Abstract
222Rn was used to assess river–groundwater interactions within Castel di Sangro alluvial aquifer (Italy). The effectiveness of results obtained through this indicator was verified by also analyzing δ18O, major ions and temperature in both surface and groundwater, and carrying out piezometric head monitoring and discharge measurements. Hydrogeological investigations suggested that the river infiltrates into the aquifer in the south-eastern aquifer portion, while groundwater discharges into the river in the north-eastern portion. The latter phenomenon is supported by 222Rn data. Nevertheless, flow-through conditions cause the modelled discharge along this river reach, estimated by 222Rn data in a degassing-corrected two-component mixing model, to be greater than the measured discharge. Concerning river infiltration into the aquifer, δ18O, major ions and temperature data show that the river contribution is negligible in terms of aquifer recharge. Thus, the observed increase in 222Rn concentration in that portion of the aquifer is due to the enrichment process caused by infiltration of rainwater (222Rn free) which flows from the local divide area. Hence, in the study site, the use of only 222Rn to predict river–groundwater interactions causes some estimation inaccuracies and it must be coupled with other hydrochemical and hydrogeological parameters to gain a thorough understanding of such interactions.
Résumé
Le 222Rn a été utilisé pour évaluer les interactions nappe–rivière au sein de l’aquifère alluvial de Castel di Sangro (Italie). La qualité des résultats obtenus avec cet indicateur a été vérifiée par l’analyse de δ18O et des ions majeurs, par le contrôle de la température dans les eaux de surface et souterraines et par des mesures de niveaux piézomètriques et de débits. Les investigations hydrogéologiques ont suggéré que la rivière recharge la nappe dans la partie sud-est de l’aquifère, tandis que la nappe alimente la rivière dans la partie nord-est. Ce dernier processus est appuyé par les valeurs de 222Rn. Néanmoins, les conditions de flux continus entrainent un débit modélisé le long de cette partie de la rivière, supérieur à celui mesuré sur le terrain; le débit est estimé par les valeurs de 222Rn dans un modèle de mélange à deux composantes corrigé par le dégazeage. En ce qui concerne l’infiltration de la rivière, les valeurs de δ18O, des ions majeurs et de la température montrent que la contribution de la rivière est négligeable en terme de recharge des eaux souterraines. Ainsi, l’augmentation de la concentration en 222Rn, observée dans cette zone de l’aquifère, est causée par un processus d’enrichissement dû à l’infiltration des eaux pluviales (dépourvue en 222Rn) qui s’écoulent depuis la ligne locale de partage des eaux. En conséquence, sur ce site d’étude, l’utilisation seule du 222Rn pour évaluer les interactions nappe-rivière peut causer des imprécisions sur les estimations et elle doit être couplée à l’étude d’autres paramètres hydrochimiques et hydrogéologiques pour parvenir à une compréhension approfondie de ces interactions.
Resumen
222Rn fue usado para evaluar las interacciones río–agua subterránea dentro de la acuífero aluvial Castel di Sangro (Italia). La efectividad de los resultados obtenidos por medio de este indicador fue verificada al analizar también δ18O, iones mayores y temperatura en aguas superficiales y subterráneas, y realizando monitoreo de cargas hidráulicas y mediciones de descarga. Investigaciones hidrogeológicas sugirieron que el río infiltra el acuífero en su porción sureste, mientras que el agua subterránea se descarga en el río en la porción noreste. Este último fenómeno es respaldado por datos de 222Rn. Sin embargo, las condiciones del flujo provocan que la descarga modelada a lo largo de esta sección del río, estimada por datos 222Rn en un modelo mixto de dos componentes corregido para desgase, sea mayor que la descarga medida. Respecto a infiltración del río en el acuífero, δ18O, iones mayores y temperatura muestran que la contribución del río es insignificante en términos de recarga del acuífero. Entonces, el incremento observado en la concentración de 222Rn en esa porción del acuífero se debe al proceso de enriquecimiento causado por infiltración de agua de lluvia (libre de 222Rn) que fluye desde el área divisoria local. Por lo tanto, en el sitio de estudio el uso de solamente 222Rn para predecir las interacciones río-agua subterránea causa algunas inexactitudes de cálculo y debe ser acompañado con otros parámetros hidroquímicos e hidrogeológicos para alcanzar un conocimiento exhaustivo de tales interacciones.
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Acknowledgements
The authors are grateful to two anonymous reviewers whose insightful comments greatly improved the final version of the manuscript. The authors would like to thank also Dr. Damiano Centioli for his help with Ionic Chromatography measurements and Dr. Eng. Mario Mussi of the Stable Isotope Laboratory of the Geosciences and Georesources Institute of CNR-Pisa for stable isotope measurements (October 2003). Finally, the authors would like to thank the Servizio Idrografico e Mareografico Nazionale of Pescara for providing the meteorological data.
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Stellato, L., Petrella, E., Terrasi, F. et al. Some limitations in using 222Rn to assess river–groundwater interactions: the case of Castel di Sangro alluvial plain (central Italy). Hydrogeol J 16, 701–712 (2008). https://doi.org/10.1007/s10040-007-0263-0
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DOI: https://doi.org/10.1007/s10040-007-0263-0