD) Energy Center, grant EC-15.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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