Comparison of Soil Chemical Properties Prior to and Five to Eleven Years after Recycled Water Irrigation

Increasing demand on freshwater supplies in the arid and semiarid western United States and more stringent wastewater discharge standards have made recycled water a common water source for irrigating urban green spaces. This has created the need to study the effects of recycled water irrigation on soil chemical properties. We collected and analyzed soil samples at the commencement (in 2004) and 5 and 11 yr after recycled water irrigation on two golf courses, five metropolitan parks, and one school ground. Samples were taken at depths of 0 to 20, 20 to 40, 40 to 60, 60 to 80, and 80 to 100 cm on golf courses and at depths of 0 to 20 and 20 to 40 cm at other locations. Soil samples were tested for soil pH, soil organic matter, soil salinity (soil electrical conductivity [EC]), and exchangeable sodium percentage (ESP). Average soil EC was 0.82, 0.90, and 1.04 dS m⁻¹ in 2004, 2009, and 2015, respectively. Soil pH was 0.25 to 0.3 units higher in 2009 and 2015 than in 2004. The degree of soil pH increase was greater at deeper than at shallower soil depths. Compared with 2004, samples collected in 2009 and 2015 had 137 and 100% increases in ESP, respectively, suggesting that sodicity was of greater concern than salinity when recycled water was used for irrigation. On Golf Course I, gypsum application after aerification displaced sodium (Na) and reduced ESP at the surface depth (0–20 cm), but soil ESP increased significantly at deeper soil depths. More research is needed to develop techniques to address the risks of soil pH and ESP increases, especially in deep rootzones. CORE IDEAS: Recycled water has become a common water source for irrigating urban green spaces. Eleven years of recycled water irrigation increased soil salinity by 27% and pH by 0.32 units. Eleven years of recycled water irrigation increased soil sodium exchangeable percentage (ESP) by 101%. The degree of soil pH increase was greater at deeper than at shallower soil depths. Gypsum application after aerification reduced ESP at the surface depth (0–20 cm) only.