 |
| Titel |
Transport of environmental tracers through a karst system with a thick unsaturated zone |
| VerfasserIn |
Tobias Geyer, Jürgen Sültenfuß, Florian Eichinger, Martin Sauter |
| Konferenz |
EGU General Assembly 2010
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250037881
|
|
|
|
|
|
| Zusammenfassung |
| The transport of the environmental tracers tritium (3H), krypton-85 (85Kr) and helium (3He)
in a karst system is investigated. Differences between mean tracer ages determined in spring
water are explained by slow percolation of water through the thick unsaturated zone
reflecting the importance of slow and diffuse unsaturated flow processes in these
systems.
Mean tracer ages on the Gallusquelle spring (Swabian Alb) were determined with lumped
parameter modeling and decrease in the following order: 3H >> 85Kr > 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
results indicate a long travel time of 3H through the unsaturated zone of the karst
system.
The interpretation is supported by a two-dimensional numerical simulation of flow and
transport in a fissured matrix block that contains a thick unsaturated zone (ca. 100 m) and is
drained by a conduit. Transport simulation is performed in the sense of backtracking, i.e. the
flow field is reversed, and the boundary conditions are adapted accordingly. At any position in
the model domain, the time required for a water molecule to reach the outlet is
estimated corresponding to the "life expectancy" (Cornaton and Perrochet 2006), i.e. the
life expectancy on the outlet is zero. The simulation of life expectancy of water in
the matrix block shows (1) the importance of heterogeneities for interpretation
of groundwater ages, (2) the location of stagnant zones in areas of low hydraulic
permeability and/or low hydraulic gradient and (3) that flow through unsaturated
fissured matrix blocks may cause a considerable travel time of water through a
karst system. The travel time of water from the recharge area to the discharge point
for the shown example is about 15 years with a travel time of water through the
unsaturated zone of 10 years (Geyer 2008). This result reflects the variation of
estimated ages for different tracers sampled at the Gallusquelle spring. Additionally,
we demonstrate 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
small.
Literature
Cornaton, F., Perrochet, P. (2006): Ground-water age, life expectancy and transit time
distributions in advective-dispersive systems: 1. Generalized reservoir theory. – Advances in
Water Resources 29 (9): 1267-1291.
Geyer, T. (2008): Characterisation of flow and transport in karst aquifers at catchment
scale, Ph.D. diss., Georg-August-Universität Göttingen, 103 pp. |
|
|
|
|
|
|