Characteristics of hydrochemistry and nitrogen behavior under long-term managed aquifer recharge with reclaimed water: A case study in north China.

Due to the quality difference between reclaimed water and natural groundwater, managed aquifer recharge (MAR) with reclaimed water may pose environmental risks. A river infiltration of reclaimed water for groundwater recharge in north China has been in operation for over 10 years. To investigate the actual impact on native groundwater under long-term MAR, 10-year monitoring data of recharge water and groundwater were analyzed. Due to the effect of recharge, the hydrochemical type of groundwater rapidly changed from Ca-Mg-HCO into Na-HCO which was the type of recharge water. Cl was used as a conservative tracer in a physical mixing model, and the mixing was concluded to be dominant in the groundwater hydrochemical change under long-term MAR. The hydraulic travel time to the 30 m depth was determined to be about 6.5 months by obtaining the best-fit linear cross correlation between the concentrations of Cl in recharge water and those in groundwater. In application of this method, the monitoring wells should be located downstream and as close as possible to the recharge site (e.g., <50 m). Based on the travel time, behaviors of total nitrogen (TN), NO-N, NO-N, and NH-N were determined by attenuation factor (A). As the main nitrogen compound, NO-N was well attenuated under high hydraulic load, resulting in the A > 1, with an attenuation rate of 99.6%. The A < 1 of NH-N indicated the additional input of NH-N in groundwater. Fluctuations of NH-N in recharge water exceeded 4 mg/L changes sorption equilibrium, resulting in the sorption/desorption of NH-N in soil-groundwater system. The concentration of NH-N in groundwater increased in the later period of monitoring. The overall attenuation rate of NH-N was 26.3%. These findings contributed to improving the environmental benefits of this MAR site and provided guidance for other similar projects.