What was said above is pretty much right on.
Distilled water is pretty much 7.0 or neutral if kept in an air tight container. Spring water is usually a little on the alkali side of things from the dissolved rock minerals and usually about 7.5 - 8.0 as it is usually for drinking and any more alkali and it would be too bitter and not really suitable for drinking.
In the summer and with the rains, acid rain could be a contributor to the well water’s change in pH but so could the heat along with the rain that brings in fresh microbes along with any plant matter and all this could be creating enough microbial activity decomposing the plant matter, this could account for the change in acidity alone.
Spring water can be used but the added mineral content needs to be accounted for, it could be good as many of your micro nutrients and needed minerals could be added to your plant this way, however in some situations it could cause problems because it is adding things, in some nutrient feeding programs they may recommend using distilled or reverse osmosis water with practically 0.00 EC/PPMs (Parts Per Million of dissolved solids/minerals in the water). These nutrient systems have all the minerals and many nutrients near the maximum and adding any more from tap water, well water, or spring water may throw the balance of things off and knock the pH way off or add a build up of too much of one of the minerals and maybe cause a toxicity for your plant.
So in the end, distilled water is in most cases is best if you don’t want to change the pH or add anything to the feeding of the plant besides just pure water. If you are growing in soil with a minimal or low dose fertilizer regimen, spring water might be a good thing. In hydro you probably don’t want to use spring water as everything you need should be added with the nutrient system you are using. You can use tap water in hydro, you just need to adjust you nutrients with the fact that your starting PPM will be higher and that it will already have some minerals in it and so will not be able to add as much as if you were using distilled or R/O water.