If you could have the water tested that would be a great first step NM. I seriously doubt that this situation has been caused by anything coming from a dog that has been introduced into the water. I have been going to the springs for many years and people from time to time will bring thier dogs into the pools and I've never had a rash problem like this before. I think the irritant is more likely as DCR described, related to some change in the chemical composition of the water. I doubt also that its some sort of biological agent. Who will solve this mystery :-)