Improvement of evaluation of water age and submarine groundwater discharge: a case study in Daya Bay, China

The water age in coastal waters can be estimated by a simplified formula defined by activity ratio of radium isotopes when neglecting the effects of four factors including recirculated seawater (RSGD), open sea water end-member, sediments and rivers. Although this formula has been widely used, sometimes it is applied without checking the assumptions of neglecting the effects of the above-mentioned four factors. Here an attempt is made to give a generalized formula for estimating water age explicitly incorporating all the above-mentioned effects. The formula is then applied in Daya Bay, China by comprehensively using all the radium quartet (^{223,224,226,228}Ra) data to assess the water age and submarine groundwater discharge (SGD). Data analyses indicate that the factors such as RSGD, open sea water end-member, sediments and rivers should be included in the general radium model when there are various radium sources and their contributions are unknown. It is found that in Daya Bay, neglecting the effects of RSGD underestimates the water age by 25.5-45.7% and neglecting the effects of open sea water end-member overestimates the water age by 120-130%. The SGD-derived fluxes of nutrients and trace elements are significantly higher than those from local rivers. SGD can support approximately 63-70% of the total primary production. Overall, this study emphasizes again the importance of the general radium model and enhances accuracy in estimating water age and SGD. Our results also reveal that SGD significantly influences coastal primary production in Daya Bay and other similar aquatic ecosystems.