It might be possible to pick up some PH tape and test the water in that capacity first. I might add that this is not the same type of red bumps that some people develop because they are allergic to the algae. I've never had this happen at any other hot springs due to the algae. I think we are all having the same kind of reaction and we've definitely been no where near chiggers! LOL
Keep in mind, when I used an acid solution to rinse in, the reaction was almost non-existant after soaking for an extended period of time.