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Earthquakes have a significant impact on groundwater environments as well as human life. However, identifying active and affected zones from seismic events using isotopic and microbial diversity indicators remains a challenging frontier. To validate the applicability of this coupled method for real-time analysis, we analyzed thoron ( 220 Rn), radon ( 222 Rn), microbial community compositions, and hydrochemistry in groundwater samples during the 2017 Pohang earthquake for the first time. We observed the detection of 220 Rn in groundwater right before the aftershocks, with a high correlation to 222 Rn concentrations. This indicates that 220 Rn and 222 Rn can serve as reliable seismic indicators for real-time analysis. The microbial data can assist in identifying affected groundwater zones, particularly when real-time detection of 220 Rn is not feasible. At the phylum level, Peregrinibacteria and Firmicutes were only found in samples with detected thoron. At the genus level, hydrogen-oxidizing or sulfur-oxidizing bacteria could serve as indicators of active zones. Two statistical analyses, self-organizing map (SOM) and principal component analysis (PCA) using hydrochemical parameters, also correlated with the results from these coupled indicators. This study demonstrates the theoretical and practical applicability of 220 Rn, 222 Rn, and microbial community compositions as new multi-faceted ecological indicators, whether for real-time analysis or otherwise., (© 2024. The Author(s).)

Dense non-aqueous-phase liquids (DNAPLs) represent one of the most hazardous contaminants of groundwater, posing health risks to humans. Radon is generally used to trace DNAPLs; however, external factors, such as rainfall or stream water, can influence its efficacy. To overcome these limitations, this study pioneered the integration of radon and microbial community structures to explore DNAPL tracing and natural attenuation in the context of seasonal variations for human health risk assessments. The results showed that a radon tracer can estimate DNAPL saturation in the source zone, especially during the dry season when radon deficiency predominates. However, samples exhibited mixing effects during the wet season because of local precipitation. Moreover, bioremediation and low health risks were observed in the plume boundary zone, indicating that microbial dechlorination was a predominant factor determining these risks. The abnormal patterns of radon observed during the wet season can be elucidated by examining microbiological communities. Consequently, a combined approach employing radon and microbial analysis is advocated for the boundary zone, albeit with a less intensive management strategy, compared with that for the source zone. This novel coupling method offers a theoretical and practical foundation for managing DNAPL-contaminated groundwater., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Groundwater is an important human resource. Daejeon in South Korea faces severe water quality issues, including radon, uranium, and fluoride pollution, all of which pose health risks to humans. With climate change, threats to potable water, such as heavy rain and typhoons, have become common. Therefore, examining the seasonal effects on groundwater quality and resultant health risks is important for understanding the mechanisms of different hydroclimatological conditions to enable the implementation of sustainable management plans in radon-contaminated groundwater areas. However, this issue has not yet been studied. To bridge this gap, in this study, major ions and microbial community structures were employed and groundwater quality index (GWQI) were calculated with hazard index based on limits set by the World Health Organization (WHO) to investigate the hydrochemical characterization and to assess pollution levels. The results showed that the rainy season had distinct hydrochemical characteristics with high correlations between radon and fluoride, and most groundwater samples collected after the typhoon had characteristics similar to those collected during the dry season, owing to the flow path. Furthermore, the microbial diversity and hazard quotient (HQ) values of fluoride revealed that pollution worsened during the dry season. All of the calculated effective dose values of radon exceeded the threshold limit set by the WHO, despite the low GWQI. Infants and children were particularly susceptible to radon-contaminated groundwater. The statistical results of self-organizing map (SOM) suggested that radon analysis was sufficient for public health intervention in the rainy season; however, in the dry season, combined analyses of radon, fluoride, and microbial diversity played important roles in health risk assessment. Our study presents a comprehensive understanding of radon-contaminated groundwater characteristics under seasonal effects and can serve as a reference for other similar zones to provide significant insights into the effective management of radon contamination., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023 The Author(s). Published by Elsevier Ltd.. All rights reserved.)

Significant changes in groundwater systems were observed after the 2017 Mw 5.5 Pohang earthquake. Groundwater level changes cannot be explained solely by poroelastic responses to the earthquake. In this study, we analyzed hydrogeochemical data, including environmental isotopes (18O, 2H, 87Sr/86Sr, and 222Rn) of water samples as well as time‐series data of groundwater level, temperature, and electrical conductivity. Principal component analysis (PCA) and physical modeling of pore pressure changes were also used. Some anomalies in time‐series data could be explained by the distributions of pore pressure changes based on modeling. However, the other wells, which did not have the correlation between time‐series data and modeling results, exhibited abnormal patterns in hydrogeochemical and isotope data. Stiff diagrams showed differing patterns of temporal change among groundwater wells. These diagrams and isotopic anomalies revealed possible mechanisms underlying the aftereffects of earthquakes, including saline water mixing, water‐rock interactions, deep fluid upwelling, and bedrock fracture opening. These anomalous phenomena were related to the clusters derived by PCA and depended on the distance to the epicenter, faults, and sea water. This study highlights that a single measurement is insufficient to reliably interpret groundwater responses, whereas a combined hydrogeochemical and physical interpretation may provide critical information for clarifying anomalies related to earthquakes. Plain Language Summary: An earthquake can have a significant impact on the groundwater system. Understanding groundwater response mechanisms related to the earthquake is important for managing subsurface systems. In general, changes in groundwater level based on physical characteristics have been commonly used as an indicator to understand earthquake impact. However, the changes in chemical or hydrogeological phenomena can be more helpful for comprehensive understanding about that. We analyzed water chemistry and environmental isotopes along with physical parameters for interpretation, which were used in a few previous studies. Based on our observations, we suggest the occurrence of earthquake‐related mechanisms such as saline water mixing, water‐rock interactions, deep fluid upwelling, and bedrock fracture opening. Our findings highlight the need to include chemical parameters with physical parameters for a broad interpretation of earthquake‐related mechanisms in groundwater systems. Key Points: Groundwater responses to the Pohang earthquake were analyzed to identify and manage subsurface systemsHydrochemistry and isotopes suggested the possible mechanisms about fluid pressure and properties related to the earthquakeThe groundwater anomalies could be explained by combined analysis of hydrogeochemical data and modeling for pore pressure changes [ABSTRACT FROM AUTHOR]

• The groundwater responses to the earthquakes were analyzed using hydrogeochemistry. • The SOM results showed that groundwater wells were grouped into 4 patterns. • The groups having distinct responses were explained by appropriate mechanisms. • The subsurface conditions can be characterized by groundwater observations. The September 12, 2016 Gyeongju earthquakes (M L = 5.1 of foreshock and M L = 5.8 of mainshock) had significant effects on groundwater systems along the Yangsan Fault System composed of NNE-trending, right-lateral strike-slip faults in Korea. Hydrological changes induced by the earthquakes are important because no surface ruptures have been reported and few earthquakes usually occur in Korea. The main objective of this research was to propose a conceptual model interpreting the possible mechanisms of groundwater response to the earthquakes based on anomalous hydrogeochemical data including isotope concentrations with lithostratigraphic classification. For this, annual monitoring data (groundwater level, temperature, and electrical conductivity) and collected data (hydrochemical parameters, radon-222, and strontium isotopes) were used. Groundwater level anomalies could be attributed to the movement of the epicentral strike-slip fault. Radon concentration data showed the potential of groundwater mixing processes. Strontium anomalies could be related to the lithology and stratigraphy of the bedrock, reflecting the effect of water–rock interaction. Using a Self-Organizing Map (SOM) statistical analysis, associations of hydro-geochemical characteristics among groundwater wells were interpreted. By combining the grouped results of the SOM with lithostratigraphic unit data, 21 groundwater wells were classified into four groups, each corresponding to different hydrogeological behaviors. A new comprehensive conceptual model was developed to explain possible mechanisms for the hydrological and geochemical responses in each group, which have been respectively identified as water–rock interaction, mixing of shallow and deep aquifers via sea water intrusion, bedrock fracture opening related to strike-slip fault movement, and no response. [ABSTRACT FROM AUTHOR]

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• A new multi-tracer approach (isotopes, hydrochemistry, and microbial data) was applied. • Strontium and radon isotopes are suitable to quantify mixing ratios. • Microbial diversity showed distinctive communities in mixing zones. • Tracers reflected the influence of anthropogenic factors to the mixing zones. • Groundwater-surface water mixing processes should be considered for effective water resource management. Investigating mixing processes between groundwater and surface water is important to manage groundwater system. Especially, because the study site is located around a groundwater heat pump (GWHP) system at a riverside area, external effects such as an operation of GWHP system and/or river level fluctuations can disturb the water system. It can result in artificial mixing of two water bodies, thereby changing original hydrogeochemical and biological characteristics. For this research, a multi–tracer approach combining strontium isotopes, 222Rn, microbial communities, and hydrochemical composition was undertaken for nineteen water samples to quantify how and to what extent mixing may occur due to external processes. Before this approach was applied, the physical experiment using flowmeter was also conducted for defining the effects of dam discharge rates to the flow system. Estimated mixing ratios based on radon tracer showed spatio–temporal variations, influenced by the dam discharge rate, seasonal effects, and GWHP. The average mixing ratio values by strontium tracer were in accordance with the results of radon tracer, suggesting that a dual-tracer approach would be a more reliable means. Mass–transfer calculation indicated that spatio–temporal variations in strontium isotopes were affected by hydrological events. Results from the microbial community using a heat map and principal coordinate analyses also supported the mixing process characteristics of the study area. Based on the results, the interactive and dynamic mixing occurred in the riverside area in relation to external factors causing hydraulic disturbances, and the mixing could be estimated from the combined application of isotopic tracers with microbial community. [ABSTRACT FROM AUTHOR]