Temporal Variations of Submarine Groundwater Discharge into a Tide-Dominated Coastal Wetland (Gaomei Wetland, Western Taiwan) Indicated by Radon and Radium Isotopes

Submarine groundwater discharge (SGD) is evidenced around Taiwan, but the seasonal/temporal changes of SGD have not been fully examined. Here, we report a time-series investigation of SGD into a tide-dominated coastal wetland, the Gaomei Wetland, located to the south of the Da-Chia River’s mouth, western Taiwan, by using environmental tracers (222Rn, 224Raex, 228Ra, δD, and δ18O). Our results showed that regardless of dry and wet seasons, the 222Rn activities in coastal waters were high at low tide but low at high tide. It represents the continuous input of 222Rn-enriched groundwater. However, the 224Raex and 228Ra activities showed seasonal changes with tide conditions. In the dry season, the 224Raex and 228Ra activities in coastal waters were low at low tide but high at high tide; whereas in the wet season, an opposite relation was observed with quite high 224Raex and 228Ra activities in the low-tide waters. Coupled with the lower δD and δ18O values of coastal and pore waters in the dry season, in comparison to those in the wet season, it is suggested that these phenomena probably reflected a seasonal difference in the main SGD component with fresh SGD in the dry season, but saline ones in the wet season. Based on a 222Rn mass balance model, the estimated SGD fluxes into the Gaomei Wetland varied with tidal fluctuations and ranged from 0.2 to 25 cm d−1 and from 0.1 to 47 cm d−1 for the dry and wet seasons, respectively. A slightly high SGD flux occurring during the wet season at spring tide, implied a stronger tidal pumping coupled with a larger hydraulic gradient between land and sea. In this study, we demonstrated that the variation of SGD into the Gaomei Wetland is not only controlled by the seasonal changes of groundwater recharge, but also by the tidal pumping process.