Bourbon - Water
Fresh spring water is needed for whiskey production so the starch in the grain can be cooked and the developing sugar extracted. Distilleries were therefore built near springs that yielded enough water.
Kentucky and Tennessee are located on a large limestone layer that filters the water very well. In valleys where the limestone layer is broken, you can find springs that carry superbly clear water ideal for whisky production.
Some of today's distilleries are located in industrial areas (e.g. Buffalo Trace). They don't use spring water but local supply water, which is demineralised or deionised.