Radon (²²²Rn) as a geochemical tracer for submarine groundwater discharge (SGD): Measuring techniques, source to sink...

This review uses radon (²²²Rn) to investigate the submarine groundwater discharge (SGD) to analyze its role as a geochemical tracer and the interplay within the coastal marine systems. The scope encompasses current measurement practices, source-to-sink processes, mass balance models, and the practical limitations of ²²²Rn in SGD research. The ²²²Rn serves as an excellent tracer due to its nature of inertness, ample half-life, portable measurements, irregular distribution within groundwater and seawater, which enables estimation of SGD fluxes and, therefore, enables understanding the variability of groundwater discharging to the marine system. Assessment of freshwater fluxes into coastal waters is critical due to their spatial and temporal variations. However, this is conceivable through measuring the ²²²Rn levels in freshwater and marine system interaction zones. Over the last three decades, SGD studies using ²²²Rn isotopes have gained popularity among oceanographers, hydrogeologists, hydrologists, ecologists, and biologists. This review is aimed to i) describe the fundamental features of SGD and its approaching changes over the last 30 years, ii) outline the applications of ²²²Rn isotopes in the SGD research, especially in the aspect of locating ²²²Rn enriched locations and RAD7 time series data measurements, iii) elaborate mass balance conceptual models and factors neglected earlier, iv) Clarify the boundaries of the box model, specifically detailing where pore water or groundwater is considered as an end member in the quantification of SGD flux and v) present existing gaps and uncertainties considerations to developing the SGD research in future. This scientific review provides a thorough framework and practical constraints for estimating SGD fluxes through systematically using ²²²Rn isotope techniques across various coastal environments and regions around the globe. The review highlights the vital importance of SGD in managing coastal ecosystems, particularly in regulating nutrient inputs, trace metals, and other pollution indicators and also underscores the essential need for long-term protection of delicate coastal freshwater systems.