Answer:
all of them must be cleaned and sanitized
Heavy industry, manufacturing and agriculture are three of the major causes of pollution of the Great Lakes system. The production of persistent organic pollutants (POPs) such as PCBs, DDT and dioxins, had previously been of principal concern. This does not account for all of the pollution, but are a majority of the blame. Chemical pollution, Plastic pollution, and Nutrient pollution have been devastating to The Great Lakes.
The causes of freshwater pollution are varied and include industrial wastes, sewage disposal, runoff from farmland, cities, and factory effluents, and the build-up of sediment. Emissions from factories and vehicles are released into the air.
Therefore, answers should be:
Industrial waste, sewage disposal and, runoff.
