Something's in the water...

Today's 6 hour ride was the worst long ride that I've had during my Kona training. I dedicate so much time and effort to my training and to be struggling with nutrition at this point is ridiculous. I've tried tweaking everything and nothing seems to be working. On every long ride, with the exception of two rides (echo lake and Mt. Evans), I've had stomach problems about three to four hours into the ride. At first I just figured it was adapting to the altitude, then I figured it was dehydration, and now I have no clue. I'm doing nothing different from my Ironman training for Ironman Arizona; my breakfast is the same, my nutrition supplements are the same, my equipment is identical, my clothes are the same, my bike position is identical...nothing is different. Sure the routes are a little more challenging out here, but my legs are fine. Right now my legs are hardly sore, but my stomach is still going crazy. The last variable that I can think of is the water of Boulder County. An hour or so after refilling my bottles at a gas station or water fountain is typically when my stomach starts to go bad. Could it really be the water? For my sanity and my Ironman, I hope it is...