Submarine Groundwater Discharge (SGD), i.e. any flow across the sediment-water interface (Burnett et al. 2003), is now recognized as the major source of continental water to the global ocean, thus exerting a major control over coastal water composition (Kwon et al. 2014). State-of-the-art methods are being used to evaluate, for the first time, the occurrence and magnitude of SGD in Ría de Vigo. Landsat Enhanced Thematic Mapper (ETM+) thermal infrared (TIR) imagery is used to obtain medium-to-high resolution sea surface temperature maps of the ria (Wilson and Rocha 2012). These are used to identify warm groundwater plumes within the system. Thermal imagery is complemented with basin-scale surveys where surface water samples are collected for the determination of the activity of 222Rn and 226Ra. Surveys cover the entire shoreline of the ría, with special focus on the areas potentially affected by fresh SGD as inferred from thermal imagery. Additionally, the activity of 222Rn and 226Ra is also determined in the aquifer systems surrounding the ría, beach porewaters, sediment incubations, atmosphere and the adjacent coastal sea. The system revealed as naturally enriched in 222Rn due to the widespread presence of granitic basement rocks in the region. Although variable, high 222Rn activities (up to 105 Bq m-3) are detected in boreholes surrounding the ría. Concomitantly, high 222Rn activity (>400 Bq m-3) is measured in certain areas of the ría. Closer to the shore, 222Rn time series in surface waters of selected beaches showed activities higher than 4000 Bq m-3 mainly restricted to low tide, suggesting high local SGD rates there. The information gathered (mass balance for excess 222Rn, i.e. that not supported by internal 226Ra decay, thermal imagery) is used to perform a first volumetric estimation of total SGD in Ría de Vigo

SUBACID (SUBmarine groundwater discharge (SGD) impact on coastal ACIDification processes in contrasting European Atlantic Shores: towards securing ecosystem services and food production) has received funding from the Irish Research Council and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 713279 through the CAROLINE program (CLNE/2017/210)

Seminario Ibérico de Química Marina (SIQUIMAR), Vigo (Spain), 20-22th June 2018

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