Evaluation of karst groundwater quality in Beijingand its eco-environmental effects

Objective As an important water supply source in Beijing, Karst groundwater played an irreplaceable role in the security of urban water supply and ecological environment improvement in Beijing city. Methods Based on the laboratory data of 278 karst groundwater samples collected systematically in Beijing karst water system in June 2021, the single factor quality evaluation and multi-factor comprehensive evaluation of karst water were carried out with karst water system as the unit, and the organic indexes were counted and analyzed for the first time. Results (1) The medians of pH, TDS and TH concentration of karst water in Beijing are 7.69, 334.77 mg·L^-1 and 262.01 mg·L^-1 respectively, which are characterized by weak alkalinity, low salinity and low hardness. The concentration of TH, TDS, SO₄^2- and Cl^- in Daxing-Tongzhou karst water system and Xishan karst water system are significantly higher than those in other karst water systems. (2) From large to small, the detection rate of unconventional indicators is:benzopyrene (4.32%) > tribromomethane (3.60%) > xylene (totalamount) (2.52%).The distribution of organic matter detection is relatively scattered and should be given sufficient attention.(3) The results of groundwater quality evaluation show that the total proportion of class I-III karst water quality in Beijing is 82.01%, with an exceeding rate of 17.99%. The exceeding points are mainly concentrated at the boundary between mountainous areas and plain area.The overstandard rate of each system was Daxing-Tongzhou (75.00%) > Beiwu (60.00%) > Changping (18.18%) >Xishan (16.94%) >Shunping (16.00%) >Qianjiadian-Jiuduhe (15.56%) > Yanqing (11.76%). Among them, Fe, TH, NH₃-N, Mn, SO₄^2-, F^- andNO₃-N are the main indexes affecting groundwater quality in Beijing. (4) In order to protect andimprovethe karst water ecological environment practically, methods such as strengthening water conservation to increase recharge, control point source and non-point source pollution to reduce pollutant input, continue monitoring and give play to early warning, and widely publicize and enhance public awareness should be appliedsystematically in the future.