On the use of orchards to support soil aquifer treatment systems

Soil aquifer treatment (SAT) is a practice used to enhance groundwater storage through intermittent percolation of treated wastewater effluent in designated infiltration basins. Due to proximity to urban regions, land availability for SAT infiltration basins is a limiting factor. Furthermore, with the growing population, SAT systems are faced with an increase in effluent volumes meant for recharge. The present study experimentally explores, for the first time, the feasibility of the short-term flooding of a citrus orchard with secondary effluent, as an alternative for an additional dedicated infiltration area for SAT, namely agricultural soil aquifer treatment (Ag-SAT). Orange trees were planted in two different agricultural setups, on flat soil and atop a ridge. Sporadic intermittent winter flooding experiments, lasting 24 and 48 h, were conducted. Volumetric water content (VWC) and oxidation-reduction conditions were continuously monitored. Concurrently, water samples were collected and analyzed for total organic carbon and nitrogen species, along with leaf health measurements. Results were compared to an adjacent control plot, where no flooding with effluent was applied. Contaminant removal rates under the ridge setup resembled active SAT basins. Moreover, chemical analysis of the water samples and VWC readings demonstrated that higher water quality and faster root zone aeration (following flooding) were obtained under the ridge, which appears to be the better Ag-SAT setup. According to a principal component analysis (PCA), the dissolved oxygen explains 75% of the variability of effluent chemistry under the ridge, illustrating that oxic conditions prevailed in this setup. This study demonstrates that while many other concerns still need to be addressed, using agricultural plots as recharge basins for SAT during the winter appears to be a promising way to supplement recharge basins while having no impact on tree health.