of the 39 lakes in the Kurtna Lake District. The release of mine water into the environment has changed the natural movement of surface water. As a result of mining activities, groundwater moves towards the excavation cavities. A 220-kilometre (85-square-mile) underground water body that holds over of water has formed in eight abandoned underground mines: Ahtme, Kohtla, Kukruse, Käva, Sompa, Tammiku, No.2 and No.4. The process of pumping water from the mines introduces oxygen via aeration, thereby oxidising the rock's pyrite. Pyrite contains sulfur, and one consequence of its oxidation is the introduction of significant amounts of sulphates into mine water. This has had a negative impact on water quality in five lakes in the Kurtna Lake District. In some lakes, sulphate levels have increased tens of times compared to the pre-mining period. Suspended mineral matter in the mine water pumped into these lakes has changed the composition of the lakes' sediments. However, it has been found that this disturbance diminishes over time; studies show that sulphates and iron in mining water decrease to levels that meet drinking water quality standards about five years after mine closure. The process and waste waters used in shale oil extraction contain phenols, tar, and several other environmentally toxic products. Power stations use water as a coolant and for hydraulic transportation of oil shale ash to the ash heaps. Narva power stations use of water from the Narva River annually for cooling. For ash transportation, generated oil shale ash is mixed with water at a ratio of 1:20 and the resulted mixture, known as "ash pulp", is pumped to the heaps. Consequently, the transportation water becomes highly alkaline. The total volume of formed alkaline water is . Another source of water pollution is aqueous leachates from oil shale ash and spent shale. About of toxic leachate from the Narva ash heaps inflows annually to the Narva River and further to the Gulf of Finland. Before the closure of old semi-coke heaps in Kohtla-Järve and Kiviõli, an additional of leachates reached via the Kohtla and Purtse rivers to the Baltic Sea annually. The toxicity of leachate is mainly caused by the alkalinity and sulphides; leachate also includes chlorides, oil products, heavy metals, and PAHs which are carcinogenic. Air emissions Oil shale-fired power stations pollute air with the fly ash and flue gases like carbon dioxide (), nitrogen oxides (), sulfur dioxide (), and hydrogen chloride