[Assessment of groundwater quality of different aquifers in Tongzhou area in Beijing Plain and its chemical characteristics analysis].

In order to evaluate the groundwater quality of Tongzhou area in Beijing Plain and to discuss the characteristics of its distribution by the view of hydrochemistry, a total of 151 groundwater samples, collected within study area in the dry period of 2008 according to the geological and hydrogeololgical condition of Tongzhou area, were classified as shallow, middle and deep groundwater, respectively. Based on the data, the groundwater quality was evaluated by the method of F value. The mean and variance of main chemical constituents of groundwater samples were presented. Almost all the quaternary groundwater of Chaobai river pluvial fan belonged to the alkaline water type. The evaluation results based on the analysis results showed that from shallow to deep, the quality of groundwater in Beijing became better. The total areas of groundwater belonging to class IV and V area were 884 km2, 599 km2 and 94 km2 respectively for shallow, middle and deep groundwater. The evaluation results showed that the main exceeding chemical constituents were TDS, hardness, NH4(+), F(-) and total Fe. Most exceeding samples belonged to middle and deep aquifers. The main types of shallow groundwater were HCO2-Ca x Mg- and HCO3 x Cl-Ca x Na x Mg, while the chemical types of mid-deep groundwater were mostly HCO3-Na x Ca- and HCO3 x SO4(2-) -Na x Ca type due to the increased Na(+), SO4(2-) and Cl(-) concentration. Study results showed that the quality of shallow groundwater became worse mainly due to human activities. The deterioration of groundwater quality in mid-deep aquifers was due to both human activities and natural occurrence of poor-quality water.