Evaluating Spatiotemporal Variations of Groundwater Quality in Northeast Beijing by Self-Organizing Map

As one of the globally largest cities suffering from severe water shortage, Beijing is highly dependent on groundwater supply. Located northeast of Beijing, the Pinggu district is an important emergency-groundwater-supply source. This area developed rapidly under the strategy of the integrated development of the Beijing-Tianjin-Hebei region in recent years. It is now important to evaluate the spatiotemporal variations in groundwater quality. This study analyzed groundwater-chemical-monitoring data from the periods 2014 and 2017. Hydrogeochemical analysis showed that groundwater is affected by calcite, dolomite, and silicate weathering. Self-organizing map (SOM) was used to cluster sample sites and identify possible sources of groundwater contamination. Sample sites were grouped into four clusters that explained the different pollution sources: sources of industrial and agricultural activities (Cluster I), landfill sources (Cluster II), domestic-sewage-discharge sources (Cluster III), and groundwater in Cluster IV was less affected by anthropogenic activities. Compared to 2014, concentrations of pollution indicators such as Cl-, SO42-, NO3-, and NH4+ increased, and the area of groundwater affected by domestic sewage discharge increased in 2017. Therefore, action should be taken in order to prevent the continuous deterioration of groundwater quality.