Test Site 1. - Sokolov
The Sokolov lignite mining area is located in the North-West of the Bohemia province, west of the town of Karlovy Vary, close to the German border. The area is largely affected by mining activities: open casts and dump sites, please see image below.
The Sokolov basin is part or the Eger rift that was strongly rifted into numerous horsts and grabens (Rojík et al, 1998) and gave rise to several brown coal deposits in Czech Republic, Germany and Poland.
The Sokolov basin, dating from the Oligocene to Miocene epochs, extends over 8 to 9 km in width and 36 km long with a total area of about 200 km2. The basin comprises 60 % volcanic ejecta resulting from faults and volcanic cones and 40 % sediments. It is bordered by a complex SW – NE faulting system and is cut by NW – SE faults. The basin is limited to the North by the Lipniza faulting system. Hydrothermal fluids have been circulating along the faults where silicification and sulphur are found, and exposure of the latter can result in Acid Mine Drainage.
Lignite is found only in the western part of the basin and comprises three coal seams. The Josef coal seam represents the lower one, just over the basement rocks. It is very rich in sulphur (up to 5%) and arsenic (60 – 70 ppm). The seam Anežka is of more recent genesis than the Josef seam and is developed only in the western part of the basin. The Josef and Anežka seams have been exploited in particular in the Medard open pit. The Antonin seam is currently exploited in the Jiří open pit, and contains up to 8 % sulphur, together with arsenic. Thick (130 – 200 m) overburden is represented by the clays of the Cypris Formation (Burdigalian), dominantly kaolinite at base, passing to illite and montmorillonite (smectite) to the top. The Cypris Formation is capped by limestone.Thick (130 – 200 m) overburden is represented by the clays of the Cypris Formation (Burdigalian), dominantly kaolinite at base, passing to illite and montmorillonite (smectite) to the top. The Cypris Formation is capped by limestone (Rojík et al., 2003, 2004).
The Lomnice pit was exploited in the early 20th century. The coal contained about 5.5% sulphide. The Medard pit has been exploited over 90 years, with a rate of eight million tonnes of lignite being extracted every year. Most of the overburden from the past has been dumped over the basement rocks, north of the Chodov – Lomnice road. The dump is 36 m thick on average and is mainly made up of clays from the Cypris Fm. Some levels are rich in organic matter, giving rise to brown levels. The clays contain few pyrite and ferro-magnesian minerals, responsible for magnetic anomalies.
In 2007, 10 million tonnes of lignite were extracted and 30 million tonnes of overburden material were removed.
The mining of brown coal is accompanied by several environmental problems, including:
- Local changes in morphology, landscape and drainage as well as degradation of land use due to dumping of material.
- Erosion of bare or thinly-vegetated dump slopes.
- Acid-mine drainage (AMD) and discharge of highly-mineralised water from mine dumps and contamination of surface and subsurface water.
- Vegetation stress due to contamination – air, soil, water.
The area is largely affected by AMD due to the presence of sulphur:
- In the brown coal itself (5 to 8% pyrite in the coal).
- In the hydrothermal deposits along the faulting system that borders the basin and that is affected by the exploitation.
AMD affects the mine water in the former-exploited open pits, with low pH values (pH = 2.2) measured with the presence of Na-Jarosite. Several abandoned pits (Lomnice, Medard, Marie, etc.) present intensive AMD and low-pH water, both on the pit slopes and at the pit lakes. AMD also occurs locally on dump sites.
The former-exploited areas are undergoing various rehabilitation procedures, including overburden backfilling of the open pits, re-vegetation of dumps and the construction of recreational areas such as golf courses. Sokolovská uhelná closure planning includes an artificial lake, filling the former Medard open pit and extending over 500 hectares with a 50-metre depth, for recreational purposes.
Rojík, P., Galek, R., and Pašava, J. (1998) Sokolov lignite basin. In Excursion Guide, 8th Coal Geology Conference, P. 1 – 69, Faculty of sciences, Charles University, Prague
Rojík, P., (2003) New stratigraphic subdivision of the Tertiary in the Sokolov Basin in Northwestern Bohemia, Journal of the Czech Geological Society, 49/3, pp 173 – 186
Rojík, P. (2004): Tectonosedimentary development of the Sokolov Basin and its interaction with the territory of the Krušné hory Mts. MS PhD Thesis. Charles University.
Satellite view of the Sokolov mining area and its location in the Czech Republic