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Groundwater springs in karstified carbonate aquifers are known to transport carbon, nutrients and trace elements to the coastal ocean. The biogeochemical significance of submarine karstic springs and their impact on coastal primary production are often difficult to quantify. We investigated several karstic springs, including the first-order Port-Miou spring, in an urbanized watershed that is also severely impacted by sewage effluent (Calanques of Marseille-Cassis, France). Karstic springs were elevated in major nutrients and bioactive trace metals over Mediterranean seawater, with relatively low concentration ranges. Groundwater NO3was likely derived from atmosphere-aquifer interactions, while DOC:DON ratios reveal that NO2and NH4 + was autochthonously produced during mixing between karst groundwater and seawater. Submarine groundwater discharge (SGD) during March 2018 (wet season, baseflow conditions) was 6.7 ± 2.0 m 3 s -1 for the entire investigated coastline, determined from simultaneous 224 Ra and 226 Ra mass balances. The contribution of groundwater PO4 3-, the major limiting nutrient of the Mediterranean Sea, sustained only 1% of primary production adjacent to sewage outfall, but between 7 and 100% of the local primary production in areas that were not impacted by sewage. Groundwater and seawater Fe:DIN and Fe:DIP ratios suggest that Fe was not a limiting micro-nutrient during the period of study, where bioactive trace metal fluxes were dominated by sewage and atmospheric deposition, although excess Fe from groundwater may locally enhance N fixation. Groundwater solute fluxes may easily vary by a factor of two or more over time because karst aquifers are sensitive to precipitation, as is the case of the regional carbonate karstified aquifer of Port-Miou, highlighting the critical importance of properly characterizing nutrient and trace metal inputs in these coastal environments.

The Radium Delayed Coincidence Counter (RaDeCC) is one of the most extensively used equipment for measuring 223 Ra and 224 Ra activities in water and sediment samples. Samples are placed in a closed He-circulation system that carries the Rn produced by the decay of Ra to a scintillation cell. Each alpha decay recorded in the cell is routed to an electronic delayed coincidence system which enables the discrimination of 223 Ra and 224 Ra. In this study, the measurement and quantification methods using the RaDeCC system are assessed through analyses of registered data in different RaDeCC systems worldwide and a set of simulations. Results of this work indicate that the equations used to correct for 223 Ra and 224 Ra cross-talk interferences are only valid for a given range of activities and ratios between isotopes. Above certain limits that are specified in this study, these corrections may significantly overestimate the quantification of 223 Ra and 224 Ra activities (up to~40% and 30%, respectively), as well as the quantification of their parents 227 Ac and 228 Th. High activities of 226 Ra may also produce an overestimation of 224 Ra activities due to the buildup of 222 Rn, especially when long measurements with low activities of 224 Ra are performed. An improved method to quantify 226 Ra activities from the buildup of 222 Rn with the RaDeCC system is also developed in this study.Wethus provide a new set of guidelines for the appropriate quantification of 223 Ra, 224 Ra, 227 Ac, 228 Th, and 226 Ra with the RaDeCC system.Plain Language Summary In the last decades, there has been a growing interest in using radioactive isotopes to evaluate environmental processes. Their concentrations in environmental settings can reveal information about provenance, path, time, and duration. In this scenario, the research in the techniques to measure isotopes from samples has played a key role. In 1996, the launching of the Radium Delayed Coincidence Counter (RaDeCC) facilitated the fast and precise measurement of Ra isotopes, which provide information on land-ocean interaction processes (e.g., groundwater discharge to the sea and coastal residence times). Nowadays, this detector has become a fundamental tool for oceanographers, geochemist, and hydrologist among other scientific communities. Nevertheless, when the RaDeCC system was released, its quantification limits were not provided, and the recommendations on its use were mostly qualitative. More than 20 years later, we address these questions in a study that contains a comprehensive analysis of the RaDeCC counting mechanism and the determination of the limits of quantification. This study should serve as guidance for the measurement and quantification of Radium isotopes for the scientific community using the RaDeCC system. Key Points: • The maximum quantification limits for 223 Ra and 224 Ra are 200 and 100 cpm in the total channel, respectively • Limits for the quantification of 223 Ra and 224 Ra are provided when measurements are influenced by cross-talk and 222 Rn buildup...

Subterranean estuaries are biogeochemically active coastal sites resulting from the underground mixing of fresh aquifer groundwater and seawater. In these systems, microbial activity can largely transform the chemical elements that may reach the sea through submarine groundwater discharge (SGD), but little is known about the microorganisms thriving in these land-sea transition zones. We present the first spatiallyresolved characterization of the bacterial assemblages along a coastal aquifer in the NW Mediterranean, considering the entire subsurface salinity gradient. Combining