Ecological risks and sustainable utilization of reclaimed water and wastewater irrigation

Inherent uneven distributions of the already limited water resources have led to serious shortages of agricultural irrigation water in China. Municipal and industrial wastewaters have been employed to relief the demand for agricultural irrigation across the nation. Historically, both untreated raw wastewater (including substandard treated effluents) and properly reclaimed water have been involved. Since the 1940s, wastewater irrigated croplands gradually expanded, and by 1998 it increased to 3.62 million hectares. Meanwhile, the water quality of the applied wastewater incrementally improved. Today, the reclaimed water that meets the regulatory standards is replacing the untreated wastewater. Water reuses have drawn the central government's attention. Establishing systematic and comprehensive reuse programs across the nation have become a focus in the most recent five-year national strategic planning. The pace of reclaimed water irrigation practices will accelerate especially in the water deficient areas north of the Yangtze River. In this paper, we analyzed the ecological risks of heavy metals, salts, nitrogen, organic pollutants especially emerging contaminants, and pathogens of irrigating croplands with wastewater and reclaimed water. We found that the soils receiving wastewater irrigation might readily be polluted by the heavy metals carried in the applied water, and the extents of pollutions were dependent on origin of the water and texture of the soils. Heavy metals and organic pollutants that accumulated in the wastewater irrigated soils affected the diversities of soil microbial communities and the activities of soil enzymes. Under long-term wastewater irrigation, soil salinization occurred and led to reductions of soil permeability due to high dissolved salt contents of the applied water. Because of soil salinity accumulation, Cl-and NO- 3 in groundwater would easily exceed the water quality standards. In addition, the quality and yields of crops also could be harmed by excessive salinity and heavy metals in the soils. In the receiving areas of wastewater irrigation people were exposed to potentially greater health risks than those otherwise due to pathogens in the wastewater. From the agronomic point of view, the primary issue in using properly reclaimed water in irrigation is on salts and nitrogen management. Reclaimed water tends to have higher levels of total dissolved minerals and nitrate. Over the long-term reclaimed water irrigation could cause salinity and nitrate levels in soils and groundwater aquifers to exceed the regulatory standards. When salinity of the irrigated soils rose as water evaporated and salts were left behind, plants will experience increasingly severe salt injuries in terms of stunted growth and reduced yield potentials. The added nitrogen of reclaimed water often caused imbalances of nutrients in the plants growing medium and caused nitrate to pollute the groundwater. Under the reclaimed water irrigation settings, heavy metal, pathogens and organic chemical pollutants posted little risk to receiving soils and underlying groundwater aquifers. Drawing from the experiences of using reclaimed water in the United States, European Union, and Australia, reclaimed water might be safely used in crop and landscape irrigations if the practices were properly managed to minimize risks of salts buildup in and nitrogen leaching down the soil profile. It illustrated that the risks of environmental pollution in soil and groundwater receiving reclaimed water irrigation were considerably less than those receiving wastewater irrigation. Moreover, reclaimed water is cost effective and ecologically beneficial. Based on the 2010 data, the present reclaimed water reuse program in Beijing generated a net annual benefit of 712 million RMB and the benefits to costs ratio was 1.7 and this ratio would rise had more wastewater were reused. Environmental benefits of reclaimed water irrigation included reducing pollutants discharge into surface water bodies, improving soil quality, and etc. Undoubtedly, reclaimed water irrigation should be encouraged nationwide. At last, for safe and sustainable uses of reclaimed water irrigation in China, we recommend: (a) following the best management practices in irrigation management, (b) improving soil quality appropriately, (c) monitoring nutrient, salt, and pollutant inputs to ensure proper plant growth and prevent salts and pollutants buildup in the soil profile, and (d) separating the application sites and the surroundings by providing setback distances or buffer zones.