Effects of low-salinity reclaimed wastewater irrigation on the hydraulic properties and microstructure of subtropical red soil

Abstract: Domestic wastewater is characterized by a single and stable source, good biodegradability, and low salinity, but high chemical oxygen demand (COD) in densely populated areas, such as campuses and communities. Therefore, there is a great potential to reuse, due to the low cost of treatment. A sustainable water recycling system can be expected to effectively alleviate the seasonal drought and water shortage with high quality in South China. However, accumulative irrigation with low-salinity reclaimed wastewater can cause a change in soil structure and hydraulic properties, and then result in irreversible damage to the soil. This study aims to explore the effects of low-salinity reclaimed wastewater irrigation on the hydraulic properties and microstructures in subtropical red soil. The campus's domestic wastewater was also treated for irrigation. Two irrigation modes were applied, including continuous reclaimed wastewater irrigation (WW), and alternating reclaimed wastewater and distilled water irrigation (AWW). In addition, the distilled water continuous irrigation was set as the control treatment (CK). An outdoor simulated soil column test was carried out to determine the soil salinity and sodicity, while the microstructure and hydraulic properties were under the irrigation modes. The interaction mechanism was proposed between the soil salinization, structure, and hydraulic properties. The results showed that: 1) Low-salinity reclaimed wastewater irrigation led to the decrease of water holding capacity and hydraulic conductivity of the red soil. Specifically, the water holding capacity under the WW treatment was higher than that under the AWW treatment, whereas, the unsaturated hydraulic conductivities were on the contrary. There was a small difference in water diffusivities under the two low-salinity reclaimed wastewater irrigation modes. 2) The low-salinity reclaimed wastewater decreased the field water holding capacity and wilting coefficient of the red soil, compared with the CK treatment. The available water increased by about 6.33% under the WW treatment, but decreased by 27.85% under the AWW treatment. 3) The low-salinity reclaimed wastewater increased the proportion of macropores. The proportion of effective pores and micropores increased by 1.3% and 5.0%, respectively under the WW treatment, and decreased by 4.3% and 1.1%, respectively under the AWW treatment. 4) Low-salinity reclaimed wastewater irrigation significantly increased the electrical conductivity (EC) value and Na+ concentration of the red soil (P<0.05), but significantly decreased the cation exchangeable capacity (CEC) (P<0.05), compared with the CK treatment. The soil exchangeable sodium percentage (ESP) and sodium adsorption ratio (SAR) under the AWW treatment were significantly higher by 142.4% and 120.3%, respectively (P<0.05) than that of the WW treatment, resulting in stronger clay dispersion. The soil particle morphology was also confirmed by scanning an electron microscope. 5) Principal component and Pearson correlation analysis were used to analyze the interaction between the soil structure, hydraulic properties, as well as soil salinity and sodicity. The field water holding capacity, wilting coefficient, available water, the proportion of effective pores, and micropores were significantly negatively correlated with the exchangeable Na+, ESP, and SAR, but significantly positively correlated with the CEC. The water with good quality (rainwater) can be involved in the process of low-salinity reclaimed wastewater irrigation, due to the high rainfall variability in subtropical regions, thus intensifying the destruction of reclaimed wastewater on the red soil. Therefore, much attention should be paid to monitoring the soil SAR, ESP, reclaimed water quality, and irrigation mode. The findings can provide a strong reference to formulate the reclaimed wastewater irrigation schedules in subtropical red soil areas.