I've actually seen that particular reservoir. It's one of those man made concrete basin ones so that water isn't going anywhere. It's just sitting there.

Well, then it will evaporate, it will hardly hurt anything unless there is a drought going on. What I do see as wasteful is that they might use pumps to move it around, that's a lot of diesel or electricity.

I don't know if you've ever been to Portland, but it only has three types of weather. Cloudy, rainy, and occasionally snowy during the winter. The risk is in the bacteria or viruses that may have hitched a ride on the piss wagon, not the other contents of it.