Ash fields of Estonian oil-shale-fired power plants, especially ash hydro-transport system with its large amounts of high-alkaline waters, may cause great damage to the environment. The situation is particularly bad at the Baltic Power Plant whose ash fields and sediment ponds occupy more than ten square kilometers. The samples taken from the 2nd ash field were studied to determine their structure, pressure resistance and water filtration ability. Ash field material has a stratified structure, all layers contain Ca(OH)(2) which, contacting with water, makes the latter highly alkaline. According to preliminary calculations, the ash field material binds only 10-20% of CO2 emitted at oil shale burning. Disconnection of the 2nd ash field from the ash field water-sluicing system would be the first and most practical way to reduce the amount of water to be added to the water system of ash fields. Ca(OH)(2) content of ash field material must be considered when making the projects for the 2nd ash field recultivation.