RESEARCH ON THE HYDROCHEMICAL CHARACTERISTICS OF HIGH-ARSENIC GROUNDWATER IN THE SHALLOW AQUIFER OF NORTHERN YINCHUAN PLAIN

The Yinchuan Plain in Ningxia Region is another high-arsenic groundwater area which was found in the Yellow River basin after the Hetao Plain. In order to ascertain the hydrochemical characteristics of the high-As groundwater and their influence mechanism to arsenic release and enrichment in the aquifer, this article chooses the Northern Yinchuan Plain as a typical study area for comprehensive research, including hydrogeology field survey, water sampling and test, correlation analysis of As and hydrochemical components, as well as hydrogeochemistry. The results show that the As content in groundwater of Northern Yinchuan Plain ranges from 0.2 to 177 μg/L. The pH value of high-As (more than 50 μg/L) groundwater is 7.5-8.5. The hydrochemical types are mainly HCO₃-Na·Ca, Cl·HCO₃-Na and Cl·HCO₃-Na·Ca. The E_h value is -200 to -100 mV. High contents of COD, NH4 and HCO₃^- occur in high-As groundwater in the Northern Yinchuan Plain, while the contents of NO3^- and SO₄^2- are low. After a long period of evolution, the alluvial and lacustrine aquifer in Northern Yinchuan Plain with high arsenic and rich organic matter formed alkaline-reducing groundwater environment, with special hydrochemical characteristics and a great potential of arsenic releasing. The high pH value causes arsenic desorption and release from the clay mineral surface and hydrates such as iron and manganese oxide or hydroxide. The arsenic combined with iron/manganese oxides is released into the groundwater, as part of iron and manganese oxides can be reduced into low state and more soluble compounds under the condition of high pH and low E_h. The competitive adsorption behavior between bicarbonate and arsenate/arsenite prompts the arsenic desorption of aquifer.