A common form of water pollution is runoff of fertilizer from fields. In the central part of the US, known as the "Bread basket" (think of places like Illinois, Iowa, Kansas, and so on), farmers use a great amount of chemical fertilizer to ensure large crops of corn and soybeans. Since fertilizer is relatively cheap, farmers have an incentive to over-apply, as opposed to under-applying it. Any fertilizer that is not absorbed by the plants runs off with rainfall into rivers, and these rivers all end up flowing into the Mississippi River. As you go downstream, the concentration of the Mississippi river increases. The result is that in the Gulf of Mexico, just off the coat of Louisiana, there is a large "dead zone' several hundred square miles in size which is caused by the excess fertilizer in the water. This fertilizer accelerates the growth of algae in the water, and the algae consumes all of the oxygen in the water, meaning that nothing else can live in the water. One of the reasons that the damage from the recent oil spill in the Gulf was not larger was that it occurred in this "dead zone", so there was not a lot of marine life to kill to begin with. This situation creates a benefit for everybody who consumes corn or soybeans - which is, basically, all of us, in the form of cheap food, but it has a significantly negative effect on the biodiversity of the Gulf of Mexico, with secondary effects that are scarcely understood by humans.