The introduction of harmful substances into the environment is better known as _____________.
There are two sources of pollution. __________ ____________ are those such as a pipe entering a stream from a wastewater treatment facility. _____________ _____________ are less easy to determine, but can include farms, septic systems and roads.
_________ ___________ occurs in some areas of the world, typically downwind of an industrial area, and can be detrimental to plants, water and animals.