After a week of trying to part with green tides in two outdoor swimming pools, Olympic officials over the weekend wrung out a fresh mea culpa and yet another explanation—neither of which were comforting. According to officials, a local pool-maintenance worker mistakenly added 160 liters of hydrogen peroxide to the waters on August 5, which partially neutralized the chlorine used for disinfection. With chlorine disarmed, the officials said that “organic compounds”—i.e. algae and other microbes—were able to grow and turn the water a murky green in the subsequent days. The revelation appears to contradict officials’ previous assurances that despite the emerald hue, which first appeared Tuesday, the waters were safe.
I would personally have avoided using the green pools, but that's just me.