The Supreme Court on Thursday upheld the sentences in a vote 3-2. The case is dubbed as Finland's largest environmental lawsuit by Finnish media, Xinhua news agency reported.
An appeals court had sentenced Pekka Pera, the CEO of the mining company, to six months imprisonment on probation. Two former managers were given fine penalties. The legal process began in 2015.
The Talvivaara mine produces nickel and other metals in a bioheap leaching process. The method creates major amounts of waste water and chalk that was stored in large open reservoir lakes in the mine compound.
Waterways and soil in the surrounding area were severely contaminated as the waste reservoirs began leaking soon after the mine started in 2007. The leakages included nickel, uranium and other toxic metals.
The mine is still in operation, but a state majority-owned company Terraframe took over in 2015 and corrected the environmental procedures. The original mining company changed its name and went bankrupt in 2018.
Legal experts said on Thursday the Supreme Court decision has a major importance for future mine development in Finland.
Vesa Majamaa, a former professor on environmental law at Helsinki University, told Finnish national broadcaster Yle that authorities would have to be more careful in reviewing the environmental applications by mine operators.
Majamaa noted that when Talvivaara filed for permits some 15 years ago, the regional authorities in the area were left alone in assessing them. "They did not have the expertise that the case would have required," he said to Yle.
( With inputs from IANS )