| Summary: | 碩士 === 國立臺灣大學 === 海洋研究所 === 106 === This study aims to investigate the input of groundwater to the riverine water of Danshuei River through radioisotope tracer, 224Ra, method. Water samples were collected from Guandu Wharf and downstream of Danshuei River during Nov. 3-5, 2017; while fluvial water from upstream of Danshuei River as well as fresh groundwater were sampled in January, 2018 and May, 2018. In addition, seawater samples off the Danshuei River were collected on boat R/V Ocean Researcher I during OR1-1176 cruise（2017/9/18-2017/9/24）.
The results of spatial distribution showed that the activities of excess 224Ra ranged from 6.0 to 10.1 dpm/100L with the higher excess 224Ra activities found in Bali dike and Chongyang Bridge. However, the fluvial water samples collected from upstream of Danshuei River tributary showed much lower excess 224Ra activities. It indicated that the downstream of Danshuei River might be influenced by groundwater input. As well, higher activity of excess 224Ra（12.6±1.7 dpm/100L）in seawater off the Danshuei River estuary implied an effect of SGD on coastal water. Through mass balance models of radium isotopes, the groundwater discharges were estimated approximately 103-147 m3/s.
Furthermore, the results of temporal distribution show that the activities of excess 224Ra ranged from under detection limit to 10.0 dpm/100L with the higher value（10.0±1.8 dpm/100L）at low tide and, the lower one at high tide. The activities of excess 224Ra were negatively correlated with the tidal level, indicating that input groundwater in our study area is mainly affected by tidally driven oscillation.
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