Highlands Ranch has a very-fine wastewater treatment plant. The effluent from Highlands Ranch is probably of better quality than 1/4 of the worlds drinking water.
On Union: Avoid the day after a storm: sidewalk runoff. Avoid the first big day or two of the season: All upstream crap get's scoured out of the river bed.
The water at Union isn't clean, but it "could be worse" and is above most of the metro outfall. On good days, above 600 cfs, most of the water is coming from Chatfield anyway.
And yes, there are always tampons in the water, I'm pretty sure that some women just throw them out the car window like cigarrette butts.