The number one source of pollution is non-point pollution. The reason it gets the number one spot is that it is such a problem, no one really knows just how big it is because many non-point pollution sources go undetected. But, of the ones that we do know about, the issue is mammoth. Non-point pollution is pollution that comes from an indirect source like boats that are in the ocean. They aren't dumping oil or gas into the water directly, but they are doing indirectly with their exhaust. There are factories that pollute the ocean indirectly that don't even sit oceanside.
They can be many states or many miles away from the ocean but their leakage into a river goes downstream, polluting the river and the sea. The same can be true for air pollution. Not all souces are directly sending air pollution up but may be doing so in an indirect manner like through fumes, even ones inside the building that end up being released into the atmosphere. Many pollutants are not direct but they directly effect the air, water, or ground and are huge problems that lead to the number one source of pollution.
Human activity and carelessness is the major cause of pollution, in fact no recorded pollution is evident from sources other than human activity. this activity includes industrial, domestic and leisure activity. Factories produce waste and toxic products and have had increasingly to monitor how this is disposed of.
Pesticides in agriculture have disturbed or destroyed wildlife, and individually, humans have produced waste and disposed of plastic that pollutes water and land.
I suspect that human waste, including what we use and then throw away, is the worst source of pollution . On surface water oil spills and gasoline waste floats on the surface and badly affects the water and its inhabitants. Water contaminants from fracking is also bad, and includes toxic chemicals, methane, benzene, and radiation. But there is also waste such as plastic, mindlessly chucked out of canoes, passenger ferries, or left on the beach to be washed away.