Most minerals are left behind when water evaporates, making rain freshwater rather than saltwater. This makes rainwater relatively pure, but atmospheric and surface pollution can reverse the process.
The burning of fossil fuels, such as coal, injects sulfur dioxide into the atmosphere. Nitrous oxide from automobile exhaust lingers in the air, too.
When sulfur dioxide and nitrous oxide combine with humidity—evaporated water from the ocean, for instance—they create two very destructive acids: sulfuric acid and nitric acid. The once-pure rain that falls back to Earth contains these acids, which can kill life and destroy Earth’s crust. The common name for this is acid rain.
Not only does acid rain hurt the environment, but when polluted water first evaporates, it leaves behind impurities (just as ocean water leaves salt behind).
Those impurities follow their own cycle of sorts, seeping into groundwater and traveling in rivers to the ocean, where the water evaporates again, leaving toxic remains to further pollute the water supply.