There are many different kinds of aquifers . The water in an aquifer can react with the rock surrounding it. This is caused by ions coming out of the rock into the water.
Some rocks contain sulfates.
When the groundwater changes in composition its behavioral properties also change. An acidic water may react with or cause other materials to dissolve that otherwise would not. Even rain has effects. If acid rain is present in an area, then the water entering the ground is already acidic and may react with the surrounding rocks and materials. A chemical indicator is something that changes color depending on its surroundings.