This article talks about the 3 kinds of pollution.
Air pollution
Air pollution is started by factories, industry, farms and some airports. Air pollution causes your sims to leave the city, since people don't want dirty air and breathing in it. To prevent it, avoid gas plants and go on with the wind power, and maybe later you can use the solar power plant. Another way is to place parks and trees in your city. Enough of them will reduce air pollution.
Water pollution
Caused by farms and industry. Farms bring in more pollution than industry because of too much fertilisation. If you construct pipes in areas where pollution is, later your enviroment advisor will complain about the pollution, and then ask you to build a water treatment plant. Ways to prevent them are by avoiding the pipes from getting in the polluted zone and building a water treatment plant away from it. But remember to connect the pipes to the treatment plant or the water will not be able to get there.
Traffic pollution
This kind is yet the most to be seen. High congestion causes the transportation advisor to complain and making you either upgrade them (if it's a street which is the problem) or make a street intersection(If it's a road).