What are the four main sources of water pollution?

In the previous answers the four causes are pathogens.
organic and inorganic contaminants, macroscopic and thermal pollutants. Pathogens are
microorganisms that can cause and spread diseases. An example for this would be
eutrophication. When we use fertilizers and other substances on farmlands some of the
chemicals contaminate nearby water sources through run-off.  This causes the
phytoplankton in the water to grow rapidly and this leads to algal bloom. Algal bloom is
not good as it uses up all of the oxygen that is needed by the flora and fauna of the
water. It blocks the sunlight that might be needed for some photosynthetic plants and
organisms. The algae might even produce toxins that are very harmful and can affect any
animal that feeds on them. So that is one way the water can be polluted and dangerous
for consumption.

Organic compounds that pollute the water
include detergents, pesticides, fuels, and some types of industrial wastes
(organic).

Inorganic compunds would include industrial
inorganic wastes like sulphur dioxide which is emitted from industries. By-products of
industrial procedures fall in this category too.

Macro
means large. So macroscopic would be large types of debris. Oil spills are on large
scales. Shipwrecks are also in this category. Sometimes trash we dump on the street can
end up in the oceans and that would be a form of macroscopic pollution. Basically
pollution we can see.

Thermal pollution is caused mainly by
industries. Usually the temperature is increased due to the reaction of some chemicals
with the water. Chemicals like acid can release large amounts of heat when reacted with
water.