Seems crazy to me. Assuming it’s an open air reservoir, imagine birds and other wildlife will do their business in the water, or right on the shore, so that it will seep into the water.
And imagine leaves from trees, pollen, dust, sticks, and other items end up in the water too.
Assuming the water is treated before it’s pumped out to customers, one person urinating is not going to pollute the water supply. Certainly not pollute it any more than can happen from animals or other sources.
Then there's the reality check. People are imperfect. They sneak into resevoirs to swim and some even pee in the water. Streams that feed the resevoir most probably get runoff from lawn fertilizers (the worst) and overflow septic etc.
Nobody is in charge of common sense anymore.