The role of a thick unsaturated zone for transport of environmental tracers in karst systems
Abstract
In this study transport of the environmental tracers tritium (3H), krypton-85 (85Kr) and helium (3He) in a karst aquifer system is interpreted. Differences between mean tracer ages determined on a karst spring are explained by slow percolation of water through a thick unsaturated zone reflecting the importance of slow and diffuse recharge in these systems. Mean tracer ages were determined with lumped parameter modelling and they range between a few and several tens of years. The different ages depend on the analytical method and decrease in the following order: 3H >> 85Kr > 3H/3He. Since 3H is part of the water molecule it enters a karst system via precipitation, i.e. the mean 3H age is a measure of water flow through the whole karst system, including the unsaturated and saturated zone. The mean 85Kr age and 3H/3He age are measures of time since groundwater recharge arrived at the water table. Therefore our findings indicate a long travel time of 3H through the unsaturated zone of the karst system. This interpretation is supported by a two-dimensional numerical simulation of flow and transport in a fissured matrix block that contains a thick unsaturated zone and is drained by a conduit. The simulation provides an insight into the distribution of life expectancy of water, i.e. the time remaining before a water particle leaves the aquifer, in a heterogeneous medium. It demonstrates that depending on boundary conditions, the unsaturated zone of a karst system may provide a large water storage since the porous matrix can be expected to be close to saturation and the volume fraction of fissures and conduits is very small. Literature Geyer, T. (2008): Characterisation of flow and transport in karst aquifers at catchment scale, Ph.D. diss, Georg-August Universität Göttingen, 103 pp.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.H11A0787G
- Keywords:
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- 1804 HYDROLOGY / Catchment;
- 1847 HYDROLOGY / Modeling;
- 1875 HYDROLOGY / Vadose zone