It usually contains ions and compounds picked up from underground, land surfaces, pollution, and other human impacts. Hardness is one test that helps us determine the amount of certain dissolved minerals. Though methodology for hardness tests can vary to account for different ions, most simple tests focus on just calcium and magnesium. Hard water simply has excessive amounts of the polyvalent cations in them.
Dolomites are rich in magnesium.
These rocks are found in Western Pennsylvania, thus affecting the hardness levels in our water. If the cations responsible for making water hard are not calcium and magnesium, but are iron, sulfate, chloride, manganese, or aluminum, etc. Water that has entered waterways directly without soaking into the ground, will be significantly softer. Collected rainwater is usually soft because it has not interacted with any geological sources of the cations. Soft water is also derived from the drainage of igneous rocks, because these rocks don't weather very easily, don't release many cations, and don't always contain calcium and magnesium.