Although arsenic-contaminated groundwater in the Datong Basin has been studied for more than 10 years, little has been known about the complex patterns of solute transport in the aquifer systems. Field monitoring and transient 3D unsaturated groundwater flow modeling studies were car- ried out on the riparian zone of the Sanggan River at the Datong Basin, northern China, to better un- derstand the effects of groundwater flow on As mobilization and transport. The results indicate that ir- rigation is the primary factor in determining the groundwater flow paths. Irrigation can not only in- crease groundwater level and reduce horizontal groundwater velocity and thereby accelerate vertical and horizontal groundwater exchange among sand, silt and clay formations, but also change the HS concentration, redox conditions of the shallow groundwater. Results of net groundwater flux estimation suggest that vertical infiltration is likely the primary control of As transport in the vadose zone, while horizontal water exchange is dominant in controlling As migration within the sand aquifers. Recharge water, including irrigation return water and flushed saltwater, travels downward from the ground surface to the aquifer and then nearly horizontally across the sand aquifer. The maximum value of As enriched in the riparian zone is roughly estimated to be 1 706.2 mg.d-1 for a horizontal water exchange of 8.98 m3.d-1 close to the river and an As concentration of 190 μg.L-1.
