Hydrochemical characteristics and genetic analysis of shallow groundwater in Fengtai District, Beijing

Based on 69 sets of groundwater quality monitoring data collected in adjacent water periods from 2020 to 2021 in Fengtai District, Beijing.Piper chart, Gibbs chart, chlor-alkali index, ion proportion analysis, correlation analysis, factor analysis and other statistical analysis methods are used to analyze the hydrochemical characteristics,sources of main ion and causes of groundwater in the study area.The results show that, The groundwater quality of Fengtai District is generally poor, especially about east of the Yongding River. During the dry period in 2021, the proportion of groundwater in monitoring wells is 21.74%, 42.03% and 36.23%, and the proportion of polluted groundwater monitoring wells in east of the Yongding River and west of the Yongding River is 96.15% and 23.53% respectively.The HCO3·Ca-Mg type water in the west of the Yongding River is mainly affected by rock weathering. The groundwater of east of the Yongding River, Cl·Na-Mg water accounts for the largest proportion, which is 40.38%. Influenced by rock weathering and evaporation, the cation alternation adsorption of groundwater in Fengtai District is weak. The result of factor analysis method shows that, the load of pH, TH, NO3−-N and Ca2+ is higher on F2, the load of Na+ and K+ is higher on F3, and the load of TH and Ca2+ is greater than 0.5 on F1 and F2. The analysis shows that F1 is caused by the comprehensive effect of natural and human influencing factors. Natural factors including dissolution of carbonate and silicate,whinc give priority to carbonate. F2 is mainly caused by human influencing factors, including industrial production, domestic sewage, pesticide,fertilizer application accumulation,historical pollution from sewage irrigation and seepage pit. F3 is mainly due to natural factors, mainly due to the dissolution of rock salts and other sedimentary rocks containing sodium salts and potassium salts.The number of groundwater monitoring wells whose f is greater than zero is 53, accounting for 76.81% of the total number. The number of groundwater monitoring wells whose f is greater than zero accounts for 94.34% and 5.66% respectively in east of the Yongding River and west of the Yongding River.In addition to the natural influencing factors such as groundwater recharge, runoff and drainage conditions, the production and discharge of industrial enterprises,domestic sewage and agricultural production activity in the region are also important human influencing factors which differentiate the groundwater chemistry and quality between the east and west of the Yongding River,however,the latter has a greater impact.