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CONSEQUENCES OF OPEN-PIT COAL MINING ON THE GEOHYDRAULICAL
SYSTEM (CASE STUDY)
Institute
of Rock Structure and Mechanics, AS CR
V Hole?ovickách 41, 182
09 Praha 8, Czech Republic
Phone:
+420-2-3121748, Fax: +4202/6880649 e-mail:
trckova @irsm.cas.cz
ABSTRACT
During the brown coal
exploitation in the Sokolov basin, the regime of the gas-rich thermal confined groundwater is frequently
affected. Since the water inrushes in the shaft Jáma V of the mine Marie in the
beginning of this century, the thermal water had been drained from this shaft in order to enable the coal to be exploited by underground mining.
During the last decade, the thermal water is pumped from the coal open-pit
mine Jirí in order to provide the geotechnical
safety of open-pit mining. This contribution reviews the information obtained
by the evaluation of regime measurements in boreholes within the region
affected by pumping - formation and development of depression cones, changes of the water chemism in relation
with drainage.
Keywords: Thermal confined
groundwater, reduction of the confined level, depression cone, longitudinal
profiles, water chemism
INTRODUCTION
The brown-coal basin of Sokolov lies in the
neighbourhood of the Karlovy Vary Spa and bedrock consists of granites of the
Carlsbad Pluton and crystalline shists. The basal complex of strata Staré Sedlo is characterized by an irregular
development of sandstones,
conglomerates and quartzites.
The oldest representative of the organogenic sedimentation is the coal seam
Josef with thickness of up to 10 m. In the overlying rock of the seam Josef, an
impermeable volcanodetritic strata series, consisting of tuffs, clays and
claystones with admixtures of tuffitic materials is deposited. This strata
series forms the confining layer of the mineralized thermal gas-rich water with confined level, partly bound to
the seam Josef, but namely to the strata series Staré Sedlo and upper part of the crystalline complex..
Water temperatures vary within the interval of 250-400
C, the total dissolved solids (TDS) between 6000-9000 mg.l-1. The
water belongs chemically to mineral waters
of the "Carlsbad type" (Dvorák
1990). As to the gas content, the thermal water is unsaturated, containing up
to 4500 mg.l-1 of the carbon
dioxide and 135 mg.l-1 of
the nitrogen. The saturation pressure varies
within 0.1-0.75 MPa (Jetel, 1972). Above the volcanodetritic layers with thickness of 40-80 m, the main coal
seam Antonín is deposited (thickness of up to
50 m). The roof stone consists of
Cypris claystones with thickness of up to 135 m. An illustrative vertical section through the
coal basin is included in the article of
J. Trcková (in these
Proceedings).
Towards the end of the last
century and in the first half of the present century, the seam Josef has been
exploited by underground mining. The
water-bearing strata Staré Sedlo and
crystalline rocks were hazardeous for the mining activities namely at sites,
where the base of the seam Josef was
separated from the water-bearing bedrock by only thin clay layers (or
directly deposited on underlying strata series and crystalline rocks),
and where an overpressure of thermal water effected. The exploitation took
place under such conditions at the beginning of this century, when important
water inrushes with subsequent flooding of mine openings occurred.
The coal seam Antonín is
exploited by open-pit mining. During
mining, the gas-rich thermal
water effects overpressure on the impermeable underlying strata of the
coal seam and endangers the stability of the open-pit bottom. Owing to the dip
of these strata, the situation arises, when
the mine bottom becomes situated
deeply below the confined level of thermal water.
Sediments of the Sokolov
basin are dislocated by significant tectonic faults of the general direction
NE-SW and several transversal and
oblique discontinuities, which reduce the strength of the volcanodetritic strata.
From the viewpoint of the geotechnical safety of mining, the most critical
situation exists in the open-pit mine
Jirí,
where the uncovered overpressure of thermal water would attain, without local
reduction of the confined level of this
water, up to 0.6 MPa at the uncovered bottom.
INTERFERENCES
INTO THE GROUNDWATER REGIME
Until the year 1990, the
groundwater regim within the basal aquifer and namely in the aquifer of the
seam Josef was decisively affected by
pumping of the thermal water from the shaft Jáma V of the mine Marie, which became
the drainage centre of the Sokolov basin already in the beginning of our
century (Fig.1). After the thermal water
inrush into the mine Marie in the Octobre 1901, when the mine waters attained the level of the seam
Antonín
and flooded the mine workings, the drainage of the seam Josef has begun. From
the shaft Jáma
V, the thermal water of the "Carlsbad type" was pumped out in the volume of
5-30 l.s-1. The water level
in the shaft Jáma
V was maintained, by pumping, at the level of 328 m above sea level. The form of the depression cone was observed in boreholes in the environs of
the pumping centre. In these boreholes, both the movements of water and that of the gaseous phase were observed. The
condition formed shortly after the water inrush remained stable, with only
small changes, until recent time, when the shaft Jáma V was used for drainage
(Fig. 2). Long-term measurements of water levels in boreholes proved a
considerable independence of the aquifer of the Josef seam, where the
depression cone, caused by pumping of the thermal water on the shaft Jáma V becomes
significantly felt (Pazdera, 1980).
During the last years,
prior to plugging operations of the old flooded mine workings in the seam Josef,
some "wild" leakage of water due to the untightness of the
watertight curtain around the shaft walls took place. This made the possibility
to obtain exact data of the yield of pumped waters more difficult.
Plugging was realized in the years
1990-1991. This terminated the pumping period of water from the shaft Jáma V and its
liquidation by flooding has been started.
In spite of the fact that
the pumping of the thermal water from the shaft Jáma V has been in operation
uninterrupted during 85 years, the confined level of water in boreholes into
the strata Staré
Sedlo and crystalline complex has not been practically affected within the
broader environs of the shaft.
Fig.1.
Scheme of the Sokolov basin. 1 -
open-pit mine bottom, 2 - coal face 3 - overburden, 4 - internal dump
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Fig.2.
Development of the depression cone with the pumping centre in the shaft Jáma V of the
mine Marie |
Fig.3.
Development of the depression cone in the open-pit mine Jirí |
PUMPING IN THE OPEN-PIT
MINE JIRÍ
In 1989, the reduction of
the pressure head of the thermal water of the basal aquifer in the area of the
open-pit mine Jirí was
initiated (Fig.1). The pressure reduction by pumping of thermal water is required not only for the prevention of
water leakage by untightness of the underlying rock, namely in the
neighbourhood of tectonic faults, but also to reduce deformation of the mine
bottom below the allowed value at
sites, where significant overpressure of thermal water exists (Skorepová, 1998).
Under extremely adverse conditions, a bottom break-through and flooding of the
open-pit mine could happen.
Simultaneously with the
proceeding local drainage, a system of observation boreholes is realized in the
mine area and its neighbourhood, where changes of the level of thermal water,
temperature, content of carbon dioxide
and the chemism of thermal water are
recorded regulary (Pazdera, 1980-1995).
Under the open-pit bottom, drainage wells are used for local reduction
of the confined level of this water.
After starting the
reduction of the confined level of water, the pumped water volume varied around
16.0 l.s-1. Actually, due to
complex geological conditions (deepest deposition of the exploited coal seam),
the quantity of pumped water was increased up to 32.0 l.s-1. At the observation boreholes on the mine bottom,
the reduced confined level of water varied in 1997 around 320 m a.s.l.,
which is by almost 40 m lower than
prior to the commencement of pumping.
From regular regime measurements of levels in observation boreholes into basal aquifer Staré Sedlo and crystalline complex not only
within the area of the open-pit mine Jirí, but also in boreholes in the near and
more distant environs of the mine, the quick development of the depression cone
becomes evident (Fig. 3). Lowering of
pressure heads can be observed at ever
increasing distances from the drainage centre. This is confirmed also by longitudinal profiles of the thermal
water pressure heads, drawn through the open-pit mine Jirí and boreholes in its environs. There,
lowering of the pressure head was observed in the borehole HJ2, which lies in the distance of more than 7 km from
the mine Jirí, too (Fig.
4). During the last 6 years, the level decreased by more than 5 m.
Fig.4.
Longitudinal profiles - confined level of the thermal water in the open-pit
mine Jirí and its
neighbourhood
WATER CHEMISM IN RELATION
TO DRAINAGE
Chemical analyses of water
from inrushs in the mine Marie proved
their relationship with the chemical composition of the thermal water of
Karlovy Vary Spa. Owing to the subsequent long-term decrease of the yield
of the Karlovy Vary springs
(1902-1907), the assumption was
formulated of the connection of thermal water in the basin with springs in
Karlovy Vary. Due to this assumption,
long-term regular observations of changes of chemism, temperature and carbon dioxide contents during the
underground and namely the open-pit mining of coal have been realized. Until
the actual time, the connection of thermal water could neither be proved, nor disproved.
Investigations in the
adjoining areas (Cheb basin, Slavkovský les Mts) brought information about the
extension of mineral waters of the "Carlsbad type" (Hanzlík, Krásný 1998, ?mejkal, Paces 1992). Such
a type is characterized by the composition of Na-SO4-HCO3-Cl,
which changes according to geological position, affecting both the
mineralization and ratios of individual
ions (Dvorák 1990). This
is evident in the region of the open-pit mine Jirí and its environs, where hydrocarbonates
prevail mostly over sulphates in both the drainage and observation boreholes.
An alteration of the ratio of ions in water is conditioned by the inflow of the
descending oxidation water from the shallow aquifer into deeper parts of the
basin. This water inflows mostly from
SW (Jetel 1994). The process can be accelerated by the drainage effect of
open-pit mining of coal and by intensive drainage of thermal water from the
basal aquifer. For the scope of preventive protection of thermal water in
Karlovy Vary, changes of the chemism of thermal water are regularly observed at
the spillway of drainage wells (Fig. 5). Fig. 6 illustrates changes of the chemism of this water at
selected boreholes, related to the drainage intensity (l.s-1).
Changes are expressed in coefficients of SO4/Na and Cl/Na (mg.l-1)
as characteristic components of mineral water of the "Carlsbad type". Contents
of Na show only small changes. For comparison, water characteristics from the
borehole HU-1 (A) bored in the centre of
occurrence of thermal water in the basin (1960), water from the mine
Marie (B) and water from the spring Vrídlo (C) in Karlovy Vary are illustrated.
Boreholes, which have been used for drainage purposes for longer than 7 month, were chosen to this purpose. Water
pumping from individual wells is not uniform, depending on the local structural
conditions.
It results from the diagram
in Fig. 6 that a significant correlation between the pumping intensity of
thermal water and its chemism changes cannot be proved. A minimum variance is
shown in the ratio of chloride and sodium ions within the entire set of
drainage wells. The ratio of sulphate
to sodium ions shows a somewhat higher
variance in individual wells. A certain dependence on the pumping
intensity of thermal water can be seen in the borehole OV36, where contents of
main ions decreased by 20% during the drainage period, i.e. from August 1995 to
January 1997. The TDS of the pumped water varies between 6000-9000 mg.l-1.
depending on the position of borehole in relation to flow paths of the thermal
water within the basal aquifer. The temperature of the pumped water varies from
330 to 360C.
The position of the
borehole OV13 represents the extension of thermal water with the chemism Na-SO4,
with TDS of about 1000 mg.l-1 and temperature 290C. An
identical water type is found in the borehole OV15 with TDS of 3300-3600 mg.l-1 and temperature
of 310C, which suggests the
position closer to the centre of occurrence of thermal water. These boreholes
were not used for intensive pumping of thermal water. In this water, the effect
of descending sulphate waters from the
basin borders to the centre from NE
can be traced (Jetel 1964).
Fig.6.
Changes of thermal water chemical composition to the drainage. A - water from borehole HU-1, B - water from the mine Marie, C - water from spring Vrídlo
CONCLUSION
Results of systematic
regime measurements, carried out on long term both on boreholes in environs of
the shaft Jáma V and in the area of the open-pit mine Jirí and its
neighbourhood, confirm the opinion of Pazdera (1980) about the separation of
the water-bearing collector of the seam Josef from the basal aquifer of the
strata Staré
Sedlo and the crystalline bedrock. It is assumed that the aquifer of the seam
Josef has not its own infiltration territory, being probably supplied by water from the basal
water-bearing collector. This results from the different development of the
depression cones in the environ of the shaft Jáma V and in the open-pit mine Jirí. While
long-term pumping of water from the seam Josef
in the shaft Jáma V did practically not affect the confined levels in
wells to the strata Staré Sedlo and crystalline bedrock
within wider environs of the shaft, the
ten-years lasting pumping of water from the bedrock of the seamj Josef in the
open-pit mine Jirí produced a
significant reduction of confined levels not only in the neighbourhood of the
open-pitg mine, but also in very distant boreholes, such as the borehole HJ2.
Actually, when the drainage centres in the open-pit mine were shifted - with
progressive mining - towards W and thus
move away from both the borehole HJ2 and Karlovy Vary, the level in the
borehole HJ2 stopped decreasing. It may be assumed that the hydrological
conditions stabilized and the water supply to the basin suffices to cover the
quantity of the pumped water. This assumption is meanwhile not sufficiently
verified due to the very short period
during which the decrease of the
piezometric head in the borehole HJ2 has been observed. There may exist the danger, that further pumping of water in
the open-pit mine Jirí
could expand the depression, and that even in the direction of Karlovy Vary.
The intensive drainage of
gas-rich thermal water from the basal aquifer does not show up, until now, by
changes of its chemical composition, resp.on the decrease of the characteristic elements and the TDS.
This results suggests the autonomy of the territory of occurrence of thermal
water in the basin in spite of significant interventions into hydrogeological
conditions due to the open-pit mining of coal.
This work was funded by the
Grant Agency of the Czech Republic
(Grant No. 205/97/0783 "Impacts of brown-coal mining on the water-bearing system of Sokolov coal
basin with aspects of Karlovy Vary
thermal springs protection"). The authors gratefully acknowledges for this
support.
REFERENCES
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Western Bohemia. Fysiatrický vestník, 68, No. 1,
237-244 (in Czech)
Hanzlík J., Krásný J. (1998) Brine occurrence in the Czech-German border
region and their palaeohydrogeology.
Proc.Hard Rock Hydrogeology in the Bohemian Massif. 3rd
Workshop,28-31.Oct.1998, Windischeschenbach, (in print)
Jetel J. (1964) Hydrochemical zoning of the central part of
the Sokolov basin in Western Bohemia. Vestník
ÚÚG
XXXIX, 381-383 (in Czech)
Jetel J (1972)
Hydrogeology of the Sokolov basin (Function of rocks, hydrogeochemistry,
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New information on the hydrogeology of the Sokolov region. Geol.Pruzkum, 22,
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Pazdera A. (1989, 1990, 1991, 1992, 1993, 12994, 1995).
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?mejkal
F.,Paces
T. (1992) Origin of the "Carlsbad type" mineral water. Geologický pruzkum, Vol.34,
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