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| Titel |
Assessment of transport parameters in a karst system under various flow periods through extensive analysis of artificial tracer tests |
| VerfasserIn |
J. Doummar, A. Margane, M. Sauter, T. Geyer |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250067269
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| Zusammenfassung |
| It is primordial to understand the sensibility of a catchment or a spring against contamination
to secure a sustainable water resource management in karst aquifers. Artificial tracer
tests have proven to be excellent tools for the simulation of contaminant transport
within an aquifer before its arrival at a karst spring as they provide information
about transit times, dispersivities and therefore insights into the vulnerability of a
water body against contamination (Geyer et al. 2007). For this purpose, extensive
analysis of artificial tracer tests was undertaken in the following work, in order
to acquire conservative transport parameters along fast and slow pathways in a
mature karst system under various flow conditions. In the framework of the project
“Protection of Jeita Spring” (BGR), about 30 tracer tests were conducted on the
catchment area of the Jeita spring in Lebanon (Q= 1 to 20 m3/s) under various flow
conditions and with different injection points (dolines, sinkholes, subsurface, and
underground channel). Tracer breakthrough curves (TBC) observed at karst springs
and in the conduit system were analyzed using the two-region non-equilibrium
approach (2NREM) (Toride & van Genuchten 1999). The approach accounts for the
skewness in the TBCs long tailings, which cannot be described with one dimensional
advective-dispersive transport models (Geyer et al. 2007). Relationships between
the modeling parameters estimated from the TBC were established under various
flow periods. Rating curves for velocity and discharge show that the flow velocity
increases with spring discharge. The calibrated portion of the immobile region in the
conduit system is relatively low. Estimated longitudinal dispersivities in the conduit
system range between 7 and 10 m in high flow periods and decreases linearly with
increasing flow. In low flow periods, this relationship doesn’t hold true as longitudinal
dispersivities range randomly between 4 and 7 m. The longitudinal dispersivity
decreases with increasing flow rates because of the increase of advection control over
dispersion and increasing dilution. Therefore variance of the TBC is controlled on the
hand by dispersivity during high flow periods and on the other hand by increasing
mobile phase in low flow periods due to an increase of the portion of immobile
zones (pools and ripples) as water level decreases. For tracer tests with injection
points at the surface, longitudinal dispersivities are found to be of higher ranges
(8–27 m) and highly reflective of the compartments in which the tracer is flowing
(unsaturated rock matrix, conduits or channel). The comparison of tracer tests with
different injection points shows clearly that the tailing observed in some of the
breakthrough curves is mainly generated in the unsaturated zone before the tracer
arrives to the main channel draining the system and decreases gradually within the
channel.
Geyer, T. , Birk S., Licha T., Liedl R., Sauter M. (2007): Multitracer Approach to
Characterize Reactive Transport in Karst Aquifers. Groundwater, Vol. 35, No 1.
35-45.
Toride, N., Leij, F.J., van Genuchten, M.T., 1999. The CXTFIT code (version 2.1) for
estimating transport parameters from laboratory or field tracer experiments. U.S. Salinity
Laboratory Agricultural Research Service, U.S. Department of Agriculture Riverside,
California. Research Report 137. |
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