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Lagged rejuvenation of groundwater indicates internal flow structures and hydrological connectivity |
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| Authors: | Tamara Kolbe Jean Marçais Jean-Raynald de Dreuzy Thierry Labasque Kevin Bishop |
| Institution: | 1. Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Science, Uppsala, Sweden;2. Institut de Physique du Globe de Paris, Université de Paris, Paris, France
INRAE, UR RiverLy, F-69625 Villeurbanne, France;3. Géoscience Rennes—UMR 6118, Université de Rennes, Centre National de la Recherche Scientifique (CNRS), Rennes, France Observatoire des Sciences de l'Univers de Rennes (OSUR)—UMS 3343, Université de Rennes, Centre National de la Recherche Scientifique (CNRS), Rennes, France;4. Géoscience Rennes—UMR 6118, Université de Rennes, Centre National de la Recherche Scientifique (CNRS), Rennes, France |
| Abstract: | Large proportions of rainwater and snowmelt infiltrate into the subsurface before contributing to stream flow and stream water quality. Subsurface flow dynamics steer the transport and transformation of contaminants, carbon, weathering products and other biogeochemistry. The distribution of groundwater ages with depth is a key feature of these flow dynamics. Predicting these ages are a strong test of hypotheses about subsurface structures and time-varying processes. Chlorofluorocarbon (CFC)-based groundwater ages revealed an unexpected groundwater age stratification in a 0.47 km2 forested catchment called Svartberget in northern Sweden. An overall groundwater age stratification, representative for the Svartberget site, was derived by measuring CFCs from nine different wells with depths of 2–18 m close to the stream network. Immediately below the water table, CFC-based groundwater ages of already 30 years that increased with depth were found. Using complementary groundwater flow models, we could reproduce the observed groundwater age stratification and show that the 30 year lag in rejuvenation comes from return flow of groundwater at a subsurface discharge zone that evolves along the interface between two soil types. By comparing the observed groundwater age stratification with a simple analytical approximation, we show that the observed lag in rejuvenation can be a powerful indicator of the extent and structure of the subsurface discharge zone, while the vertical gradient of the age-depth-relationship can still be used as a proxy of the overall aquifer recharge even when sampled in the discharge zone. The single age stratification profile measured in the discharge zone, close to the aquifer outlet, can reveal the main structure of the groundwater flow pattern from recharge to discharge. This groundwater flow pattern provides information on the participation of groundwater in the hydrological cycle and indicates the lower boundary of hydrological connectivity. |
| Keywords: | CFCs groundwater age stratification groundwater recharge hillslope storage Boussinesq equations Krycklan subsurface discharge subsurface hydrological connectivity |