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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...

Lateral CH4 inputs to Arctic lakes through groundwater discharge could be substantial and constitute an important pathway that links CH4 production in thawing permafrost to atmospheric emissions via lakes. Yet, groundwater CH4 inputs and associated drivers are hitherto poorly constrained because their dynamics and spatial variability are largely unknown. Here, we unravel the important role and drivers of groundwater discharge for CH4 emissions from Arctic lakes. Spatial patterns across lakes suggest groundwater inflows are primarily related to lake depth and wetland cover. Groundwater CH4 inputs to lakes are higher in summer than in autumn and are influenced by hydrological (groundwater recharge) and biological drivers (CH4 production). This information on the spatial and temporal patterns on groundwater discharge at high northern latitudes is critical for predicting lake CH4 emissions in the warming Arctic, as rising temperatures, increasing precipitation, and permafrost thawing may further exacerbate groundwater CH4 inputs to lakes.