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Oil Shale, 2006, Vol. 23, No. 1 ISSN 0208-189X
pp. 3–14 © 2006 Estonian Academy Publishers
THE IMPACT OF INFILTRATION DAM ON THE
GROUNDWATER REGIME IN THE KURTNA
LANDSCAPE RESERVE AREA
I. VALGMA
*
, H. TORN, K. ERG
Mining Department of Tallinn University of Technology
Ehitajate tee 5, 19086 Tallinn, Estonia
The area of Kurtna Landscape Reserve is situated between oil shale mines.
This area is an important part of the Estonia deposit, and the located mining
conditions there are good. Narva surface mine pumps groundwater from the
area of Kurtna Lakes. It is able to minimize the influence of surface mining.
Testing of mining technology and hydrogeological modelling show that mine
front may be closed for stopping water flow instead of leaving an open trench
by the border of the area of the lakes. According to modeling, hydraulic
conductivity of the dam must remain 0.1 m/d to avoid sinking of water level
in the lakes, and filtration basins must be supplied with water in an amount
of 7000 m
3
/d as yearly average. As the result, the landscape will be
reclaimed, overall look and shape will be smoothed. Abandoned fields of peat
milling will be reclaimed, and their fires will be avoided.
Introduction
The influence of the power and mining industry on Kurtna Lakes located
in the centre of the oil shale mining area in North-East Estonia has been a
discussion object for the last fifteen years. Oil shale mines surround the area
of Kurtna Lakes. There are 40 lakes in a 30-km
2
area above a 70-m deep
buried valley [1]. Two mines – Estonia and Narva – exert the greatest
influence on this. Both mines pump out water from the area and lead it back
to the lakes or rivers in the same area. The question is how much the mining
influences protected lakes and species. Besides Narva surface mine
neighbouring the lakes has claimed the permission to pump groundwater
from the lake area (see Fig. 1). Surface mining was stopped at a distance of
2 km from the protected area since the problem had not been solved, thus the
mining company asked independent research groups to perform correspond-
ing analyses and to test the mining technology used in this area.
*
Corresponding author: e-mail ingoval@cc.ttu.ee
I. Valgma, H. Torn, K. Erg
4
Fig. 1. Location of the Kurtna Landscape Reserve, mining section
and infiltration dam
There are two main reasons for mining oil shale in the area. The first
reason is oil shale resource. Local reserves represent an important part of the
Estonia Oil Shale Deposit [2]. Energy rating of oil shale is one of the best
among the potential mining areas reaching 44 GJ/m
2
,
and depth is relatively
low (10 to 15 m) compared to 22 m in the rest part of the surface mining area
[3, 4]. This resource is of great economical importance because when mined,
total costs of mining will remain stable. On the other hand, pressure on
mining oil shale in unsuitable areas will become actual in the future [5].
From these aspects, the application for mining permit is reasoned.
Since Estonian main oil shale-fired power plant has been renovated, and
new boilers require oil shale of a more stable quality than the former ones,
the need for oil shale mining remains actual for at least 25 years, which
corresponds to the resource in current minefields [6]. New power units
operate applying a new, fluidised-bed technology that guarantees less impact
on the environment. Total resources of oil shale in the Estonia deposit
guarantee operating of power plants for 60 years [2, 5].
Due to low oil shale quality in the most part of the deposit and additional
environmental restrictions, the quantity of mineable oil shale is not as great as
it seems. Compared to 50% total loss of oil shale resource in the case of
underground room-and-pillar mining, the loss in open cast mines reaches 30%
[7]. The losses are due to differences in official and actual resources and oil
The Impact of Infiltration Dam on the Groundwater Regime in the Kurtna Landscape Reserve Area
5
shale remaining in supporting, protective and barrier pillars. As for the usage
of resources, surface mines are more valuable [8]. The main problems con-
cerning mining fields are related to mining conditions and environmental
restrictions. Surface mining is reaching depth limit, while underground mining
is confronted by low quality of oil shale, bad roof conditions and environ-
mental restrictions [4, 9]. For these reasons continuation of mining in the
section neighbouring the area of Kurtna Landscape Reserve is advisable.
Mining in the test section should be performed under continuous monitoring
for calibrating dynamic modelling with groundwater software.
Influence of oil shale mining on the area of Kurtna Landscape
Reserve
The influence of mining on the lakes has been investigated from the
hydrogeological aspect, recommending the usage of infiltration basins and
regulation of water flow [10–14]. Water chemistry has been investigated
proceeding from the effect of oil shale mining on sulphate content of water
[15]. The influence on the landscape and plants has been studied by analys-
ing plant species and mining waste [16–19]. The set of water wells and lakes
in the water monitoring program has been set to analyse changes in eco-
logical situation. Unfortunately, it has not given a clear answer to the ques-
tion about the influence of mining on the groundwater flow in the area. The
reason for that has probably been complexity of situations and lack of an
interested party who would evaluate all the aspects concerning this region.
The closest lakes (Kastjärv, Aknajärv, Jaala, Kihljärv, Nootjärv, Valge-
järv and Virtsiku) are located in a distance 1.5 to 3 km from the front of
Narva surface mine. Data of observation wells show that the level has not
been remarkably changed due to surface mining operation during the last
five years. All these lakes are located in the area of boggy, glaciolacustrine
and -fluvial deposits. Drawdown of groundwater has been formed due to an
intensive consumption of groundwater at the water intake in the central part
of the Vasavere buried valley. Quaternary aquifer is an unconfined water-
bearing stratum. The values of porosity and permeability of Quaternary
aquifer depend to a large extent upon the degree of sand cementation. Con-
sequently, these values are generally expected to be much higher for the
central part of the valley than for the slopes. A study of the samples shows
that porosity values exceeding 33% are common for the central part, whereas
those less than 20% are usual for the southern part of the valley. Similarly,
intergranular permeabilities average 2500–2700 m
3
d
–1
m
–2
in the central part
and drop to 10–50 m
3
d
–1
m
–2
at the border of the valley [20]. Seasonal
factors, changing flow and various forms of recharge may all produce
fluctuations in the water level of about 1 m.
I. Valgma, H. Torn, K. Erg
6
32
33
34
35
36
37
38
39
40
1999 2000 2001 2002 2003 2004
Water level, m
Fig. 2. Sinking of water level shows sand inflow to the trench and
quick normalisation of the state after closing the flow. Water wells
B 6-1 ja B 6-2 are located between Lake Kastjärv and the dam
Fig. 3. Technogenic valley formed after sand flow into the trench
in March 1999
Water table of L. Valgejärv lowered 2 m in 1984, caused by water usage
for stopping peat fires. For the year 1996, water level was normalised again.
Only one remarkable event happened in 1999 when sand basement of the
peat field flew into mining trench (see Fig. 2, 3). This happened because the
spoil was piled on ice, and when ice melted sand spoil became unstable. The
water-table diagram shows that original water level was restored quickly
after closing sand inflow.
The Impact of Infiltration Dam on the Groundwater Regime in the Kurtna Landscape Reserve Area
7
Mining technology
The influence of Narva surface mine at the east side of the lakes’ area
could be minimised. First, the mine front can be closed for stopping water
flow instead of leaving an open trench in the border the area of lakes.
Second, the overburden material that is used for closing the trench can be
piled in such a way that it has lower permeability than soil in the nature.
Besides, a part of that area is covered with abandoned fields of peat milling.
Thickness of the residual peat layer reaches up to 3 m being a good material
for decreasing permeability of the final dam material. For testing these
assumptions, a test section was planned and designed by Mining Department
of Tallinn University of Technology in 1997. The purpose was to test
whether the filtration dam will decrease the sinking of water level in the
area. Test mining in this section started in 1998. The idea originated from
dam-piling experiences in the same mine where a dam has been piled with
careful dumping and mixing of overburden material (see Fig. 4). This
resulted in accumulation of water behind the dam, a water body now called
Lake Vesiloo named after the designer of the dam. Water level in the upper
lake has remained stable for 45 years, which proves permeability of piled
overburden material consisting of limestone, clayey sand and peat.
For evaluating the influence of mining on a larger area, a modflow-
groundwater model was set up by independent company AS Maves in co-
operation with Mining Department of Tallinn University of Technology
[19]. The model is supported by continuous monitoring of water wells and
mine-dewatering data.
For closing the existing open trench, placement of mine front and stripping
technology were changed. The direction of mine front had to be changed by
45 or 90 degrees (see Fig. 5). This could enable building of an infiltration
barrier between the lakes and the mine at the end of the trench where only a
30-m-wide pit would be temporarily opened for water infiltration.
36.1m
30.3m
Spoil
5.8m
Bank
Mine bottom
Infiltration dam
Vesiloo lake
Fig. 4. Cross-section of infiltration dam in the mined-out
area of Viivikonna section. Water level in Lake Vesiloo
has remained 6 m higher from water level in lower lake
for 45 years
I. Valgma, H. Torn, K. Erg
8
Fig. 5. Layout of water inflow to the mine applying old technology
(above), new technology (below)
Dragline ES-10/70 is used for piling the dam. Ordinary selective piling of
the overburden will be finished at 50 m from the border. Beginning from this
point, the material will be disposed homogeneously. Different materials are
dumped on top of each other. This should guarantee low permeability of the
dam. Important is that the material be dumped from the maximum height of
dumping position of the dragline and onto different locations. The width of the
dam should be at least 25 m, and the high wall should be covered with a
mixture of clayey material and peat. Stripping productivity in the section will
be decreased by several factors. Dragline’s cycle time increases because of
hauling of the bucket to the maximum height of the dump, repositioning of the
boom in every cycle, and careful monitoring of homogenisation of the spoil.
Besides, technology of seam extraction in the dam area is affected by many
factors. Oil shale interlayer C/D has to be hauled away from the location of the
dam because of its high swelling value (up to 200%). Hydraulic conductivity
The Impact of Infiltration Dam on the Groundwater Regime in the Kurtna Landscape Reserve Area
9
of this loose material could reach 1000 m/d. Alternatively, the seam has to be
extracted non-selectively, leaving a 30-cm limestone layer in the output
material. The overall productivity will decrease because the trench is short –
700 m, the optimum length being 1.5 km [21]. This concerns organising
stripping, haulage and dam operations in a short section. Optimum length of
the section was achieved by positioning the trench at 45 degrees instead of
planned 90° in relation to the original North-South direction. (see Fig. 5)
The groundwater model was made on several assumptions. Groundwater
discharge from the mine should be within certain limits to keep the decrease in
the water level of the lakes below the agreed limits. According to the model,
hydraulic conductivity of the dam material should not exceed 0.1 m/d.
Besides, infiltration basins which are located between the mine front and the
lakes should be fed with water in an amount of 7000 m
3
/d for complementing
soil water of the surroundings and keeping water level in the area stable.
Dam material consists of silty fine-grained sand whose modulus of
hydraulic conductivity k = 0.1–0.8 m/d depending on density and compaction
index of the material, and the content of clay particles. Provided that the test
section was piled according to the design, the modulus of hydraulic con-
ductivity of the material in this section could be in limits k = 0.1–0.2 m/d. In
addition, the overburden contains moraine (k = 0.01–0.05 m/d), fine-grained
sand (k = 0.5–2 m/d) and loose broken limestone (k = 100 m/d). Spoil material
dumped conventionally is characterised by k = 0.5–50 m/d. It is assumed,
basing on the experience gained from the test section, that fine material will
fill spaces in coarse material decreasing permeability of the bank dam. The
given solution will not work with drains that could form in the case of piling
C/D layer, or with open dewatering tunnels under oil shale bed. The influence
of mining on water level and quality of lake water near the test section is
unnoticeable that proves the suitability of the technology. However, the
conditions will become more complicated in southern direction (see Fig. 6).
There are peat, sand and moraine layers in the cross section of the
Quaternary sediments and Ordovician limestone in hard overburden, causing
high permeability of the spoil. An increase in the thickness of limestone
seam in southern direction causes most of the dumping problems in dam
building. Additional amounts of sand have to be scraped from aside,
probably rehandling the overburden, and, in addition, compactors should be
used for achieving proper modules of hydraulic conductivity.
Dam piling was modelled applying the geometric model that is used for
determining the ultimate pit depth for draglines [9]. The model yields figures
about suitability of draglines, need for rehandling and additional scraping,
and also differences in the final height of the ground (see Figures 7 and 8).
The test section has shown that it is difficult both organisationally and
technologically to establish all these parameters. For evaluating the
effectiveness of the dam in the southern part, several tests on density,
compression and permeability of the spoil material have to be performed and
compared with data obtained using the hydrogeological model.
I. Valgma, H. Torn, K. Erg
10
12
17
22
27
32
37
42
47
Height, m
Soft overburden
13.9 15.5 13.6 10.8 10.9
Hard overburden
5.14.28.09.912.1
Oil shale seam
2.82.82.82.82.8
Bottom layer
23.2 22.3 19.2 17.5 13.6
1-North 2 3 4 5- South
Fig. 6. Thickness of limestone overburden increases in southern direction causing
dumping difficulties for draglines and higher hydraulic conductivity of overburden
material
Fig. 7. Pit at the end of the trench before 1997. Limestone pile on the
bottom of the trench forms under spoil drainage channels
If the technology of dumped spoil fails, the compactors on the spoil and a
geomembran barrier on the bank wall made using contour blasting, or a clay
barrier must be applied.
The Impact of Infiltration Dam on the Groundwater Regime in the Kurtna Landscape Reserve Area
11
0
10
20
30
40
-100 -80 -60 -40 -20 0 20 40 60
Dragline center
Spoil Bank
Length, m
Fig. 8. Pit layout and modelling of surface height using geometrical model
Different materials and constructions are widely used to reduce hydraulic
permeability at exploitation of mines, quarries and waste depositories.
Depending on purposes, the barriers, dams, sheet piles or cut-off walls of
different permeability could be constructed. In Estonia, hydraulic barriers are
designed and constructed around Sillamäe Radioactive Waste Depository
(bentonite slurry cut-off wall) and Tallinn old municipal landfill (vinyl
sheet-pile wall). Watertight clay barriers are widely used in construction of
new landfills.
Vertical slurry cut-off walls
Slurry cut-off walls are vertical walls constructed by excavating a trench
and simultaneously filling the trench with a bentonite slurry. The bentonite
slurry forms a thin (typically
3 mm) filter cake of low hydraulic con-
ductivity (<10-8 cm/s) on both sides of the trench. The filter cake minimises
slurry loss from the trench, stabilises native soil on the side walls of the
trench, and provides a plane for slurry stabilisation in the excavated trench.
The bentonite slurry contains typically 4% to 7% (w/w) sodium bentonite
mixed with water.
Three main types of slurry walls, used to locate polluted groundwater, are
soil-bentonite (SB) walls, cement-bentonite (CB) walls, and composite
slurry walls (CSW). Soil-bentonite slurry walls are constructed by displacing
bentonite slurry in the excavated trench by backfilling with a mixture of
bentonite slurry and excavated trench spoils. Cement-bentonite (CB) walls
are constructed by using a mixture of cement and bentonite slurry to
maintain the stability of the excavated trench; i.e. no backfill materials are
required. Therefore, CB walls are typically constructed in the case when
suitable backfill materials are not available. Composite slurry walls (CSW)
are constructed simply by inserting a geomembrane into the slurry in the
trench.
Height, m
I. Valgma, H. Torn, K. Erg
12
Alternative passive barriers
Aside from slurry cut-off walls, other passive vertical barriers include
walls constructed using deep soil mixing or jet grouting using chemical
grouts (e.g., silicates, resins, and polymers), grout curtains, and sheet-pile
walls. Although these technologies are used extensively in more traditional
geotechnical engineering applications, such as dams and construction
excavations, none of these technologies have been used extensively as
passive containment barriers for remediation of contaminated land.
A comparison of the construction costs for the vertical barriers shows that
the costs associated with used material of alternative barriers is typically
greater and the construction rates lower than for the more traditional SB and
CB slurry walls.
Biobarriers
The concept of using bacteria to form biofilm barriers, or biobarriers, in
otherwise highly permeable media (e.g., sands) through plugging or fouling
the massif to reduce the migration of contaminant plumes has recently
gained attention. Reductions in hydraulic conductivity from one to three
orders of magnitude have been reported for a variety of porous media using
many types of bacteria and different treatment methods, including stimula-
tion of indigenous bacteria (biostimulation) and injection of full-size living
and dead bacteria as well as ultramicrobacteria (bioaugmentation). A significant
additional research must be performed before biobarriers can be used
routinely for practical application.
Conclusions
In the test section of the Estonia deposit near Kurtna Lakes the influence
of mining on the water level in lakes is low. The technology of dam
construction has been proved. However, geological and geotechnical condi-
tions will be more complicated southwards. According to modelling,
hydraulic conductivity of the dam must remain 0.1 m/d to avoid sinking of
water level in the lakes, and filtration basins must be supplied with water in
an amount of 7000 m
3
/d as yearly average. After oil shale mining east of the
area of Kurtna Lakes the area will be recovered as
o landscape will be reclaimed, overall look and shape will be smoothed
o abandoned fields of peat milling will be reclaimed and their fires
avoided.
Acknowledges
This study was supported by ESF Grant 5913 Usage of mined-out areas.
The Impact of Infiltration Dam on the Groundwater Regime in the Kurtna Landscape Reserve Area
13
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Presented by E. Reinsalu
Received January 21, 2005
