A study of the spatial variation in groundwater discharge to river using ²²²Rn in the lower reaches of the Malian River

Quantifying groundwater-river interactions is critical for protection and management of water resources. Radon was utilized as a tracer to determine the distribution of groundwater discharge in the lower reaches of the Malian river in northwestern China. A mass balance model for river ²²²Rn is established and it provides a detailed longitudinal profile of the groundwater discharge rates. The results show that groundwater ²²²Rn activities are almost 1 order of magnitude higher than those of river water and the ²²²Rn activity in the hyporheic zone water is governed by the mixing with groundwater and river water and ²²²Rn inputs from sediments, which indicates that the groundwater from the Cretaceous aquifer discharged to the Malian river in the whole sections of the lower reaches with a total of 4.5 m³/s, accounting for 73.2% of the total river flux. The discharge rates vary spatially along the river length with higher rates in the upper and lower reaches, while a weaker interaction exists in the middle sections. The groundwater discharge resulting from the model is subject to several uncertainties including variability in groundwater ²²²Rn activity, uncertainties of ²²²Rn inputs from the hyporheic zone and gas transfer coefficient. Despite the uncertainties of the model, when correctly applied, the ²²²Rn tracer method can provide detailed information on the spatial distribution of groundwater discharge, especially in the gaining rivers.