Sources and migration of nitrate in groundwater in the Xishan karst aquifer in Beijing

The Xishan karst groundwater is an important source of water supply in Beijing. During the process of regional hydrogeological survey, it was found that the nitrate concentration of a few groundwater samples in the recharge area of the karst groundwater was high, however, the nitrate concentration in the surrounding karst groundwater did not change significantly (the average concentration was 18.81 mg/L), which was characterized by a point source of nitrate. The distribution characteristics and influence factors of nitrate in the karst aquifers are of great significance. In this paper, the distribution and possible sources of nitrate in the karst groundwater are studied by using the results of karst hydrochemical and isotopic analyses and the contribution rate of each possible source is calculated by using the IsoSource software. The results show that the main source of nitrate in the karst aquifer of the Junzhuang- ongfengtun runoff area and the Junzhuang-Gucheng-Yuquanshan Spring runoff area comes from domestic sewage, soil organic nitrogen mineralization and atmospheric sedimentation, and the contribution rates are 37.1%, 36.3% and 26.6%, respectively. Nitrate in the karst groundwater near Tanzhishi-Sijiqing is affected by infiltration of aquaculture waste water, nitrate content in the runoff area is low, and denitrification degradation occurs. The nitrate of the Yongding River is not the main source in the Xishan karst groundwater. The results are of important reference value for the control of nitrate in the Xishan karst groundwater.