Why did the Rio 2016 pool water turn green?
Although not strictly alkalinity, this article quotes a changed pH level and an expert gives two possible explanations


  One: A low or high pH can bring out minerals. "Calcium would show itself as cloudy water, copper would show up as a blue-green," he says.
  
  Two: A too-high pH would render the chlorine ineffective, which could then allow for the proliferation of algae. (Hernandez says it's doubtful, though, that the green color is mainly from algae. It typically doesn't bloom that fast in such large pools.)