Long-term effect of wastewater irrigation on nitrate in groundwater in the North China Plain.

Urban wastewater has been used for irrigation in regions where water is scarce. Re-use of wastewaters is therefore a challenging possibility for managing regional water resources. It is important to re-assess the long term impacts of wastewater irrigation practices on regional groundwater quality. Wastewater irrigation practices in the North China Plain (NCP) after the 1950s were reviewed. With urbanization in the NCP, huge quantities of wastewater were disposed of from cities and used as an alternative source of irrigation water and fertilizer for agricultural development. Average annual precipitation in the NCP is <600 mm, and 80% of the irrigation water used in the croplands near big cities is urban wastewater, causing a serious problem in surface water and groundwater quality. Nitrate in groundwater due to wastewater irrigation is discussed, based on a case study in the North China Plain. It was found that the wastewater has an effect on water quality in the local groundwater flow system, and concentrations of nitrate in most wells with a depth of <40 m exceed the drinking water standard set by WHO. In the croplands irrigated with wastewater, nitrate concentrations range from 50 to 130 g m 3 , but in the croplands irrigated with groundwater away from the canal, nitrate concentrations are <35 g m 3 . Wastewater irrigation was found to control nitrate distribution in groundwater, and ought to be used carefully to protect both soil and groundwater from nitrate pollution.