Abstract
The natural radioisotope radon (222Rn) has been widely used as a tracer for quantifying submarine groundwater discharge (SGD). Applicable equipment for continuous radon-in-water detection using a radon-in-air monitor includes the “RAD AQUA” and the “Water Probe” (both Durridge Inc., USA). Although both devices have been applied to measure 222Rn concentrations in coastal water over time periods from hours to days, the monitoring results for the same water body may differ due to the different physical driving forces for 222Rn degassing from the water, which will result in an inaccurate SGD estimate. Here the two approaches were simultaneously applied for monitoring 222Rn concentrations at the same site in the same surface-water body (Daya Bay, China). The SGD fluxes resulting from both approaches were evaluated and compared, based on the obtained time series and a 222Rn mass balance model. The results demonstrate that the average 222Rn concentration measured by the Water Probe is only ~30% of that detected by means of the RAD AQUA. The mean SGD flux estimated based on 222Rn time series measured with the Water Probe is 69% of that detected by means of the RAD AQUA. The findings of this study suggest that the RAD AQUA can capture more accurately the signals of variation of radon concentration, and is a better approach for continuously monitoring radon and quantifying SGD in coastal zones. Thus, previous studies based on 222Rn data from the Water Probe for estimating SGD and the associated nutrient flux may need re-evaluation.
Résumé
L’isotope radioactif naturel du radon (222Rn) est fréquemment utilisé comme traceur pour quantifier les sorties d’eau souterraine sous-marines (SGD). L’instrumentation adaptée pour la détection en continu du radon dans l’eau inclut le « RAD AQUA » et le « Water Probe » (les deux produits de Durridge Inc., USA). Bien que ces appareils aient été utilisés pour mesurer les concentrations en 222Rn dans les eaux côtières sur des périodes de temps horaires à journalières, les résultats de suivi d’une même masse d’eau peuvent différer à cause des différents processus physiques contrôlant le dégazage du 222Rn de l’eau, ce qui va résulter en une estimation erronée du SGD. Dans la présente étude, les deux approches ont été appliquées simultanément pour le suivi des concentrations en 222Rn sur le même site dans la même masse d’eau de surface (Baie de Daya, Chine). Les flux de SGD issus des deux approches ont été évalués et comparés sur la base des séries temporelles obtenues et d’un modèle de bilan de masse en 222Rn. Les résultats démontrent que la concentration moyenne en 222Rn mesurée avec le “Water Probe” correspond à seulement ~30% de celle détectée par le “RAD AQUA”. Le flux moyen de SGD basé sur les séries temporelles mesurées avec le “Water Probe” correspond à 69% de celui détecté à l’aide du “RAD AQUA”. Les résultats de cette étude suggèrent que le “RAD AQUA” peut restituer de manière plus précise les signaux de variation des concentrations en radon, et qu’il s’agit d’une meilleure approche pour le suivi en continu du radon et pour quantifier le SGD dans les zones côtières. De fait, les études précédentes basées sur des données de 222Rn mesurées avec le “Water Probe” pour l’estimation du SGD et des flux en nutriments associés pourraient nécessiter une réévaluation.
Resumen
El radioisotópo natural del radón (222Rn) ha sido ampliamente utilizado como trazador para cuantificar la descarga submarina de aguas subterráneas (SGD). El equipo aplicable para la detección continua de radón en agua utilizando un monitor de radón en aire incluye el "RAD AQUA" y la "Water Probe" (ambos de Durridge Inc., EEUU). Aunque ambos dispositivos se han aplicado para medir concentraciones de 222Rn en aguas costeras a lo largo de períodos de tiempo de horas a días, los resultados del control para la misma masa de agua pueden diferir debido a las diferentes fuerzas físicas que impulsan la desgasificación de 222Rn del agua, lo que dará lugar a una estimación inexacta de la SGD. En este caso, los dos enfoques se aplicaron simultáneamente para el seguimiento de las concentraciones de 222Rn en el mismo lugar y en la misma masa de agua superficial (Daya Bay, China). Se evaluaron y compararon los flujos de SGD resultantes de ambos enfoques, basándose en las series temporales obtenidas y en un modelo de balance de masa de 222Rn. Los resultados demuestran que la concentración media de 222Rn medida por la sonda de agua es sólo ~30% de la detectada por medio del RAD AQUA. El flujo medio de SGD estimado a partir de series temporales de 222Rn medidos con la sonda de agua es el 69% del que se detecta mediante el RAD AQUA. Los resultados de este estudio sugieren que el RAD AQUA puede capturar con mayor precisión las señales de variación de la concentración de radón, y es un mejor enfoque para monitorear continuamente el radón y cuantificar la SGD en las zonas costeras. Por lo tanto, los estudios previos basados en datos de 222Rn de la sonda de agua para estimar la SGD y el flujo de nutrientes asociados pueden necesitar una reevaluación.
摘要
天然放射性同位素氡(222Rn)是广泛用于量化海底地下水排放(SGD)的示踪剂。使用空气中氡监测器进行连续水中氡检测的适用设备包括“RAD AQUA”和“水探针”(均为Durridge公司,美国)。虽然这两种装置已用于测量沿海水中从数小时到数天时间段的222Rn浓度,但同一水体的监测结果可能因为222Rn从水中脱气的物理驱动力不同而不同,这将导致SGD估计不准确。这两种方法同时用于监测同一地表水体(大亚湾,中国)同一位置的222Rn浓度。基于所获得的时间序列和222Rn质量平衡模型,评估和比较了两种方法估算的SGD通量。结果表明,水探针测得的平均222Rn浓度仅为RAD AQUA检测到的平均浓度的约30%。基于用水探针测量的222Rn时间序列估算的平均SGD通量是通过RAD AQUA检测到的69%。该研究的结果表明,RAD AQUA可以更准确地捕获氡浓度变化的信号,是连续监测沿海地区氡和量化SGD的更好方法。因此,需要重新评估之前基于水探针法的222Rn数据估算SGD和相关营养通量的研究。
Resumo
O radioisótopo natural radônio (222Rn) vem sendo amplamente utilizado como um marcador a fim de quantificar a descarga submarina de água subterrânea (DSAS). Os equipamentos apropriados para detecção contínua do radônio em água, “RAD AQUA” e o “Water Probe” (ambos da Durridge Inc., EUA), utilizam um monitor de radônio no ar. Apesar dos dois dispositivos estarem sendo aplicados para medição de concentrações de 222Rn em água costeira em períodos de horas a dias, os resultados do monitoramento para o mesmo corpo de água podem diferir devido às diferentes forças físicas do 222Rn de desgaseificação da água, o que resultará em uma estimativa imprecisa de DSAS. As duas abordagens foram simultaneamente aplicadas para o monitoramento das concentrações de 222Rn numa mesma superfície d’água na baía de Daya na China. Os fluxos da DSAS resultantes em ambas as abordagens foram avaliadas e comparadas de acordo com série temporal obtida e o modelo de balanço de massa do 222Rn. Os resultados demonstram que a concentração média de 222Rn medido pelo equipamento Water Probe é apenas ~30% do detectado utilizando o RAD AQUA. O principal fluxo estimado de DSAS baseado na série temporal de 222Rn medido usando o Water Probe é 69% do detectado pelo RAD AQUA. Este estudo sugere que RAD AQUA poderia capturar mais assertivamente os sinais de variação da concentração de radônio , e é a melhor abordagem a fim de monitorar constantemente este elemento e quantificar a DSAS em zonas costeiras. Deste modo, estudos anteriores com o equipamento Water Probe baseados em dados de 222Rn para estimar a DSAS e o fluxo associado de nutrientes precisariam ser reavaliados.
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Acknowledgements
The authors thank An An, Kai Xiao, Xiaoting Lu, Meng Zhang, Shaohong Li, and Zongzhong Song for their field work.
Funding
This work was supported by the National Natural Science Foundation of China (Grant Nos. 41430641, 41702266, 41890852) and the National Basic Research Program of China (“973” Program, Grant No. 2015CB452901). This work was partially funded by the Guangdong Provincial Key Laboratory of Soil and Groundwater Pollution Control (No. 2017B030301012), and State Environmental Protection Key Laboratory of Integrated Surface Water-Groundwater Pollution Control.
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Wang, X., Li, H., Zhang, Y. et al. Submarine groundwater discharge revealed by 222Rn: comparison of two continuous on-site 222Rn-in-water measurement methods. Hydrogeol J 27, 1879–1887 (2019). https://doi.org/10.1007/s10040-019-01988-z
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DOI: https://doi.org/10.1007/s10040-019-01988-z