Geochemical processes occurring in Maardu mining waste rock dumps, Estonia, are reviewed on the basis of Estonian practice and recent modelling studies. Oxidation of Dictyonema shale disposed close to the steep slopes leads To air pollution and destruction of newly formed vegetation during first decades after disposal. New mineral phases are formed at high temperatures and potential pyritic acidity is eliminated. iii central par＇s of the dump, low-temperature (below 40 degrees C) oxidation of pyrite leads to a sequence of geochemical reactions in interaction with dumped material, including conversion of illite to smectite, precipitation of K-jarosite, gypsum and ferric oxyhydroxide, as well as formation of leachate with high sulphate content and anomalously high Mg/Ca ratio. Hydrochemical modelling showed that low-temperature oxidative leaching of the shale would cause regional surface and ground water contamination with sulphate for centuries. A possibility exists, that limestone buffering capacity becomes locally depleted inside the clump, leading to the breakthrough of acidic and metal-rich waters. Maardu study assists also in development of environmentally friendly technology for shale handling in future.