Impact of Irrigation on Arsenic Mobilization in Groundwater from the Hetao Plain, Northern China: Evidence from Cl/Br Ratios and Stable Isotopes

The Hetao Plain, located in western Inner Mongolia, China, has been used for irrigation since the second century BC. Sixty-five samples were collected, including fifty-nine groundwater and six surface water samples, for hydrochemical and oxygen and hydrogen stable isotope analysis to assess the impact of irrigation on arsenic mobilization in groundwater in the study area. The total dissolved arsenic concentration in groundwater and surface water ranged from 3.2 to 764.8 μg/L and from 6.2 to 11.2 μg/L, respectively, generally exceeding 50 μg/L in groundwater, where the reducing environment prevails. The primary groundwater recharge source was a shallow aquifer that receives a considerable amount of irrigation water. The high arsenic content in groundwater was attributed to hydrochemical processes caused by vertical leaching of dissolved halite from the unsaturated zone, which was determined based on the molar Cl/Br ratios analysis. The oxygen and hydrogen stable isotope analysis of groundwater demonstrated the mixing between the groundwater and Yellow River water. An association between lateral recharge and mixing, evaporation, leaching, vertical mixing, and arsenic enrichment in groundwater was established based on the correlation between Cl^– concentration and δ¹⁸O values. In an anaerobic groundwater environment, nitrate from nitrogen fertilizers indirectly oxidized As(III) to As(V).