Abstract: Radon isotope was utilized as a tracer to determine the river-groundwater interaction and explore the spatial variation and controlling factors of ²²²Rn activity in river water and groundwater in the lower reach of the Malian River. Isotope gradient was proposed as an indicator of the magnitude of river water and groundwater interaction. The results implied that ²²²Rn activity was considerably lower in river water than that in groundwater. The highest ²²²Rn activity in groundwater was found in the Cretaceous aquifer, followed by the loess aquifer, the quaternary aquifer, and river water in sequence. Moreover, ²²²Rn activity in groundwater was controlled by lithology, geological formation and redox conditions and lithology was the most important factor, whereas ²²²Rn activity in river water was influenced by groundwater discharge. The distribution of ²²²Rn isotopic gradient provided a detailed profile of river water and groundwater interaction. A positive index indicated that groundwater had entered surface water between those sites of R02-R03, R03-R04 and R13-R14, while a weaker interaction in other sites.

Key words: river-groundwater interaction, ²²²Rn activity, distribution features, isotopic gradient, Malian River