Investigation of seasonal variations in submarine groundwater discharge using radium isotopes under drought conditions in northwestern coastal Taiwan
Journal
Journal of Hydrology
Journal Volume
649
Start Page
132450
ISSN
0022-1694
Date Issued
2025-03
Author(s)
Abstract
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, 224Raex, and 228Ra) and stable isotopes (δ18O and δD). Coupled short-lived radium isotopes (223Ra and 224Raex) 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 224Raex, 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 224Raex 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.
Subjects
Radium apparent age
Radium isotopes
Source of groundwater
Stable isotopes
Submarine groundwater discharge
Taoyuan-Chungli Tableland
Publisher
Elsevier BV
Type
journal article