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hydraulic
investigation of Artesian Groundwater in Eastern-Styria (Austria) - a Field
Study in the Feistritz and Safen Valley
ALEXANDER ARCHIMANDRITIS & VILMOS VASVARI
Technical University Graz, Austria
Institute for Hydraulics and Hydrology
Mandellstrasse 9, A-8010 Graz, Austria
Phone: ++43/316/8736262, Fax: ++43/316/8736264
e-mail: archimandritis@hydro.tu-graz.ac.at
Abstract
Since the
end of 19th century artesian confined groundwater has played a primary role for
the rural potable water supply in Eastern Styria (Austria) because this
groundwater is well protected against antropogenic influences by impermeable
layers.
In order
to get planning documents for the management of these groundwater numerous
investigations, based on geological, geophysical, hydrological and hydraulic
data, were executed.
By means of this data and by the analysis and interpretation of the
piezometric level hydrograph of the surveying places we have tried to simulate
the groundwater flow. The recorded piezometric level hydrograph made it
possible to predicate on the distribution of the storage coefficient in the
investigation area by way of the calculation of the barometric efficiency. The
available data permitted only the creation of two-dimensional flow models.
The model
of the Safe Valley affirms the assumption that the horizons covered in the
investigation area represent a multiaquifer and that there doesn't exist a
direct hydraulic connection between the different horizons. In the Feistritz
Valley a single horizon could be defined by pumping tests. For this horizon the
simulation of the steady contribution was executed.
Keywords:
Artesian well, confined aquifer, piezometric level hydrograph, barometric
efficiency, groundwater modelling,
Due to the complexity of confined groundwater systems the development
requires the systematic execution of different investigations. The complexity
of confined Aquifer in the eastern Styria Tertiary period basin arises mainly
as a result of their limnial fluvial history of the origin.
This article is about the data acquisition and the analyses made for
flow modelling, as well as the model development and the determination of the
general flow conditions which are based on it present. In order to be create
suitable planning documents for the water supply, the Institute for Hydraulics
and Hydrology of the Technical University of Graz has made an extensive
hydraulic investigations on behalf of the Referate for Water Economic Planning
of the Styrian Federal State Government. These investigations covered among
other things also the systematic processing and analysis of the piezometric
level hydrographs of the observations wells.
Because of the complicated geological alternated stratification of
permeable and impermeable layers, the seepage into the aquifer couldn't be
investigated yet. Therefore we tried to characterise the process of the
alimentation from the evaluation and the analysis of the piezometric levels
hydrographs
The
shallow-being situated confined groundwater deposits in the Eastern-Styria
possess a great importance for the domestic and local drinking water supply,
particularly in the rural space. The present situation is marked by substantial
deficits in the management of the ground-water resources. Especially the
private sector shows an extremely wasteful handling of the artesian
groundwater. This situation conflicts with the requirements of a future potable
water supply in this region and requires thus a fundamental reorganisation.
The
confined aquifer relevant for the potable water supply indicates into the
central Eastern-Styria a deep of approx. 30-350 m. The shallow-being situated
confined groundwater supply is out of the question for the drinking water
supply because of their strong contamination, while the groundwater under
approx. 350 m depth is not suitable without previous handling for the potable
water supply due to their considerable mineralization and the increased
temperature. The confined aquifer within the depth range from 30 to 300 m
belongs to Pannonian and Sarmatian formation. The development of the Pannonian
layers had been limnian fluvial, while that one of the Sarmatian layers had
been fluviomarine. Therefore the aquifers situated in the Pannonian show a
strong local variation (inhomogeneity).
In the
area of the Alpine piedmont the Pannon predominantly consists of limnian and
fluvial deposits whereby the tertiary main groove is traversed by the today's
drainage system of the rivers and their feeders in a more or less pointed
angle. The exact position of the buried groove is difficult to determine, since
there had been local displacements, maeander, old branches. As far as the areas
outside of the old river courses it concerns itself with the deposits of the
Pannons around insignificant fine sand lenses in limnial, clayey-silty layers
of partly considerable thickness. The Sarmat which is under the Pannon consists
mainly of brackish water deposits of clayish and gravely layers up to carbonic
positions. The locally different formation and the interlocking of strata can
be seen as the result of the raid and retreat phases of the sea. Palaeozoic
carbonate rocks and Phyllite form the base of the Tertiary period basin. Under
the Palaeozoic is the crystalline basement, whose rocks are also exposed at the
northern basin edge. Both for aquifers in pannonian and sarmatian sediments the
possibility of the recharge exists by Infiltration of surface waters, if these
layers exposed at the surface in the area of today's streamlets. Where the
layers of Baden crop out near the surface as limes or limestones delete, a
locally intensified replenishment of the confined groundwater is given both
directly by precipitation and by Infiltration of surface waters.
The hydrogeological
profiles of the Feistritz- and Safen Valley and the isoline card of the
Pannonian layer thickness show that the aquiferous horizons in this region are
alternating situated in the Pannonian and in the Sarmat. In the Safen Valley
they mainly belong to the Sarmat. In the Feistritz Valley they are
predominantly part of the Pannon. (H. ZOJER, 1987; J. E. GOLDBRUNNER et
al., 1994)
The spreading of
groundwater horizons in the Pannon can not be compared with those in the
Sarmat, where long-range connected surfaces occur. In the Pannonian layers the
aquifer is often locally limited in accordance with fluvial sedimentation and
coarse-grained deposits. That the evenly said acknowledges the difficulty
spacious localisation of the spreading of the aquifer due to a long range network
of observation wells.
Fig. 1 General map of the investigations area with observation network.
Database
The
executed investigations are based on geological, geophysical, hydrological and
hydraulic data.
In the
test drillings of the authority's of water resources management (Fig.1)
geophysical borehole measurements (gamma log, self-potential log, resistance
log) as well as pumping tests and overflow tests under artesian conditions
after drilling of a well have been made. By the evaluation of the pumping test
data the hydraulic characteristic values of the opened horizons such as the
transmissibility and in rare cases also the storage coefficient have been
determined. Because of the short duration of the pumping tests and the limited
draw-off, only in the middle Feisritz valley a direct hydraulic connection
between the drillings have been determined, since the drillings are situated in
a distance of 2 to 5 km from each other.
The
geophysical logs were correlated in order to determine the propagation of the
water-bearing horizons and the connections between the drillings.
Further
more, at certain wells water level recorder were installed, in order to record
the variations of the piezometric level continuously and on a long-term basis and to be able to deduce eventual trends
from these recordings (Fig.2). Most piezometric level hydrographs show seasonal
fluctuation, where the amplitudes amount to about 0.15 to 1.50 m.
Fig. 2 Piezometric level hydrograph in observation
well Geiseldorf in the period
between 1980 and 1997
The
recorded piezometric level hydrographs permit also the determination of the
storage coefficient with the barometric efficiency and estimating the porosity.
(DE WIEST, 1965).
|
|
S storage
coefficient H saturated
thickness g specific
weight of water b
compressibility of water BE barometric
efficiency |
For the
determination of the barometric efficiency, the statistical method of
CLARK (1967) was used. The barometric efficiency of the confined aquifers
are between 5 and 15 %, but they show no significant dependence of depth.
The
piezometric level hydrographs were correlated both with each other, and with
the run-off hydrograph of the surface waters, as well as with the measurement
of different gages. In addition to that the daily averages of the piezometric
level and the daily averages run-off and the daily precipitation were
consulted. The investigations did not prove any significant relation between
the piezometric level hydrographs of neighbouring wells, but some drillings
show a certain dependence of the piezometric level from the surface run-off.
The investigation concerning the correlation of the daily precipitation and the
daily averages of the recorded piezometric levels, produced contradictory
results. Taking response times (0-10 days) into consideration the calculation
resulted in strongly varying coefficients of correlation. In this way neither
the alimentation could be localised nor a significant influence of the
precipitation could be observed. Through the analysis of the
"precipitation efficiency (PE)", ÜBERWIMMER (1992) had succeeded in
proving a possible statistical relation between precipitation and piezometric
levels. At the same time he had found an appropriate explanation for the
continuous renewal of the groundwater resources. The total amount of water,
taken from the groundwater reservoirs by local and private wells had formed an
important part of the investigations, because the represent the output of the
entire system.
Hydrogeological model
The
geological exploratory works, the well sections, the geophysical borehole
measurements, as well as the hydraulic investigations lead to the conclusion
that the groundwater reservoirs in central Eastern Styria represent a
multiaquifer formation.
In the
basin inside the aquifers are seperated. They are only at the rimes of the
basin interlocked, and therefore in direct hydraulic connection. The existence
of geological windows in the basin inside is possible, so that the layers,
which are cropping out, can be alimentated by the surface water.
The
circulation of confined groundwater is determined by the gaining stream, which
represents the natural and usually not recognisable drainage of artesian
groundwater system. In the Eastern Styrian basin the function as gaining
streams can only be assumed by the rivers. This requires a hydraulic connection
between horizons of different depths, by migration of groundwater in sediments
of certain permeability, up to surface streams. The migration doesn't depend on
the permeability of the aquifer, but of the effective velocity rate, which
results from the reduced discharge. If there is a difference between the
specific discharge, determined by the gradient and the permeability, and the
retention period, the withdrawel of a relatively high quantity of water is
possible, because nature doesn't take advantage of this conductivity (ZOJER,
1987).
Numerical
groundwater flow models
Because of
the available data base we had to renounce the development of a multi-layer
model, because the hydraulic connection of the horizons as well as the boundary
conditions for each horizon could not be proved. Therefore for the works area
of the Feistritz valley and the Safen valley a single layer model was been
developed. For this model the opened horizons has been summarised as an
equivalent aquifer by presuming an average depth.
Feistritz
valley
In the
middle Feistritz valley there is a hydraulic connection between four wells.
This is proved by the fact that draw-off attempts show mutual influences(Fig.
3).
Fig.3:
Flow net in the middle Feistritz valley with the steady draw-off of 4l/s and
increased inflow.
For this
horizon the steady draw-off which had been determined by pumping tests have
been confirmed and the recharge could be localised (NW) and quantified as a
function of the draw-off. The model calculations permit quantitative statements
about the expansion of the aquifer in E-direction (BERGMANN et al., 1993).
The model for the
Safen valley (Fig. 4) shows that the inflow in the northern model area comes
from NW direction (at the basin rim from precipitation) whereas the inflow in
the middle area comes from W-SW-direction (from the valley of the Feistritz). The
southern model area had to be assumed as a hydraulically separated system. With
the withdrawal presently permitted and a sufficient supply in the model area a
steady state flow would come up after 18-22 months.(VASVARI, 1996)
Conclusion
Because of
the lack of data the models has been hydraulically simplified. Nevertheless
they made it possible to estimate the underground water conversions. Moreover
they do tell us in which areas further wells are to be established, in order to
be able to describe the boundary conditions of the groundwater flow more
precisely. The most important condition for a 3D - modelling is the knowledge
of the distribution of the hydraulic conductivity relating to the depth of the
aquifer - systems. This can be determined with the help of the resistivity of
the rocks by correlating it with the results of the pumping tests.
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