Chemical Characteristics and Evolution of Groundwater in the Yongding River Alluvial Fan of Beijing Plain

This study intends to find out the evolutionary mechanism of groundwater geochemical field and groundwater cycle in the Yongding River alluvial fan of Beijing.Based on the horizontal distribution of geochemical characteristics and groundwater evolutionary process in typical profiles,the authors have reached the following conclusions:1) From the recharge area to the discharge area of Yongding River alluvial fan,different zones of groundwater chemical types can be recognized.The chemical type of phreatic groundwater changes from Ca-HCO3 to Ca·Mg-HCO3 and Mg·Na-SO4 while the type of deep groundwater changes from Ca·Mg·Na-HCO3 and Ca·Na-HCO3 to Na-HCO3 and Na-HCO3·SO4.Groundwater with chemical types of Ca·Na-HCO3·SO4,Na·Ca-Cl or Ca·Mg-SO4 in the recharge area have been affected by human activities.In general,the distribution of groundwater chemical zones is consistent with that of groundwater flow zones,especially in the unconfined and moderate-deep confined aquifer.2) The groundwater chemical field in different depths is affected by water-rock interaction,which includes the dissolution/deposition of sulfate,silicate,carbonate and halite and the cation exchange of Ca-Na and Ca-Mg.3) From the recharge areas to the downstream discharge areas of the Yongding River alluvial fan,the groundwater velocity decreases gradually while the amount of dissolution/deposition for silicate,carbonate,sulfate and halite grows gradually.Therefore,the renewable velocity of groundwater decreases.