Contrary to popular belief, rainwater is in no way pure; it's filled with chemicals. Even without industrial pollution, atmospheric carbon dioxide dissolved into clouds makes rainwater at least slightly acidic.
Indiana's rainwater is almost ten times more acidic than natural rainwater. There are a lot of power plants and other factories that burn coal to make energy in this Midwestern state.
Burning coal releases sulfur and nitrogen oxides into the atmosphere, which make rain more acidic.
Luckily, the acid content isn't high enough to harm humans, and it doesn't affect the taste of the water, either. But acid rain can damage lakes, streams, and trees at high elevations. Also, acid rain accelerates the decay of buildings, statues, and sculptures.