Influence of irrigation practices on arsenic mobilization: Evidence from isotope composition and Cl/Br ratios in groundwater from Datong Basin, northern China

Environment isotopes (δ18O and δ2H) and Cl/Br ratios in groundwater have been used to trace groundwater recharge and geochemical processes for arsenic contamination in Datong Basin. The arsenic concentrations of groundwater samples ranged from 0.4 to 434.9 μg/L with the average of 51.2 μg/L, which exceeded China’s drinking water standard (10 μg/L). All the groundwater samples are plotted on or close to the meteoric water line of the δ18O vs. δ2H plot, indicating their meteoric origin. The relationship between δ18O values and Cl/Br ratios and Cl concentrations demonstrate that leaching and mixing are the dominant processes affecting the distribution of high arsenic groundwater in this area. The observed non-linearity in the trend between δ18O and arsenic concentration is due to combined effects of mixing and leaching. The similarity of the trend in Cl/Br ratios and δ18O values for high arsenic groundwater demonstrate that extensive leaching of irrigation return and salt flushing water flow could be the dominant process driving arsenic mobilization in the groundwater system. Moreover, the long term irrigation practice can cause the drastic change of the biogeochemical and redox condition of in the aquifer system, which in turn promotes the mobilization of arsenic. Therefore, groundwater pumping for irrigation in this area of waterborne endemic arsenic poisoning should be under strict control to protect groundwater quality in this area.