How are radiotracers shaping the research in submarine groundwater discharge?

The use of radiotracer techniques has been a fundamental tool for characterizing fluxes of solutes and water flows into the coastal ocean driven by submarine groundwater discharge (SGD). Indeed, the scientific interest in the use of radionuclides as tracers of SGD started developing in the late 90s when high activities of Ra isotopes and 222Rn in the coastal ocean were associated with groundwater inputs. Since then, the number of articles published about SGD has considerably grown and the technical improvements in radiotracer methods have often been accompanied by concurrent scientific advances in the understanding of the process. Although current research in SGD is conducted through multiple techniques (direct measurements, hydrological, geophysical, and geochemical techniques), the use of tracers such as Ra isotopes and 222Rn continues to be the most used and widespread method. Therefore SGD estimates are likely to be highly dependent on the methodological biases associated with radiotracer techniques. The aim of this study is to evaluate the main biases and assumptions relative to the use of Ra isotopes and 222Rn in SGD studies through a meta-analysis of the published academic literature. The results of this work highlight that a significant number of SGD studies using radionuclides as tracers are based on erroneous assumptions or inaccurate calculations leading to unreliable SGD quantifications, thus preventing its use for comparison with other studies or extrapolating from local to regional-global scale. These results also emphasize that the SGD community should seek comparison, reproducibility, and multiapproach studies that help to understand the complexity of SGD in multiple sites and bridge the gap between different quantification methods.