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Research article - Peer-reviewed, 2020

Lagged rejuvenation of groundwater indicates internal flow structures and hydrological connectivity

Kolbe, Tamara; Marcais, Jean; de Dreuzy, Jean-Raynald; Labasque, Thierry; Bishop, Kevin

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 km(2) 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

Published in

Hydrological Processes
2020, volume: 34, number: 10, pages: 2176-2189
Publisher: WILEY

Authors' information

Kolbe, Tamara
Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment
Kolbe, Tamara
Freiberg University of Mining and Technology
Marcais, Jean
Univ Paris
de Dreuzy, Jean-Raynald
Univ Rennes
Labasque, Thierry
Univ Rennes
Swedish University of Agricultural Sciences, Department of Aquatic Sciences and Assessment

Associated SLU-program

Acidification

Sustainable Development Goals

SDG6 Clean water

UKÄ Subject classification

Oceanography, Hydrology, Water Resources

Publication Identifiers

DOI: https://doi.org/10.1002/hyp.13753

URI (permanent link to this page)

https://res.slu.se/id/publ/105692