Hydrographic proxies for submarine groundwater discharge in the Jiulong River estuary and global perspectives.

Submarine groundwater discharge (SGD) significantly impacts most coastal waters. However, its quantification, depending on chemical tracers/proxies, limits its parameterization in numerical models. This study explored the hydrographic proxies of SGD in the Jiulong River estuary (JRE) using 226Ra and 228Ra as SGD tracers. Our results showed significant monthly fluctuations in the flux of SGD, with a peak in June and a minimum in April. On average, the flux of SGD was equivalent to 10 ± 1.67 % of the concurrent river discharge, with the area-normalized rate of 0.007 ± 0.017 to 0.13 ± 0.04 m/day. Positive SGD response to river discharge implies a connection with the surface runoff of the shallow aquifers. Furthermore, the flux of SGD presented a significant negative correlation with the return flow factor and flushing time of the estuary. The radium activities in the estuary were positively correlated with water depth, indicating that SGD was not driven by tidal pumping. Instead, physical mixing in low to middle salinity regions predominated such behavior of radium. Our results indicate that river discharge, flushing time and return flow factor may serve as hydrographic proxies of SGD in the JRE and potentially be applicable in parameterization of SGD in numerical models in similar coastal ecosystems. Globally, a positive correlation between SGD flux and river discharge emphasizes the latter as a general proxy in estuaries.