A HYDROGEOLOGICAL MODEL OF THE OPEN-CAST MINE TAMNAVA-WEST FIELD ( KOLUBARA COAL BASIN , SERBIA ) HIDROGEOLOŠKI MODEL

The Kolubara Coal Basin is the largest coal-producing area in Serbia. It is administratively comprised of several open-cast mines: Field A, Field B, Field C, Field D, Field E, Field F, Field G, Veliki Crljeni, Šopić-Lazarevac, Tamnava West Field, Tamnava – East Field, Radljevo, Zvizdar, Ruklade, and Trlić. The paper focuses on the largest active open-cast mine: Tamnava West Field. A study and conclusions about the hydrogeologic functions of the lithologic strata, and insight into their spatial interactions, are extremely important for hydrogeologic modeling of a given area. The main purpose of a hydrogeologic model is to derive conclusions about the spread of the aquifers in the study area, their interactions, and groundwater flow conditions. Based on geologic and hydrogeologic exploration, and analysis and synthesis of the collected data, the paper describes hydrogeologic modeling of the open-cast mine Tamnava – West Field. The software used in the research provides a portrayal of the 3D distribution of the aquifers present in the study area. As such, the hydrogeologic model constitutes a basis for developing a hydrodynamic (mathematical) model of the groundwater regime.


INTRODUCTION
Kolubara Coal Basin (KCB) is the strongest link of the Electric Power Industry of Serbia.Since World War II, it has produced more than 580,000,000 tons of coal.Every other kilowatt in Serbia comes from KCB's lignite (brown coal).A daily extraction rate of as much as 100,000 tons of coal ensures reliable operation of the thermal power plants Nikola Tesla and Veliki Crljeni.
Among the major recent hydrogeologic exploration activities at Tamnava -West Field are those reported in Polomčić et al. (2016) and Đinđić et al. (2016).
With regard to the topic of this paper, many researchers worldwide have addressed hydrogeologic systems of ore deposits, groundwater flow and processes that affect the hydrogeology of mining sites, including: Dimitrakopoulos et al. 2000;Rapantova et al. 2007;Kamkar-Rouhani, 2008;Rühaak et al. 2009;Niedbalska et al. 2011;Wels et al. 2012;Brown and Trott, 2012.

STUDY AREA
KBC occupies a land area of about 600 km 2 .It is located some 50 km southwest of the capital Belgrade (Fig. 1) and encompasses the area around the middle and lower courses of the Kolubara River and its tributaries: the Tamnava (left) and the Peštan (right).Tamnava -West Field falls within the municipalities of Lajkovac and Ub, near the villages of Kalenić, Mali Borak, Radljevo and Skobalj.To the west and south, the mine borders on the exploratory field Radljevo, and to the east on Tamnava -East Field.The northern boundary is natural, where the coal bed lenses out.

Figure 1 Geographic location of the study area
The geologic framework of Tamnava -West Field is comprised of Paleozoic and Mesozoic sediments within the paleo-relief.The coal basin is built up of Quaternary and Tertiary (Neogene/Pliocene) strata.In the extended zone of the study area (Fig. 2), these deposits contain alluvial and terrace riverine-lacustrine sediments made up of mediumgrain gravels and sandy gravels (river terraces and alluvial plains -riverbed sediments of the Kolubara, the Kladnica, the Turija and the Peštan).There are also other deposits, such as diluvial/proluvial and oxbow sediments -silty clays and local bog clays (alluvial plains).Quaternary sediments lie discordantly over Pontian strata.The coal seams of Tamnava -West Field are underlain by quartz sands.In addition to the coal beds, the complex coal-bearing sequence comprises sand and coaly-clay interbeds.Upper Pontian and Quaternary sediments make up the ceiling of the coal sequence.There is a triple aquifer system in the extended zone of the study area: upper, intermediate and lower.The three aquifers are all confined and feature a subartesian groundwater level.
The upper aquifer at Tamnava -West Field is largely comprised of alluvial and riverine/lacustrine terrace sediments -terrace gravels of various grain sizes in both vertical and horizontal directions.Well pumping tests have been conducted and the hydraulic conductivity found to range from 6x10 -5 m/s to 6x10 -4 m/s.
The intermediate aquifer comprised of sands is found in the extended area of Tamnava -West Field, in a part where deposited sands lie between two coal beds.These sands extend beyond the mine to the northwest, where they join the sands of the lower aquifer.
The grain-size distribution of the intermediate aquifer is relatively homogeneous in both vertical and horizontal directions, the sands are fine-grain and medium-grain, and the hydraulic conductivity is about 1x10 -5 m/s.The intermediate aquifer is overlain and underlain by coal beds.In the northern part of the mine the overlying layer is integrated into the upper aquifer.
The lower aquifer at Tamnava -West Field is comprised of Lower Pontian sediments, represented by quartz sands.These sands extend beyond the mine and reach the edges of the Kolubara Coal Basin.The sands within Tamnava -West Field are very homogenous.Their values of hydraulic conductivity is (1.3 to 2.5) x 10 -5 m/s and transmissivity T = (2.5 to 6.0) x 10 -3 m 2 /s.The aquifer floor comprises coal beds, fine Miocene sands, silt and clay.

METHODOLOGY
A hydrogeologic model is an interpretation of the dynamic and physical properties of a hydrogeologic system (American Society for Testing and Materials, 2014).According to Polomčić (2001), a hydrologic system is determined by: input elements, characteristics of the medium, processes that take place in the medium, and output elements, where the input and output elements are inter-dependent.The characteristics of a hydrogeologic system are its geometry (i.e.spatial distribution of water-bearing media) and filtration properties.Hydrogeologic modeling is schematization of the hydrogeologic system, due to the hydrogeologic and hydrodynamic complexity of the conditions, and includes modeling of the (Polomčić, 2002): -flow field (aquifer geometry), -filtration characteristics of the porous medium and hydrodynamic state of aquifer flow, -aquifer regime, which is determined by certain laws, phenomena and distribution of factors, -water balance components, -boundary conditions (inflow into and outflow from the model), and -initial conditions, determined by the piezometric head at a certain initial time.
A hydrogeologic model is the first step in groundwater modeling and precedes a hydrodynamic model.If the hydrogeologic model is not properly defined, the hydrodynamic model will not be realistic.In hydrogeologic exploration of open-cast mines (Morton and van Mekrek, 1993), the hydrogeologic model can be developed in two stages, so-called "conceptualization phases".
In the first phase, the hydrogeologic model is created using collected data from prior investigations and, as such, it constitutes a basis for determining which data are "critical".The second phase is elaboration, founded upon the outcomes of additional, more recent geological and hydrogeological research.
In essence, the hydrogeologic model shows the types of rocks, from a hydrogeological perspective, and their geometry.The focus is on aquifers and their spatial relationships.Groundwater levels, directions of groundwater flow, and recharge and discharge zones of the hydrogeologic system can also be depicted.
Such a hydrogeologic model constitutes the groundwork for the next step -conversion of the hydrogeologic model into a hydrodynamic model via model geometry (i.e.spatial distribution of hydrostratigraphic units) and hydrogeologic parameters of the porous medium, followed by specifying initial and boundary conditions and selecting the numerical method to be applied (Krešić, 2006;Krešić and Mikszewski, 2012).

RESULTS
The software used in this research, to create the conceptual hydrogeologic model, was Groundwater Vistas Advanced, version 64-Bit 6.74 b.24 (Rumbaugh and Rumbaugh, 2011).
The contours of the modeled layers were determined on the basis of data collected from numerous exploration boreholes, distributed across the study area.According to the "Report on Lignite Reserves at Tamnava -West Field of the Kolubara Coal Basin as on 31 December 2009" (Kitanović, 2010), there were 796 exploration boreholes from contour 172.5 to contour 100, whose total depth was 48,449.05m.Based on those field explorations and an assessment of the lithologic members and their hydrogeologic functions, a total of eight layers were identified along the vertical, four of which were water-bearing layers: upper aquifer (Layer 2), intermediate aquifer (Layer 5) and lower aquifer (Layers 7 and 8).The real geometry of the layers was represented by their real areal spread in both planform and elevation.Given the natural depths and positions of the geologic members, the hydrogeologic model was specified with different areal spreads in planform and different, real thicknesses of the layers.The layers, from a hydrogeological perspective, viewed from the ground surface, are listed in Table 1.

CONCLUSION
The geologic and hydrogeologic conditions of the origin of the lithologic members in the study area have determined the geologic framework and sediment structure, as well as the hydrogeologic relationships between the lithologic members.
Distinct spatial (3D) groundwater flow has been noted in the zones of contact of the alluvial sand-and-gravel sediments with the sands of the intermediate aquifer and further down with the sands of the lower aquifer, where there is vertical lithologic stratification and non-uniform horizontal spread of the lithologic members.This affected the selection of the main characteristics of the hydrogeologic model, which was comprised of eight layers along the vertical.
The developed hydrogeologic model constitutes unavoidable groundwork for the creation of a hydrodynamic model of groundwater flow, which will support a hydrodynamic analysis of the conditions for ensuring protection of the open-cast mine Tamnava -West Field from groundwater, as well as the selection of an optimal dewatering system.

Figure 3
Figure 3 3D representation of the triple aquifer system (1 -upper, 2 -intermediate, and 3lower): a) south-north cross-section, and b) view from the south.

Table 1
Polomčić et al. (2016)thologic members of the modeled layers Analysis of the boundary conditions of the hydrogeologic system included effective infiltration and boundary conditions: river, general head boundary, drainage, specified discharge -drainage wells, specified discharge -"no flow" cells, and flow barrier.Among the so-called initial conditions, which characterize the piezometric head at a certain initial time, groundwater levels of the observation network comprised of 116 piezometers (of which 54 in the upper aquifer, 22 in the intermediate aquifer, and 40 in the lower aquifer) were measured and analyzed.Both the initial and boundary conditions are also examined in detail inPolomčić et al. (2016).The schematization described above enabled the development of the hydrogeologic model of Tamnava -West Field.Supported by suitable software, the model made it possible to distinguish and show individual lithologic members and provide an unlimited number of hydrogeologic cross-sections.By way of illustration, Fig.3shows the spatial distribution of the upper, intermediate and lower aquifers from different angles.