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ESTIMATED MEAN WATER
RESIDENCE TIME (d 18O) OF
KARSTIC SPRINGS AND KARST SYSTEM DEVELOPMENT
Geology
Institute, Swiss Federal Institut of Technology, Zürich, Switzerland
Water
Management Authority, Canton of Solothurn, Switzerland
DR.
THILO HEROLD
Centre d'Hydrogéologie, Université de Neuchâtel
Rue
Emile-Argand 11
2007
Neuchâtel, Switzerland
thilo.herold@chyn.unine.ch
The Weissenstein Anticline is located 100 km northwest of Bern, in the
south-east part of the Folded Jura (Switzerland). The anticline contains two
large aquifers, the Dogger limestone and the Malm limestone. The two aquifers
are separated by impermeable layers. The anticline is hydrogeologically bounded
by the Weissenstein-tunnel in the west and the Oensingen gorge in the east. Geometric
modelling and the results of the multiple tracer experiments showed that pre-
and synorogenic faults strongly influence the pattern and interconnection of
karst systems.
During the folding of the anticline, the erosion started at preferential
locations which were connected to preexisting fault systems. Thus we suspect a
paleo-geomorphologic pattern which influenced the development of limestone fracture
systems in present day karst systems.
To test this hypothesis, the largest karst springs in the Weissenstein
Anticline were continuously measured for discharge, temperature and
conductivity for several years. In addition, weekly samples of spring waters
and the daily precipitation were measured for stable Isotpes (18O) to determine annual variations. As
demonstrated in this paper, with the Malm karst spring database, it was
possible to estimate, as the first step, the mean residence time of the spring
water. The water from individual sampling points showed characteristic
differences in residence time along the valley from east to west. Given the
spring water residence times we believe that maturity of karstic features in
the Weissenstein Anticline increases from east to west as a result of the
structure's geological and paleo-geomorphological evolution.
The Jura fold belt developed by thin-skinned tectonic activity into an
arc 390 km long and 250 km wide along the French-Swiss border. The upper part
of the belt overlies a crystalline basement complex, which was brittly deformed
during the Variscan orogeny. Late Palaeozoic sedimentation infilled troughs in
the Basement (Diebold et al. 1991). Mesozoic and earliest Cainozoic sediments
overlie the Permo-Carboniferous deposits. These later strata began to be folded
in the late Miocene as a result of the Alpine orogeny (Laubscher 1961, 1972,
1985). The structure of the anticline, synclines as well as present day active
karst systems were controlled by the reactivation of pre-existing WSW-ENE
trending Late Paleozoic structures and NNE-SSW trending early Tertiary
structures (Herold 1998).
Figure 1: Short tectonic history sketch of the study area and surrounding regions (strongly modified after Diebold et al. 1991).
Contemporaneous to the folding, erosion started at locations connected
to the tectonic structures. In the course of time, a geomorphological pattern
developed which had an increasing influence on the development of the karst
systems.
A part of the
Folded Jura was selected for additional karst hydrogeological investigations
since existing studies have already defined geological structures in the area
(Bitterli, 1990; Laubscher & Hauber 1982; Meier 1977; Thyry et al., 1994).
The area
investigated is called the Weissenstein Anticline. The anticline is 22 km long
by 8 km wide. The western part of the anticline is hydrogeologically bounded by
the Weissenstein-tunnel, and the eastern part by the Oensingen gorge.
Data collection
Quarterly
groundwater monitoring for 18O in spring water began in autumn 1993.
Since 1996 the samples have been collected weekly.
Starting
from 1995 daily precipitation was monitored for 18O. Two additional
stations were added to the precipitation network at the start of 1996.
The 18O
analyses were carried out at the Stable Isotope Laboratory of the Geological
Institute ETH-Zürich. The 18O values are expressed with reference to
the Standard Mean Ocean Water (SMOW) in units of parts per thousands (per mil).
Lumped-parameter
flow models are analytical, steady state and one-dimensional concepts
describing the transformation of a given tracer input (concentration in
precipitation or air Cin) into the tracer output (concentration at
an outflow site Cout) within a continuous flow system (Maloszewski
and Zuber, 1982; Amin and Campana, 1996). For environmental isotopes such as
tracers, this expression takes form of a convolution integral with a system
response function thereby giving the expected residence time distribution:
Cout (t) = 
Cin (t-T)
g(T) exp (-lT) dT
t = chronological time
T = residence time
l
= decay rate for radioactive isotopes
Different flow systems exist in karst regions. As Rank et al. (1992)
demonstrated for a karstic region in an alpine karst massif, slow tracer
transport through a fissured-porous system can be described by the dispersion
model. For fast tracer transport through the direct flow path in the drainage
channels, between sink holes and springs, the piston flow model can be used.
For mean
residence time modeling of karst spring water in the study area, Maloszewki's
and Zuber's, 1996, two-parameter exponential/piston-flow model was used. The
daily local 18O rainfall record has been calculated into a monthly
weightened rainfall quantity record. Precipitation values below 4 mm/d were not
considered. Furthermore this record was completed for the period January 1990
until March 1995 using monthly values from IAEA/WMO station in Bern.
18O
levels in the Hun spring, which is located at the upper end of Oensingen gorge
(fig. 4), fluctuate annually between 1996 and 1997 (fig. 2). A curve of spring
water 18O values can be modeled and fit to the observed data (fig.
3). 18O results suggest that the mean spring water residence time is
approx. 20 months.
The
Chaltbrunnen spring is located 5 km west of the Hun spring. The annual
amplitude of the 18O variation is lower, which indicates a larger
retention time compared to the Hun spring. As shown in figure 3, the modeling
could be adapted for the winter 96/97 and 97/98. It indicates an age of approx.
40 months for the mean water residence time. For winter 95/96 an adjustment was
not possible. 18O values suggest that different processes operate at
Chaltbrunnen spring. For better model
calibration, better weighting of the inputs (precipitation) would have to be
applied and considered with respect to the geological setting.
Figure 2:
Annual variation of 18O
values of the most important springs in the Malm limestone.
Figure 3:
Mean water residence time calculations (curves) fit to measured 18O
spring water values.
The Hammer
spring, 5 km west of Chaltbrunnen spring, shows no significant change in 18O
content between 1995 and 1997. Consequently, modelling of the mean residence
time using 18O is not possible. Compared with the other two springs,
the Hammer spring has a longer mean water residence time.
In the study area the tectonic history plays a key factor in
establishing the location, direction and extension of the karst drainage system
(Herold 1998). As shown in figure 4, the largest springs along the hillside of
the Weissenstein-anticline are always connected to NNE-SSW striking thrust
zones. These thrust zones even connect deep karst systems in the core of the
anticline to the limbs, cutting through thick impermeable layers.
In addition to the tectonic controls, another important factor for karst
development is the paleo-geomorphological evolution. During the folding of the
Jura Mountains, erosion started at preferential locations leading to the
stepwise development of the present gorges (e.g. Oensingen gorge) which
orthogonally crosses the anticlines. These gorges control surface water
drainage, which flows from the Jura Mountains into the Mittelland. This
evolution influenced the genesis of the karst systems, which migrated from east
(Oensingen gorge) to west, parallel to the northern side of the
Weissenstein-anticline.
Our data indicate that because the erosion started in the Gorge of
Oensingen and progressed parallel to the anticline to the west, the fault
network connected to the Gorge was the first to be drained. The flowing water
enlarged the preexisting network of fractures. For this reason, springs in this
area have a low mean residence time (Hun spring).
As erosion propagated along the valley, the western fracture systems
were progressively connected to the older part of the karst, and became drained
by the flow system within the valley and the gorge. The last fracture system to
be connected to the flow system includes the Hammer spring catchement. Because
this was the last of the fracture systems influenced, less time was available
for the karst system development. The water of this spring (Hammer spring)
flows through a highly fractured, but poorly developed karst system, so that
the highest mean residence times are observed at this location.
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