OXIDATION OF DICTYONEMA SHALE IN MAARDU MINING WASTE DUMPS

Geochemical processes occurring in Maardu mining waste rock dumps, Est onia, are reviewed on the basis of Estonian practice and recent modell ing studies. Oxidation of Dictyonema shale disposed close to the steep slopes leads To air pollution and destruction of newly formed vegetat ion 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) oxida tion of pyrite leads to a sequence of geochemical reactions in interac tion 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 hi gh Mg/Ca ratio. Hydrochemical modelling showed that low-temperature ox idative 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. M aardu study assists also in development of environmentally friendly te chnology for shale handling in future.