Here is a article from Popular Mechanix:

LESSON: RESPECT FOR DANGER
CASE STUDY /// Canyon Missteps
Who: Paul and Karen Stryker, 26-year-old newbie hikers in the Grand Canyon, during the blast-furnace days of June.

What happened: Embarking on a tough overnight hike they estimated at 18 miles (the true distance was 29 miles), the couple carried 6 quarts of water apiece.

Crucial decision: Underestimating the danger. After quickly finishing half their water, the Strykers came across a small pool. They rejected it, Karen later said, BECAUSE TADPOLES WERE SWIMMING IN IT.

What happened next: By nightfall, Paul was ill. The next day, with his condition worsening, they left the trail for what seemed to be a shortcut to safety. Their water gone, Paul became semiconscious and died that afternoon, probably of heatstroke. The next morning, Karen found her way to an emergency phone at Phantom Ranch at the bottom of the canyon. End of article.

My comments:
Note that they rejected water because it had tadpoles in it. I would think that the presence of tadpoles means that the water must be reasonably pollution free. I would still use a water purification method. The water can’t be that bad if something can live in it. I’m interested in what everybody thinks about this. Would you reject a water source because it has tadpoles living in it?