Investigation of seasonal variations in submarine groundwater discharge using radium isotopes under drought conditions in northwestern coastal Taiwan

Submarine groundwater discharge (SGD) serves not only as an additional source of water, nutrients, and other associated solutes from the land to the sea, but also as a potential freshwater resource, especially under drought conditions. Therefore, it is crucial to quantify SGD and characterize its seasonal and temporal variations. However, sparse studies have examined SGD during a drought period. In this study, we investigated the seasonal SGD in Taiwan in an extreme drought year (2021). Water samples, including coastal seawater, groundwater and riverine/ drainage water, collected from the northwestern coast of Taiwan during dry (May-October) and wet (November-April) seasons were analyzed for radium isotopes (²²³Ra, ²²⁴Ra_ex, and ²²⁸Ra) and stable isotopes (δ¹⁸O and δD). Coupled short-lived radium isotopes (²²³Ra and ²²⁴Ra_ex) in coastal seawater at four selected sites were used to calculate the Ra-derived apparent age of coastal seawater, which can represent the amount of time that a water mass spent in our boundary-defined box (i.e., residence time), and the averages displayed slightly higher in the dry season (6.48 ± 2.63 days) than in the wet season (4.69 ± 2.40 days). Based on a mass balance model of ²²⁴Ra_ex, SGD in the wet season (841.47 ± 156.61 cm d⁻¹) was 2.7 times that in the dry season (391.34 ± 79.59 cm d⁻¹). In addition, the similarity in the averaged activity ratios of ²²⁴Ra_ex to ²²³Ra in shallow (depth ≤20 m) and deep (depth >20 m) groundwater and the good linear relationship between δ¹⁸O and δD values might indicate the same origin for shallow groundwater and deep one, probably the deep aquifer contributed to SGD in the coastal area. This study provides rare observation data and valuable results related to the seasonal variation of SGD in northwestern coastal Taiwan in an extreme drought year. Importantly, it offers new insights into the contribution of groundwater from deep aquifers under drought conditions. The results can serve as an important reference for understanding the effects of drought conditions on the seasonal variation of SGD, thereby enhancing our understanding of SGD under contrasting hydrological conditions.