why does the treated water smell? (living, charge, places)

[Deserterer]

the water coming from the treatment plant is discharged into the Rio Grande. In the winter, its the only water in the Rio. I've lived in many places and I've never smelled the treated water before. It doesnt smell like sewage, but it doesnt smell good either. I love the cool wind from the north where I'm living, except that it brings the smell wafting through my open window. Its hard to describe the smell. It smells like water that's been trapped in concrete tanks for a long time. Kind of stale, perhaps a bit "off". I certainly wouldnt drink it or swim in it, but I notice it doesnt stop other people or ducks.