Fresh Groundwater Discharge as a Major Source of ⁹⁰Sr into the Coastal Ocean

The behavior and source of ⁹⁰Sr in the coastal ocean remain uncertain. Here, we investigated the distributions of ⁹⁰Sr in coastal fresh groundwater, river water, pore water, and seawater in three bays along the southeastern coast of China between 2019 and 2021 and evaluated the potential of submarine groundwater discharge (SGD) as a source of coastal ⁹⁰Sr. The ⁹⁰Sr activity in coastal fresh groundwater was higher than that in river water and seawater, while the ⁹⁰Sr activity in pore water was comparable to that in adjacent seawater. In addition, nonconservative mixing behavior of ⁹⁰Sr along the salinity gradient between river water and seawater was observed. These observations indicated that fresh SGD may serve as an additional source of ⁹⁰Sr in coastal seawater. Combining our groundwater ⁹⁰Sr data with the reported fresh SGD flux data, the estimated fresh SGD-derived ⁹⁰Sr fluxes into the three bays were comparable to or even higher than those supplied by riverine sources. These results revealed that fresh SGD is a major but overlooked source of ⁹⁰Sr in the coastal ocean. This subterranean pathway for transport of ⁹⁰Sr to the coastal ocean should be considered in the monitoring and risk assessment of coastal areas, especially those near nuclear facilities.