Riverine groundwater discharge estimation in a dynamic river corridor using 222Rn

Groundwater discharge flux into rivers (riverine groundwater discharge or RGD) is essential information for the conservation and management of aquatic ecosystems and resources. One way to estimate area-integrated groundwater discharge into surface water bodies is to measure the concentration of a groundwater tracer within the water body. We assessed groundwater discharge using Rn-222, a tracer common in many surface water studies, through field measurements, surface water Rn-222 mass balance model, and groundwater flow simulation, for the seldom studied but ubiquitous setting of a flooding river corridor. The investigation was conducted at the dam-regulated Lower Colorado River (LCR) in Austin, Texas, USA. We found that Rn-222 in both the river water and groundwater in the river bank changed synchronously over a 12-hour flood cycle. A Rn-222 mass balance model allowed for estimation of groundwater discharge into a 500-m long reach of the LCR over the flood. The groundwater discharge ranged between negative values (indicating recharge) to 1570 m(3)/h; groundwater discharge from groundwater flow simulations corroborated these estimates. However, for the dynamic groundwater discharge estimated by the Rn-222 box model, assuming whether the groundwater Rn-222 endmember was constant or dynamic led to notably different results. The resultant groundwater discharge estimates are also highly sensitive to river Rn-222 values. We thus recommend that when using this approach to accurately characterize dynamic groundwater discharge, the Rn-222 in near-stream groundwater should be monitored at the same frequency as river Rn-222. If this is not possible, the Rn-222 method can still provide reasonable but approximate groundwater discharge given background information on surface water-groundwater exchange time scales.

Automated summary

Groundwater discharge flux into rivers is important for the conservation and management of aquatic ecosystems and resources. This study used Rn-222 as a tracer to estimate groundwater discharge into the Lower Colorado River in Austin, Texas, USA. The results showed that Rn-222 in river water and groundwater changed synchronously over a flood cycle, and the estimated groundwater discharge ranged from negative values to 1570 m³/h. The study also emphasized the importance of monitoring Rn-222 in near-stream groundwater to accurately characterize dynamic groundwater discharge.