With an open-air water reservoir, I'd be much more concerned about things like Dracunculiasis (Guinea worm)[0] than urine. Thankfully, the Guinea worm poses a low risk to the US due to how easy it is to prevent[1], but it's a problem elsewhere in the world.
Dracunculiasis only occurs in four countries (and the US is not one), but the reason it spreads/persists there is due to improper practices regarding drinking water.
[0] https://en.wikipedia.org/wiki/Dracunculiasis
[1] https://en.wikipedia.org/wiki/Dracunculiasis#Cause - each generation of worms needs a human host, so even if a single water supply were contaminated, it would not be able to reproduce again unless the water supply were re-contaminated.