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Chemical and biological characteristics of the Pühajõgi and Kunda catchments were determined in 1995, 2000 During last ten years the concentration of total nitrogen in the samples of the river water from the Pühajõgi catchment has decreased on the average almost 90% since four mines were closed. The concentrations of total nitrogen in the water samples of the Kunda catchment decreased on the average ca 20% during these ten years. The conductivity values of the river water samples of the Pühajõgi catchment between 800 and 1800 µS cm -1 indicated a significant amount of ionic species in the river water. The water was microbiologically unpolluted according to the results of the analysis in 2005.

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CaS formed during the retorting process of oil shale has a hazardous influence on surface water quality. Interaction of retorted oil shale with water generates highly alkaline leachate with a high content of sulfur due to the CaS (mo•L -1 ) 2 at a temperature of 25 ℃.