Water -rock interaction simulation of groundwater in the Yongding River alluvial fan of Beijing plain

The purpose of the study is to study the evolution mechanism of groundwater geochemical field in the Yongding River of Beijing. Firstly, based on the ion composition ratio method and ion complexation model, the reasons of the groundwater chemical field were analyzed qualitatively. Then, PHREEQC software was used to perform a geochemical modeling for water-rock interaction, and analyze quantitatively the evolution process and groundwater forming mechanism. The main conclusions are as follows: 1) The shallow, moderate and deep groundwater in the Yongding River alluvial fan was subject to sulfate, silicate, carbonate and halite dissolution/deposition and the cation exchange of Ca-Na and Ca-Mg. 2) The intensity of gypsum and halite dissolution in the shallow groundwater was stronger than that in the deep groundwater, while the intensity of silicate dissolution/deposition and cation exchange in the deep groundwater were both stronger than that in the shallow groundwater. The results of water-rock interaction modeling are conductive to reveal the evolution mechanism of the groundwater chemical environment and give direct information of interactions between groundwater and aquifer environment.