I would pull a water sample if I was you. I had this happen to me over 10 years back on a pond near a surface road. Turned out the water authority had scaled the water lines one night a 1/4 mile away (uphill from the pond) and the run off had made it into the pond. We pulled a water sample and the chlorine is what led to me finding out what really had happened. I had first chalked it up to a kill off and had took the sample anyway. Good thing I did.