Optimizing Groundwater Radium End-Members in Submarine Groundwater Discharge Estimation by Incorporating Groundwater Flow Information

Submarine groundwater discharge (SGD) can be as important as river discharge for marine ecosystems. Radium (Ra) isotopes are excellent tracers of SGD estimation, and the selection of groundwater radium end-members is the most critical step in using the radium mass balance model to evaluate SGD. However, where and when to collect groundwater Ra end-members and their relationship with groundwater flow were not fully considered in previous studies. Based on field investigations and numerical simulations, this study systematically analyzed the influence of groundwater flow on the spatial distribution of 224Ra activity, and a new method was proposed to determine groundwater Ra end-members by incorporating groundwater flow information in a sandy beach at Xiaojing Bay, China. The results show that the groundwater radium activity collected near the low tide mark at low tides may be the most suitable groundwater Ra end-member. Compared with the newly-proposed method, we found that the traditional methods for determining groundwater Ra end-members such as the mean value and quartile methods may overestimate the SGD rates. Our study provides new insights into the determination of groundwater Ra end-members in a tidal beach and highlights that the groundwater Ra end-members should be determined based on groundwater flow information in SGD estimations by radium mass balance model.