| Summary: | Jiaozhou Bay, an urbanized coastal bay located in the southern part of Shandong Peninsula, China, has been deeply affected by anthropogenic activities. Here, the naturally occurring <sup>222</sup>Rn isotope was used as a tracer to assess the submarine groundwater discharge (SGD) in this bay. The time series of <sup>222</sup>Rn concentrations in nearshore seawater were monitored continuously over several tidal cycles at two fixed sites (Tuandao (TD) and Hongdao (HD)) during the dry season in spring and the wet season in autumn of 2016. <sup>222</sup>Rn concentrations in seawater were negatively related to the water depth, indicating the influence of tidal pumping. A <sup>222</sup>Rn mass balance model revealed that the mean SGD rates were 21.9 cm/d at TD and 17.8 cm/d at HD in the dry season, and were 19.5 cm/d at TD and 26.9 cm/d at HD in the wet season. These rates were about 8–14 times the discharge rates of the local rivers. Enhanced groundwater inputs occurred at HD in the wet season, likely due to the large tidal amplitudes and the rapid response to local precipitation. Large inputs of SGD may have important influences on nutrients levels and structure, as well as the water eutrophication occurring in coastal waters.
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