Estimating groundwater discharge in the dam-affected rivers using 222Rn

• Dam effect on the 222 Rn-based groundwater discharge estimation is first investigated. • The error of groundwater discharge caused by dam is quantified to approximately 60%. • The width ratio had a significant influence on the groundwater discharge estimation. • Theoretical results can optimize groundwater discharge estimation in the Dawen river. Accurate groundwater discharge estimations have significant impact on river water quality and ecology. An increasing number of dams are being built on rivers, altering river morphologies and potentially affecting groundwater discharges. However, there is little known regarding about the impact of dams on groundwater discharge estimations in rivers. In this study, the 222 Rn mass balance model, which is widely used for tracing groundwater discharge, was used to estimate the groundwater discharge in dam-affected rivers. We used a theoretical 222 Rn model to investigate the influence of the river width, width ratio (dam width/river width), sampling distance from the dam, and the atmospheric escape caused by dam release (AED) on the groundwater discharge estimation. The results indicated that the 222 Rn method would underestimate the groundwater discharge in dam-affected rivers if the dam’s influence was ignored. The estimation error of groundwater discharge increased with the width ratio and remained stable as the width ratio became greater than three. It decreased with the sampling distance from the dam, and was limitedly related to river width. The AED resulted in an additional error of approximately 10–20 % compared to that without AED. As to the base model, generally, the estimation error is less than approximately 60 %. Finally, we conducted groundwater discharge estimations for the Dawen River, a tributary of the lower reaches of the Yellow River in China, that is extensively affected by dams and that plays a vital water resource role for agricultural and industrial activities. After calibration using the results of the theoretical model based on the characteristics of the dams, the estimated groundwater discharge using the 222 Rn method was 4.26 ± 0.53 m 3 /s, which is 1.5 times greater than the uncalibrated groundwater discharge of 2.83 ± 0.37 m 3 /s.