Mercury

PCBs in the substrate sludge can not easily be
transported or distributed, which makes them stay in the soil
for a long time. Also due to their high fat-solubility, PCBs are
easily accumulated and concentrated in the organism.
Hydrolic systems including substrate sludge and soil, etc.
act as a very important deposition utensil to PCBs and other
hydrophobic organic compounds. With the disappearing of
primary pollution sources, they are likely to become the
secondary pollution sources and release PCBs to the
environment.
The settled PCBs in the environment will continuously
disperse to the oceans and cause the decreasing biotic
population and aggravate the damage to the biological
environment.