Groundwater sulphate content changes in Estonian underground oil shale mines

During the course of the studies changes in the sulphate content in the Quaternary and Ordovician groundwater within ca 220 km area of closed and working underground mines in the central part of the Estonian oil shale deposit. In the area of oil shale mines, the Ordovician groundwater is stored in the carbonate rocks of the Nabala–Rakvere, Keila–Kukruse and Lasnamäe– Kunda aquifers. The oil shale mining causes drainage of the Keila–Kukruse strata. This affects the groundwater regime. Besides, the chemical composition of the water also changes. This concerns, first of all, the content of sulphates, concentration of dissolved mineral salts and hardness of water. In the eight closed mines underground water pools have formed. The chemical composition of water in these pools differs from relevant background values. The aim of the work was to study the sulphate content of surface (lake) and groundwater in pre-mining period, but also mining and post-mining changes in the Keila–Kukruse aquifer. It was attempted to find out interfaces between underground water pools and to study the sulphate content changes in the Keila–Kukruse and Lasnamäe–Kunda aquifers in the area of closed oil shale mines in a lateral and transversal direction. The thesis is based on the databases of Estonian Oil Shale Company and Geological Survey of Estonia and on the data obtained by the author in the course of her studies. The main results may be summarized as follows: 1. the hydrogeological regime in oil shale mines is controlled by the thickness of the aeration zone, tectonical faults and fractures in the geological section, alteration of hydraulic gradients causing changes in flow direction and rate (Paper I); 2. water table drawdown predictions are generally based on the assumption that geologic materials transmit water equally in all directions (isotropically); however, the oil shale mining area is unisotropic and non-homogeneous (Paper III) and simplified approach is excluded; 3. in pre-mining time the groundwater quality was mainly affected by precipitation (Paper II). During the mining period the sulphate content increased and was up to 50 times as high as under natural conditions (2–10 mg/l)(Papers II, IV); 4. in post-mining time the mines fill with water; the content of sulphates increases sharply – 3– 4 times (1500 mg/l) during two years and then, after four years, decreases to 200 mg/l (Paper IV); 5. closing and flooding of underground mines has radically changed the groundwater forming conditions in the Lasnamäe–Kunda aquifer (Paper IV); 6. due to technogenic impact the water of closed mines is connected with the Lasnamäe–Kunda aquifer. Evidence is derived from the deflection of the content of sulphate from natural background both in the mining field and in the observation wells tapping the Lasnamäe– Kunda aquifer in the surroundings of the mining area (Ch. 4); 7. of the active and passive methods applicable to water removal from the closed mines and prevention of flooding of the surrounding area author recommend to choose overflow wells (Ahtme mine) (Ch. 5). Generally, passive treatment technologies are preferred since these are more sustainable and can be made to integrate much better into their surroundings; 8. active water level regulation methods cause mine water depletion and changes in the sulphate content of the water around the year (Ch. 5); 9. generally, the water of closed mines meets the requirements of the Drinking Water Standard of Estonia (RTL 2001/100/1369) (Ch. 5) and the recommended active treatment method can cause large-scale secondary pollution. In view of this, the author of the thesis considers special mine water treatment unpurposeful.