Like many other countries during the first half of the twentieth century, the Soviet Union did not know fully about the effects of massive industrialization on pollution. And the 30s saw a massive boom in industry, factories, mining, building literal new cities, etc., which created environmental problems that continue to this day. There’s water pollution, as well as air pollution in big cities, and some places have heavy metal contaminated soil and water. There are also problems with deforestation.

However, it should be noted that an attempt to slow this down began in the 60s/70s by the leadership of the USSR, although it hasn’t been “fixed” yet by a long shot.